Title of Invention

METHOD OF IMPROVING HIGH-VOLTAGE INSULATION IN POWER SEMICONDUCTOR DIODE USING POLYIMIDE-AMIDE COATING OVER MICA-FOIL INSULATOR.

Abstract

The present invention provides a method for improving the reverse blocking I-V characteristic of stud-type power semi conductor diode through appropriate selection of a polyimide-amide polymer coating on the surface of mica insulator that is otherwise susceptible to arcing at higher levels of voltage. The invention also describes the method of coating and the curing procedure. The thin-film coating helps to completely eliminate surface flashover on the dielectric surface thereby providing improved high-voltage insulation in the internal structure of the diode. The processing yield and the long -term product reliability of the diode are improved. The method has been adapted in industrial manufacturing process.

Full Text

2 FIELD OF INVEVTION The present invention describes a method of improving high-voltage insuiatbn within the structure of power semiconductor diode by way of adopting a suitable polymer coating over the surface of the dielectric insulator including its method of application. BACKGROUND OF THE INVNENTION It is essential that a power semiconductor diode is designed with adequate structural features that provide suitable electrical insulation between the two terminals viz. cathode and anode. A diode, which is an ON/OFP switch in simple terms of definition, is basically rated for its maximum average forward current (IFAV) and maximum peak reverse repetitive voltage (URRM) IFAV represents the allowable current-flow during ON-state when the switch is closed. URRM indicates the reverse voltage that is allowable across the switch in its OFF-state when it is opened. The design requirement for high voltage insulation arises from URRM rating. The insulating components that are used in the structure of the diode are, therefore, accordingly selected and designed with regard to the material and the dimensions. It is important that the design satisfies the other technical requirements such as the compressive strength and the operating temperature range. Conventionally, ceramic (Al2O3) or mica is chosen as the material for use in power semiconductor diode structures, as these meet the dielectric strength, temperature and strength requirements. However, where the space is a major design constraint, mica is preferred over ceramic and thin mica foils of 200-300 microns thick are used one over the other so as to build up the necessary thickness. 3 Generally, natural finish (without any protective coatings on the surfaces) is adopted for ceramic as well as mica insulators. Therefore, it is quite common that surface-related insulation problems arise during product qualification tests at manufacturers' end or during the end-use operation at users end. Whereas manufacturing yield is lowered in the former situation, long-term product reliability is a concern in the latter case. OBJECTS OF THE THE INVENTION - The object of the invention is to improve high-voltage insulation of power semiconductor diode by polymer coating. - Another object of the invention is to improve the reverse blocking characteristics of power semiconductor diode. - Further, the object of the invention is to eliminate any high voltage flash over or arcing along the surface of the insulator. - Yet other object of the invention is to provide a method to overcome the limitation of natural surface finish of an insulator. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig-1 shows overall outline diagram of the fully fabricated power semiconductor diode (250A, 2800V). Fig-2 shows schematic diagram of internal structure of diode showing the semiconductor diode chip, mica insulator and other components. The portions of mica-surface that are subjected to high-voltage are marked as 'S1' and 'S2' 4 Fig-3 shows schematic diagram of apparatus-1 used for coating polyimide-amide polymer on mica foil insulator surface. Fig-4 shows schematic diagram of apparatus-2 used for curing the coated polymer layer. Table-1 represents comparative results of measurement of reverse blocking voltage (UR) of diode. Test current (lR)=10mA, Test temperatures=25°C, 160°C. a) Mica-foils with natural finish (No coating). b) Mica-foils coated with Durimide-32A polymer film. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Physical construction of a diode may be depicted as the 250 A/2800V stud type, pig tail diode fabricated out of a semi conductor chip (20mm diameter, 1.8 mm thickness) that comprises silicon wafer (p-n junction) bonded together with molybdenum disc. FIG -1 shows the fully fabricated diode and its out line diagram and Fig - 2 illustrates the internal structure comprising, cathode (K), Anode (A), Disc Spring (1), the mica foil ring insulator (2), steel washer for anode (3), semi conductor diode chip (4), steel cup for cathode (5), copper conductor anode (6) Disc -springs (1) apply the force required to minimize the contact voltage drop and contact thermal resistance. Steel washers (3) are used for reinforcement of strength. Di-electric insulator (2) has been provided for high voltage insulation between two electrodes. The insulator is made out of a stack of two mica-foil rings each of 200-300 micron thickness, 23 mm outer diameter (OD) and 8 mm inter-diameter (ID). 5 This internal structure is encapsulated within a hermetically sealed (nitrogen-filled) ceramic to metal housing and crimped with a flexible pig-tail lead. The conventional diode under reference is subjected to undergo the following thermal and electrical tests. a) Reverse blocking voltage (UR) measured at IR = 10 mA, 25°C and 160°C. Acceptance criterion: UR > 2800V. b) ON-state voltage drop (UT) measured at IT =600A and 25°C. Acceptance criterion: UT c) Thermal resistance (RTH) between p-n junction to the diode-case. Acceptance criterion: RTH d) Long-duration high-temperature storage test with high-voltage reverse bias (16-hours, 160°C, 2800V bias). Acceptance criterion: Max. allowable change in reverse blocking Voltage (UR) The limitations of mica-foil insulators with natural surface finish have been observed in the process of the invention. Among the tests described the tests (a) and (d) are related to high-voltage. In the diode structure, the stack of mica-foil insulator rings is meant for high-voltage insulation in both vertical and horizontal directions. Vertically, it provides insulation across its thickness, as it is sandwiched between the disc-springs on the top (cathode) and the steel washer on the bottom (anode). Horizontally (in radial direction), it provides insulation through its annular length (OD-ID)/2, as the copper conductor (anode) is at the center and the steel cup (cathode) runs close to the OD. This arrangement is susceptible to arcing at higher levels of voltage in the case of horizontal insulation, when the mica-foil rings are used with natural finish. The arcing finds the path along the horizontal flat surfaces (marked as "S1" and "S2" in Fig-2) of 6 the mica-foil rings. [The annular lengths of SI and S2 are 2.3-mm and 3.5-mm respectively, which are adequately higher than the minimum air-gap distance (in Nitrogen) needed for supporting the insulation voltage without any arcing.] The phenomenon is known as surface fiashover and is attributable to the charging of local regions on the dielectric surface due to emission of primary and secondary electrons from the surface [1]. The fiashover characteristic is determined by the surface finish, the dielectric material, the magnitude of voltage, the distance of separation of polarities, etc, etc [2]. The diode pieces that exhibit arcing characteristics are classified as rejects that give rise to lowered manufacturing yield. Even in the case of qualified and accepted pieces that are passed on to customer, it is hard to predict the end-use product reliability. The limitation of natural surface-finish of mica-foil rings led to the invention of the protective coating over the mica-surface. The following technical requirements were to be satisfied in selection of the coating of the material: a) the coating shall not add up on internal structural build-up beