Title of Invention	A process for preparation of thick -film composite system useful for joining metal on insulating substrate.
Abstract	A process for the preparation of thick- film composite system by cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined, coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy and preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined area? in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint.

Title of Invention

A process for preparation of thick -film composite system useful for joining metal on insulating substrate.

Abstract

A process for the preparation of thick- film composite system by cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined, coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy and preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined area? in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint.

Full Text	The present invention relates to a process for the preparation of thick- film composite system. useful for joining metal on an insulating substrate More particularly, the present invention relates to joining two parts: a metal part such as metallic lead or metallic cap and a composite part such as a thick-film conductor consisting of metallic particles bonded by a glass matrix on an insulating substrate over a defined area of their respective surfaces using the principle of isothermal solidification. Its intended application is in microelectronic modules such as hybrid microcircuits, multichip modules etc. These modules find application in all areas which include: consumer, medical, telecommunication, automobiles, process-control, high temperature and high power applications, high reliability applications etc. In the hitherto known process for the production of microelectronic modules, it is desirable to establish a bond between two parts, a composite part such as thick film conductor on insulating substrate and a metallic part such as metallic lead or metallic cap. Microelectronic modules which are meant for commercial applications are either epoxy encapsulated, conformal coated with plastic, sealed in plastic moulded packages, or frit-sealed ceramic packages,. However, in any of the above, external leads are provided on thick film conductor at the periphery of the substrate by soldering which in turn may either be dip, reflow, using solder iron etc. In modules, which are to be used in stringent environments, metal- can packages are used but due to the high cost of metal packages, some times a metallic cap is secured td the substrate so as to isolate semiconductor devices and other components mounted on the substrate from the environment in which the module might be used. Generally, cap is secured to the substrate by soldering or welding In order to provide lead interconnection to the module and to isolate semiconductor devices and other components effectively, conductive interconnection between the lead and thick film conductor is provided thirough-holes in the substrate and these through-holes in the substrate must be sealed It has been conventional hitherto to use solder for conductive connection and solder, glass or plastic material to seal the through-holes in the ceramic substrate The above stated known processes for lead attachment in non-hermetic packaging, lead and cap attachment as well as through- hole sealing in hermetic packaging have the following disadvantages (i) Mechanical strength of the soldered interconnection is low and is an inherent property of the solder material used eg 15-20 MPa in case of tin based solders (n) The soldered joints are stable below the melting point of the solder material used e g 200- 220°C in case of lead- tin solders and 156°C in case of indium solders, (in) When the cap is secured to the substrate by welding it causes high temperature exposure on other circuit elements and use of solder for lead attachment and sealing through- holes is not advisable due to instability of solder at high temperatures, (iv) When the cap is attached by soldering and through- holes are sealed by glass or plastic even then, at temperatures used for soldering the cap to the substrate, some plastics and glasses have a negative temperature coefficient of expansion and consequently tensile stresses may be created in the substrate material In the case of ceramic substrate material this results in a disadvantage, because ceramics, although strong in compression, are relatively weak in tension. (v) The use of glasses and plastics adds one more process step: first providing a conductive connection between lead and thick film metallisation using solder and then sealing the through- hole with glass or plastic, (vi) Several other reliability problems are also associated with the presently used process e.g. outgassing, ball formation etc. The main object of the present invention is to provide a process for joining metal- to-thick- film composite system on insulating substrate which obviates the drawbacks of the hitherto known process as detailed above. Another object of the present invention is to provide an improved process which enables hermetic sealing of microelectronic modules. Accordingly the present invention provides a process for the preparation of thick- film composite system useful for joing metal insulating substrate on an which comprises : (i) cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined, (ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy, (iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint. The cleaning of the surfaces to be joined may be effected using known methods such as by boiling or ultrasonic cleaning using chemicals such as methanol, acetone etc. The low melting metallic component used may be an element such as tin, indium, bismuth or an alloy such as bismuth- indium, bismuth- tin. The coating of the low melting component such as an element or an alloy may be effected using known methods such as electro/ electroless plating, thin film deposition, or by using a foil. The preheating of the thick- film conductor on insulating substrate, if desired may be done using stage heating, oven heating. In an embodiment of the present invention, under the specified temperature, pressure and time as mentioned above, the molten low melting element reacts with thick film conductor on one end and with metal part on the other end thereby connecting the metal part and thick- film conductor on insulating substrate with intermetallic phases formed by the process of isothermal solidification. These phases are in accordance with the phase diagrams of the reacting parts and the property of the joint is governed by the property of the phases. Moreover, the above stated process also enables hermetic sealing. The following examples are given by way of illustration with accompanying drawing of the present invention and should not be construed to limit the scope of the present invention. Example 1 Fig. 1 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers it. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 µm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 2 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (3) at the periphery of the substrate (1). A strip portion (4) extending from the circular portion provides the conductor run on the substrate. Fig. 3. of the drawings accompanying this specification shows the enlarged top plan view of the lead. The metallic lead is made of nickel. The lead has a circular pad (5) and the diameter of the lead pin (6) is 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 µm over the area of the lead that is to be bonded to the substrate. The coating comprises of a low melting element tin. In order to bond the lead to the substrate the coated surface of the lead and the circular conductor pad on the substrate are brought into confronting relationship, as shown in Fig. 1 and 3 combined with the leads in registration with the circular conductor pads. The lead pads are brought into contact with the conductor pads and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor pads and consequently a good electrically conductive connection is provided between the lead and the conductor pad (see Fig. 4 of the drawings accompanying this specification showing the sectional view of Fig. 1 and 3 combined, where the substrate and lead are bonded together). Example 2 Fig. 5 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate with printed through-hole. It illustrates a substrate (1) that is formed with a thick- film conductor coating (2) that partially covers the substrate. The substrate is 96% alumina and coating comprising of gold conductor composition of thickness around 15 µm is achieved by screen printing. Printing is also done over the through-holes (9) so that a part of the material is transferred to the holes and the composition is printed on the interior surface of the through hole (10) by vacuum suction or related technique. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 6 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate with printed through-hole. The conductor comprises an annular portion (3) concentrically surrounding the through-hole (9) and at the periphery of the substrate (1). A strip portion (4) extending from the annular portion provides the conductor run on the substrate. The figure also depicts the partial through-hole printing of the conductor. Fig. 7 of the drawings accompanying this specification is an enlarged partial sectional view of the lead. The metallic lead is made of Cu coated with gold. The lead has preferably a circular head (11) and the diameter of the pin (12) is in the range of 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 urn over the area of the lead that is to be bonded to the substrate and partially on the lead pin.. The coating comprises of a low melting element indium. In order to bond the lead to the substrate the coated surface of the lead and the metallised surface of the substrate are brought into confronting relationship, as shown in Fig. 5 and 7 combined with the leads inserted through hole and in registration with the annular conductor rings. The lead heads are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor rings. Some of the molten metal (7) flows into the through-hole. This is shown in Fig. 8, of the drawings accompanying this specification which is a sectional view of Fig. 5 and 7 combined, where the substrate and lead are bonded together and consequently a good electrically conductive connection is provided between the lead and the conductor ring and the through hole is hermetically sealed. Example 3 Fig. 9 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers the substrate and also includes coating at specific locations (13) for cap attachment. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 pm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 10 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (13) on the substrate (1). Thick film conductor coating that forms the conductor run on the substrate is not shown. Fig. 11. of the drawings accompanying this specification shows the enlarged top plan view of the cap. The metallic cap is made of nickel. The cap has a circular periphery (14) of diameter 1.0 mm and a cap head (15). Each cap is provided with a coating layer (16) of thickness 5 urn over the area of the cap that is to be bonded to the substrate. The coating comprises of a low melting element tin. In order to bond the cap to the substrate the coated surface of the cap and the circular conductor ring on the substrate are brought into confronting relationship, as shown in Fig. 9 and 11 combined with the cap in registration with the circular conductor ring. The caps are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between caps and conductor rings and consequently a good contact is provided between the cap and the conductor ring (see Fig. 12 of the drawings accompanying this specification showing the sectional view of Fig. 9 and 11 combined, where the substrate and cap are bonded together and as a result hermetic sealing is achieved). The main advantages of the present invention are: (i) The mechanical strength of the interconnection is comparatively high and varies between 15 to 42 MPa depending on the material combination used, (ii) The thermal stability of the joints is also high and varies between 753 to 1173 K for above mentioned reason, (iii) When the cap is secured to the substrate by welding or soldering, it causes high temperature exposure. Since the joints and seals produced using the present invention are stable at least upto 753 K so lead attachment and sealing through- holes using this technique will not be effected by cap sealing process, (iv) In the present invention, providing a conductive connection between lead and thick film metallisation as well as sealing the cap and through- hole is done in a single process step, (v) Reduction in cost, firstly because only small amount of joining element e.g. tin, indium or bismuth is used, and secondly because its a low temperature process, (vi) The process of the present invention is also environment friendly, firstly because harmful metals like lead is not used and secondly it also avoids the use of CFC since the joints are flux free. The present invention relates to a process for the preparation of thick- film composite system. useful for joining metal on an insulating substrate More particularly, the present invention relates to joining two parts: a metal part such as metallic lead or metallic cap and a composite part such as a thick-film conductor consisting of metallic particles bonded by a glass matrix on an insulating substrate over a defined area of their respective surfaces using the principle of isothermal solidification. Its intended application is in microelectronic modules such as hybrid microcircuits, multichip modules etc. These modules find application in all areas which include: consumer, medical, telecommunication, automobiles, process-control, high temperature and high power applications, high reliability applications etc. In the hitherto known process for the production of microelectronic modules, it is desirable to establish a bond between two parts, a composite part such as thick film conductor on insulating substrate and a metallic part such as metallic lead or metallic cap. Microelectronic modules which are meant for commercial applications are either epoxy encapsulated, conformal coated with plastic, sealed in plastic moulded packages, or frit-sealed ceramic packages,. However, in any of the above, external leads are provided on thick film conductor at the periphery of the substrate by soldering which in turn may either be dip, reflow, using solder iron etc. In modules, which are to be used in stringent environments, metal- can packages are used but due to the high cost of metal packages, some times a metallic cap is secured td the substrate so as to isolate semiconductor devices and other components mounted on the substrate from the environment in which the module might be used. Generally, cap is secured to the substrate by soldering or welding In order to provide lead interconnection to the module and to isolate semiconductor devices and other components effectively, conductive interconnection between the lead and thick film conductor is provided thirough-holes in the substrate and these through-holes in the substrate must be sealed It has been conventional hitherto to use solder for conductive connection and solder, glass or plastic material to seal the through-holes in the ceramic substrate The above stated known processes for lead attachment in non-hermetic packaging, lead and cap attachment as well as through- hole sealing in hermetic packaging have the following disadvantages (i) Mechanical strength of the soldered interconnection is low and is an inherent property of the solder material used eg 15-20 MPa in case of tin based solders (n) The soldered joints are stable below the melting point of the solder material used e g 200- 220°C in case of lead- tin solders and 156°C in case of indium solders, (in) When the cap is secured to the substrate by welding it causes high temperature exposure on other circuit elements and use of solder for lead attachment and sealing through- holes is not advisable due to instability of solder at high temperatures, (iv) When the cap is attached by soldering and through- holes are sealed by glass or plastic even then, at temperatures used for soldering the cap to the substrate, some plastics and glasses have a negative temperature coefficient of expansion and consequently tensile stresses may be created in the substrate material In the case of ceramic substrate material this results in a disadvantage, because ceramics, although strong in compression, are relatively weak in tension. (v) The use of glasses and plastics adds one more process step: first providing a conductive connection between lead and thick film metallisation using solder and then sealing the through- hole with glass or plastic, (vi) Several other reliability problems are also associated with the presently used process e.g. outgassing, ball formation etc. The main object of the present invention is to provide a process for joining metal- to-thick- film composite system on insulating substrate which obviates the drawbacks of the hitherto known process as detailed above. Another object of the present invention is to provide an improved process which enables hermetic sealing of microelectronic modules. Accordingly the present invention provides a process for the preparation of thick- film composite system useful for joing metal insulating substrate on an which comprises : (i) cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined, (ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy, (iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint. The cleaning of the surfaces to be joined may be effected using known methods such as by boiling or ultrasonic cleaning using chemicals such as methanol, acetone etc. The low melting metallic component used may be an element such as tin, indium, bismuth or an alloy such as bismuth- indium, bismuth- tin. The coating of the low melting component such as an element or an alloy may be effected using known methods such as electro/ electroless plating, thin film deposition, or by using a foil. The preheating of the thick- film conductor on insulating substrate, if desired may be done using stage heating, oven heating. In an embodiment of the present invention, under the specified temperature, pressure and time as mentioned above, the molten low melting element reacts with thick film conductor on one end and with metal part on the other end thereby connecting the metal part and thick- film conductor on insulating substrate with intermetallic phases formed by the process of isothermal solidification. These phases are in accordance with the phase diagrams of the reacting parts and the property of the joint is governed by the property of the phases. Moreover, the above stated process also enables hermetic sealing. The following examples are given by way of illustration with accompanying drawing of the present invention and should not be construed to limit the scope of the present invention. Example 1 Fig. 1 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers it. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 µm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 2 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (3) at the periphery of the substrate (1). A strip portion (4) extending from the circular portion provides the conductor run on the substrate. Fig. 3. of the drawings accompanying this specification shows the enlarged top plan view of the lead. The metallic lead is made of nickel. The lead has a circular pad (5) and the diameter of the lead pin (6) is 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 µm over the area of the lead that is to be bonded to the substrate. The coating comprises of a low melting element tin. In order to bond the lead to the substrate the coated surface of the lead and the circular conductor pad on the substrate are brought into confronting relationship, as shown in Fig. 1 and 3 combined with the leads in registration with the circular conductor pads. The lead pads are brought into contact with the conductor pads and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor pads and consequently a good electrically conductive connection is provided between the lead and the conductor pad (see Fig. 4 of the drawings accompanying this specification showing the sectional view of Fig. 1 and 3 combined, where the substrate and lead are bonded together). Example 2 Fig. 5 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate with printed through-hole. It illustrates a substrate (1) that is formed with a thick- film conductor coating (2) that partially covers the substrate. The substrate is 96% alumina and coating comprising of gold conductor composition of thickness around 15 µm is achieved by screen printing. Printing is also done over the through-holes (9) so that a part of the material is transferred to the holes and the composition is printed on the interior surface of the through hole (10) by vacuum suction or related technique. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 6 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate with printed through-hole. The conductor comprises an annular portion (3) concentrically surrounding the through-hole (9) and at the periphery of the substrate (1). A strip portion (4) extending from the annular portion provides the conductor run on the substrate. The figure also depicts the partial through-hole printing of the conductor. Fig. 7 of the drawings accompanying this specification is an enlarged partial sectional view of the lead. The metallic lead is made of Cu coated with gold. The lead has preferably a circular head (11) and the diameter of the pin (12) is in the range of 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 urn over the area of the lead that is to be bonded to the substrate and partially on the lead pin.. The coating comprises of a low melting element indium. In order to bond the lead to the substrate the coated surface of the lead and the metallised surface of the substrate are brought into confronting relationship, as shown in Fig. 5 and 7 combined with the leads inserted through hole and in registration with the annular conductor rings. The lead heads are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor rings. Some of the molten metal (7) flows into the through-hole. This is shown in Fig. 8, of the drawings accompanying this specification which is a sectional view of Fig. 5 and 7 combined, where the substrate and lead are bonded together and consequently a good electrically conductive connection is provided between the lead and the conductor ring and the through hole is hermetically sealed. Example 3 Fig. 9 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers the substrate and also includes coating at specific locations (13) for cap attachment. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 pm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time. Fig. 10 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (13) on the substrate (1). Thick film conductor coating that forms the conductor run on the substrate is not shown. Fig. 11. of the drawings accompanying this specification shows the enlarged top plan view of the cap. The metallic cap is made of nickel. The cap has a circular periphery (14) of diameter 1.0 mm and a cap head (15). Each cap is provided with a coating layer (16) of thickness 5 urn over the area of the cap that is to be bonded to the substrate. The coating comprises of a low melting element tin. In order to bond the cap to the substrate the coated surface of the cap and the circular conductor ring on the substrate are brought into confronting relationship, as shown in Fig. 9 and 11 combined with the cap in registration with the circular conductor ring. The caps are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between caps and conductor rings and consequently a good contact is provided between the cap and the conductor ring (see Fig. 12 of the drawings accompanying this specification showing the sectional view of Fig. 9 and 11 combined, where the substrate and cap are bonded together and as a result hermetic sealing is achieved). The main advantages of the present invention are: (i) The mechanical strength of the interconnection is comparatively high and varies between 15 to 42 MPa depending on the material combination used, (ii) The thermal stability of the joints is also high and varies between 753 to 1173 K for above mentioned reason, (iii) When the cap is secured to the substrate by welding or soldering, it causes high temperature exposure. Since the joints and seals produced using the present invention are stable at least upto 753 K so lead attachment and sealing through- holes using this technique will not be effected by cap sealing process, (iv) In the present invention, providing a conductive connection between lead and thick film metallisation as well as sealing the cap and through- hole is done in a single process step, (v) Reduction in cost, firstly because only small amount of joining element e.g. tin, indium or bismuth is used, and secondly because its a low temperature process, (vi) The process of the present invention is also environment friendly, firstly because harmful metals like lead is not used and secondly it also avoids the use of CFC since the joints are flux free. We claim : 1. A process for the preparation of thick- film composite system usefull joing metal or an insullating substrate which comprises : (i) cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined, (ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy, (iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre-coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint. 2. A process as claimed in claim 1 wherein the low melting metallic component used is Sn, In, Bi, or an alloy selected from Bi-ln, Bi-Sn. 3. A process as claimed in claims 1-2 wherein the preheating of the thick-film conductor on insulating substrate, if desired is done using stage heating, oven heating. 4. A process for joing metal or an useless substrate the preparation of thick- film composite system useful substantially as herein described with reference to the examples and drawing accompanying this specification.

Full Text

The present invention relates to a process for the preparation of thick- film composite system.
useful for joining metal on an insulating substrate
More particularly, the present invention relates to joining two parts: a metal part such as metallic lead or metallic cap and a composite part such as a thick-film conductor consisting of metallic particles bonded by a glass matrix on an insulating substrate over a defined area of their respective surfaces using the principle of isothermal solidification.
Its intended application is in microelectronic modules such as hybrid microcircuits, multichip modules etc. These modules find application in all areas which include: consumer, medical, telecommunication, automobiles, process-control, high temperature and high power applications, high reliability applications etc.
In the hitherto known process for the production of microelectronic modules, it is desirable to establish a bond between two parts, a composite part such as thick film conductor on insulating substrate and a metallic part such as metallic lead or metallic cap. Microelectronic modules which are meant for commercial applications are either epoxy encapsulated, conformal coated with plastic, sealed in plastic moulded packages, or frit-sealed ceramic packages,. However, in any of the above, external leads are provided on thick film conductor at the periphery of the substrate by soldering which in turn may either be dip, reflow, using solder iron etc. In modules, which are to be used in stringent environments, metal- can packages are used but due to the high cost of metal packages, some times a metallic cap is secured td the substrate so as to isolate semiconductor devices and other components mounted on the substrate from the environment in which the module might be used. Generally, cap is secured to the
substrate by soldering or welding In order to provide lead interconnection to the module and to isolate semiconductor devices and other components effectively, conductive interconnection between the lead and thick film conductor is provided thirough-holes in the substrate and these through-holes in the substrate must be sealed It has been conventional hitherto to use solder for conductive connection and solder, glass or plastic material to seal the through-holes in the ceramic substrate
The above stated known processes for lead attachment in non-hermetic packaging, lead and cap attachment as well as through- hole sealing in hermetic packaging have the following disadvantages
(i) Mechanical strength of the soldered interconnection is low and is an inherent property of the solder material used eg 15-20 MPa in case of tin based solders (n) The soldered joints are stable below the melting point of the solder material used e g 200- 220°C in case of lead- tin solders and 156°C in case of indium solders, (in) When the cap is secured to the substrate by welding it causes high temperature exposure on other circuit elements and use of solder for lead attachment and sealing through- holes is not advisable due to instability of solder at high temperatures, (iv) When the cap is attached by soldering and through- holes are sealed by glass or plastic even then, at temperatures used for soldering the cap to the substrate, some plastics and glasses have a negative temperature coefficient of expansion and consequently tensile stresses may be created in the substrate material In the case of ceramic substrate material this results in a disadvantage, because ceramics, although strong in
compression, are relatively weak in tension. (v) The use of glasses and plastics adds one more process step: first providing a conductive connection between lead and thick film metallisation using solder and then sealing the through- hole with glass or plastic, (vi) Several other reliability problems are also associated with the presently used process e.g. outgassing, ball formation etc.
The main object of the present invention is to provide a process for joining metal- to-thick- film composite system on insulating substrate which obviates the drawbacks of the hitherto known process as detailed above.
Another object of the present invention is to provide an improved process which enables hermetic sealing of microelectronic modules.
Accordingly the present invention provides a process for the preparation of
thick- film composite system useful for joing metal insulating substrate on an which comprises :
(i) cleaning by known methods the defined areas of a metal part and
thick- film conductor on an insulating substrate to be joined, (ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy, (iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature
above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint.
The cleaning of the surfaces to be joined may be effected using known methods such as by boiling or ultrasonic cleaning using chemicals such as methanol, acetone etc.
The low melting metallic component used may be an element such as tin, indium, bismuth or an alloy such as bismuth- indium, bismuth- tin.
The coating of the low melting component such as an element or an alloy may be effected using known methods such as electro/ electroless plating, thin film deposition, or by using a foil.
The preheating of the thick- film conductor on insulating substrate, if desired may be done using stage heating, oven heating.
In an embodiment of the present invention, under the specified temperature, pressure and time as mentioned above, the molten low melting element reacts with thick film conductor on one end and with metal part on the other end thereby connecting the metal part and thick- film conductor on insulating substrate with intermetallic phases formed by the process of isothermal solidification. These phases are in accordance with the phase diagrams of the reacting parts and the property of the joint is governed by the property of the phases.
Moreover, the above stated process also enables hermetic sealing.
The following examples are given by way of illustration with accompanying drawing
of the present invention and
should not be construed to limit the scope of the present invention.
Example 1
Fig. 1 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers it. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 µm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 2 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (3) at the periphery of the substrate (1). A strip portion (4) extending from the circular portion provides the conductor run on the substrate.
Fig. 3. of the drawings accompanying this specification shows the enlarged top plan view of the lead. The metallic lead is made of nickel. The lead has a circular pad (5) and the diameter of the lead pin (6) is 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 µm over the area of the lead that is to be bonded to the substrate. The coating comprises of a low melting element tin.
In order to bond the lead to the substrate the coated surface of the lead and the circular conductor pad on the substrate are brought into confronting relationship, as shown in Fig. 1 and 3 combined with the leads in registration with the circular conductor pads. The lead pads are brought into contact with the conductor pads and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor pads and consequently a good electrically conductive connection is provided between the lead and the
conductor pad (see Fig. 4 of the drawings accompanying this specification showing the sectional view of Fig. 1 and 3 combined, where the substrate and lead are bonded together).
Example 2
Fig. 5 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate with printed through-hole. It illustrates a substrate (1) that is formed with a thick- film conductor coating (2) that partially covers the substrate. The substrate is 96% alumina and coating comprising of gold conductor composition of thickness around 15 µm is achieved by screen printing. Printing is also done over the through-holes (9) so that a part of the material is transferred to the holes and the composition is printed on the interior surface of the through hole (10) by vacuum suction or related technique. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 6 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate with printed through-hole. The conductor comprises an annular portion (3) concentrically surrounding the through-hole (9) and at the periphery of the substrate (1). A strip portion (4) extending from the annular portion provides the conductor run on the substrate. The figure also depicts the partial through-hole printing of the conductor.
Fig. 7 of the drawings accompanying this specification is an enlarged partial sectional view of the lead. The metallic lead is made of Cu coated with gold. The lead has preferably a circular head (11) and the diameter of the pin (12) is in the range of 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 urn over the area of the lead that is to be bonded to the substrate and partially on the lead pin.. The coating comprises of a low melting element indium.
In order to bond the lead to the substrate the coated surface of the lead and the metallised surface of the substrate are brought into confronting relationship, as shown in Fig. 5 and 7 combined with the leads inserted through hole and in registration with the annular
conductor rings. The lead heads are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor rings. Some of the molten metal (7) flows into the through-hole. This is shown in Fig. 8, of the drawings accompanying this specification which is a sectional view of Fig. 5 and 7 combined, where the substrate and lead are bonded together and consequently a good electrically conductive connection is provided between the lead and the conductor ring and the through hole is hermetically sealed.
Example 3
Fig. 9 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers the substrate and also includes coating at specific locations (13) for cap attachment. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 pm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 10 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (13) on the substrate (1). Thick film conductor coating that forms the conductor run on the substrate is not shown.
Fig. 11. of the drawings accompanying this specification shows the enlarged top plan view of the cap. The metallic cap is made of nickel. The cap has a circular periphery (14) of diameter 1.0 mm and a cap head (15). Each cap is provided with a coating layer (16) of thickness 5 urn over the area of the cap that is to be bonded to the substrate. The coating comprises of a low melting element tin.
In order to bond the cap to the substrate the coated surface of the cap and the circular conductor ring on the substrate are brought into confronting relationship, as shown in Fig. 9 and 11 combined with the cap in registration with the circular conductor ring. The caps are
brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between caps and conductor rings and consequently a good contact is provided between the cap and the conductor ring (see Fig. 12 of the drawings accompanying this specification showing the sectional view of Fig. 9 and 11 combined, where the substrate and cap are bonded together and as a result hermetic sealing is achieved).
The main advantages of the present invention are: (i) The mechanical strength of the interconnection is comparatively high and varies
between 15 to 42 MPa depending on the material combination used, (ii) The thermal stability of the joints is also high and varies between 753 to 1173 K for
above mentioned reason, (iii) When the cap is secured to the substrate by welding or soldering, it causes high
temperature exposure. Since the joints and seals produced using the present invention
are stable at least upto 753 K so lead attachment and sealing through- holes using this
technique will not be effected by cap sealing process, (iv) In the present invention, providing a conductive connection between lead and thick
film metallisation as well as sealing the cap and through- hole is done in a single
process step, (v) Reduction in cost, firstly because only small amount of joining element e.g. tin,
indium or bismuth is used, and secondly because its a low temperature process, (vi) The process of the present invention is also environment friendly, firstly because
harmful metals like lead is not used and secondly it also avoids the use of CFC since
the joints are flux free.
The present invention relates to a process for the preparation of thick- film composite system.
useful for joining metal on an insulating substrate
More particularly, the present invention relates to joining two parts: a metal part such as metallic lead or metallic cap and a composite part such as a thick-film conductor consisting of metallic particles bonded by a glass matrix on an insulating substrate over a defined area of their respective surfaces using the principle of isothermal solidification.
Its intended application is in microelectronic modules such as hybrid microcircuits, multichip modules etc. These modules find application in all areas which include: consumer, medical, telecommunication, automobiles, process-control, high temperature and high power applications, high reliability applications etc.
In the hitherto known process for the production of microelectronic modules, it is desirable to establish a bond between two parts, a composite part such as thick film conductor on insulating substrate and a metallic part such as metallic lead or metallic cap. Microelectronic modules which are meant for commercial applications are either epoxy encapsulated, conformal coated with plastic, sealed in plastic moulded packages, or frit-sealed ceramic packages,. However, in any of the above, external leads are provided on thick film conductor at the periphery of the substrate by soldering which in turn may either be dip, reflow, using solder iron etc. In modules, which are to be used in stringent environments, metal- can packages are used but due to the high cost of metal packages, some times a metallic cap is secured td the substrate so as to isolate semiconductor devices and other components mounted on the substrate from the environment in which the module might be used. Generally, cap is secured to the
substrate by soldering or welding In order to provide lead interconnection to the module and to isolate semiconductor devices and other components effectively, conductive interconnection between the lead and thick film conductor is provided thirough-holes in the substrate and these through-holes in the substrate must be sealed It has been conventional hitherto to use solder for conductive connection and solder, glass or plastic material to seal the through-holes in the ceramic substrate
The above stated known processes for lead attachment in non-hermetic packaging, lead and cap attachment as well as through- hole sealing in hermetic packaging have the following disadvantages
(i) Mechanical strength of the soldered interconnection is low and is an inherent property of the solder material used eg 15-20 MPa in case of tin based solders (n) The soldered joints are stable below the melting point of the solder material used e g 200- 220°C in case of lead- tin solders and 156°C in case of indium solders, (in) When the cap is secured to the substrate by welding it causes high temperature exposure on other circuit elements and use of solder for lead attachment and sealing through- holes is not advisable due to instability of solder at high temperatures, (iv) When the cap is attached by soldering and through- holes are sealed by glass or plastic even then, at temperatures used for soldering the cap to the substrate, some plastics and glasses have a negative temperature coefficient of expansion and consequently tensile stresses may be created in the substrate material In the case of ceramic substrate material this results in a disadvantage, because ceramics, although strong in
compression, are relatively weak in tension. (v) The use of glasses and plastics adds one more process step: first providing a conductive connection between lead and thick film metallisation using solder and then sealing the through- hole with glass or plastic, (vi) Several other reliability problems are also associated with the presently used process e.g. outgassing, ball formation etc.
The main object of the present invention is to provide a process for joining metal- to-thick- film composite system on insulating substrate which obviates the drawbacks of the hitherto known process as detailed above.
Another object of the present invention is to provide an improved process which enables hermetic sealing of microelectronic modules.
Accordingly the present invention provides a process for the preparation of
thick- film composite system useful for joing metal insulating substrate on an which comprises :
(i) cleaning by known methods the defined areas of a metal part and
thick- film conductor on an insulating substrate to be joined, (ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy, (iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre- coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature
above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint.
The cleaning of the surfaces to be joined may be effected using known methods such as by boiling or ultrasonic cleaning using chemicals such as methanol, acetone etc.
The low melting metallic component used may be an element such as tin, indium, bismuth or an alloy such as bismuth- indium, bismuth- tin.
The coating of the low melting component such as an element or an alloy may be effected using known methods such as electro/ electroless plating, thin film deposition, or by using a foil.
The preheating of the thick- film conductor on insulating substrate, if desired may be done using stage heating, oven heating.
In an embodiment of the present invention, under the specified temperature, pressure and time as mentioned above, the molten low melting element reacts with thick film conductor on one end and with metal part on the other end thereby connecting the metal part and thick- film conductor on insulating substrate with intermetallic phases formed by the process of isothermal solidification. These phases are in accordance with the phase diagrams of the reacting parts and the property of the joint is governed by the property of the phases.
Moreover, the above stated process also enables hermetic sealing.
The following examples are given by way of illustration with accompanying drawing
of the present invention and
should not be construed to limit the scope of the present invention.
Example 1
Fig. 1 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers it. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 µm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 2 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (3) at the periphery of the substrate (1). A strip portion (4) extending from the circular portion provides the conductor run on the substrate.
Fig. 3. of the drawings accompanying this specification shows the enlarged top plan view of the lead. The metallic lead is made of nickel. The lead has a circular pad (5) and the diameter of the lead pin (6) is 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 µm over the area of the lead that is to be bonded to the substrate. The coating comprises of a low melting element tin.
In order to bond the lead to the substrate the coated surface of the lead and the circular conductor pad on the substrate are brought into confronting relationship, as shown in Fig. 1 and 3 combined with the leads in registration with the circular conductor pads. The lead pads are brought into contact with the conductor pads and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor pads and consequently a good electrically conductive connection is provided between the lead and the
conductor pad (see Fig. 4 of the drawings accompanying this specification showing the sectional view of Fig. 1 and 3 combined, where the substrate and lead are bonded together).
Example 2
Fig. 5 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate with printed through-hole. It illustrates a substrate (1) that is formed with a thick- film conductor coating (2) that partially covers the substrate. The substrate is 96% alumina and coating comprising of gold conductor composition of thickness around 15 µm is achieved by screen printing. Printing is also done over the through-holes (9) so that a part of the material is transferred to the holes and the composition is printed on the interior surface of the through hole (10) by vacuum suction or related technique. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 6 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate with printed through-hole. The conductor comprises an annular portion (3) concentrically surrounding the through-hole (9) and at the periphery of the substrate (1). A strip portion (4) extending from the annular portion provides the conductor run on the substrate. The figure also depicts the partial through-hole printing of the conductor.
Fig. 7 of the drawings accompanying this specification is an enlarged partial sectional view of the lead. The metallic lead is made of Cu coated with gold. The lead has preferably a circular head (11) and the diameter of the pin (12) is in the range of 0.5 mm. Each lead is provided with a coating layer (7) of thickness 5 urn over the area of the lead that is to be bonded to the substrate and partially on the lead pin.. The coating comprises of a low melting element indium.
In order to bond the lead to the substrate the coated surface of the lead and the metallised surface of the substrate are brought into confronting relationship, as shown in Fig. 5 and 7 combined with the leads inserted through hole and in registration with the annular
conductor rings. The lead heads are brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between leads and conductor rings. Some of the molten metal (7) flows into the through-hole. This is shown in Fig. 8, of the drawings accompanying this specification which is a sectional view of Fig. 5 and 7 combined, where the substrate and lead are bonded together and consequently a good electrically conductive connection is provided between the lead and the conductor ring and the through hole is hermetically sealed.
Example 3
Fig. 9 of the drawings accompanying this specification is an enlarged partial sectional view of a hybrid circuit substrate and illustrates a substrate (1) that is formed with a thick-film conductor coating (2) that partially covers the substrate and also includes coating at specific locations (13) for cap attachment. The substrate is 96% alumina and coating comprising of silver thick film conductor composition of thickness around 15 pm is achieved by screen printing. The same is dried at temperature of 423 K for a period of 15 min. and fired at 1123 K for 10 min peak time.
Fig. 10 of the drawings accompanying this specification is an enlarged partial top plan view of the substrate. The conductor comprises a circular portion (13) on the substrate (1). Thick film conductor coating that forms the conductor run on the substrate is not shown.
Fig. 11. of the drawings accompanying this specification shows the enlarged top plan view of the cap. The metallic cap is made of nickel. The cap has a circular periphery (14) of diameter 1.0 mm and a cap head (15). Each cap is provided with a coating layer (16) of thickness 5 urn over the area of the cap that is to be bonded to the substrate. The coating comprises of a low melting element tin.
In order to bond the cap to the substrate the coated surface of the cap and the circular conductor ring on the substrate are brought into confronting relationship, as shown in Fig. 9 and 11 combined with the cap in registration with the circular conductor ring. The caps are
brought into contact with the conductor rings and by application of pressure and heat the coating melts and forms intermetallic phases (8) between caps and conductor rings and consequently a good contact is provided between the cap and the conductor ring (see Fig. 12 of the drawings accompanying this specification showing the sectional view of Fig. 9 and 11 combined, where the substrate and cap are bonded together and as a result hermetic sealing is achieved).
The main advantages of the present invention are: (i) The mechanical strength of the interconnection is comparatively high and varies
between 15 to 42 MPa depending on the material combination used, (ii) The thermal stability of the joints is also high and varies between 753 to 1173 K for
above mentioned reason, (iii) When the cap is secured to the substrate by welding or soldering, it causes high
temperature exposure. Since the joints and seals produced using the present invention
are stable at least upto 753 K so lead attachment and sealing through- holes using this
technique will not be effected by cap sealing process, (iv) In the present invention, providing a conductive connection between lead and thick
film metallisation as well as sealing the cap and through- hole is done in a single
process step, (v) Reduction in cost, firstly because only small amount of joining element e.g. tin,
indium or bismuth is used, and secondly because its a low temperature process, (vi) The process of the present invention is also environment friendly, firstly because
harmful metals like lead is not used and secondly it also avoids the use of CFC since
the joints are flux free.

We claim :
1. A process for the preparation of thick- film composite system usefull joing metal or an insullating
substrate which comprises :
(i) cleaning by known methods the defined areas of a metal part and thick- film conductor on an insulating substrate to be joined,
(ii) coating by known methods the said defined area of either the metal part or the thick- film part with an appropriate low melting metallic component selected from an element or an alloy,
(iii) preheating the thick- film conductor on an insulating substrate to a temperature below the melting point of the low melting metallic component, if desired, in order to avoid any damage to the substrate, contacting the said defined area of the metal part and thick- film conductor with atleast one of the part pre-coated as mentioned above, heating the said defined areas in direct contact with each other in such a manner so as to attain a temperature above the melting point of the low melting metallic component coating, at a pressure in the range 0.1 to 10 MPa and for a duration in the range 0.1 to 10 min depending on the intermetallics desired to be formed for making the joint.
2. A process as claimed in claim 1 wherein the low melting metallic component used is Sn, In, Bi, or an alloy selected from Bi-ln, Bi-Sn.
3. A process as claimed in claims 1-2 wherein the preheating of the thick-film conductor on insulating substrate, if desired is done using stage heating, oven heating.
4. A process for joing metal or an useless substrate the preparation of thick- film
composite system useful substantially as herein described with reference to the examples and drawing accompanying this specification.

Documents:

3347-del-1997-abstract.pdf

3347-del-1997-claims.pdf

3347-del-1997-complete specification granted.pdf

3347-del-1997-correspondence-others.pdf

3347-del-1997-correspondence-po.pdf

3347-del-1997-description (complete).pdf

3347-del-1997-drawings.pdf

3347-del-1997-form-1.pdf

3347-del-1997-form-19.pdf

3347-del-1997-form-2.pdf

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Patent Number

199610

Indian Patent Application Number

3347/DEL/1997

PG Journal Number

36/2008

Publication Date

05-Sep-2008

Grant Date

15-Dec-2006

Date of Filing

21-Nov-1997

Name of Patentee

Council of Scientific and Industrial Research

Applicant Address

Rafi Marg, New Delhi - 110 001.

Inventors:

#	Inventor's Name	Inventor's Address
1	Pramod Kumar Khanna	Scientists, Central Electronics Engineering Research Institute, Pilani-333031.
2	Satish Kumar Bhatnagar	Scientists, Central Electronics Engineering Research Institute, Pilani-333031.

PCT International Classification Number

B23K 1/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA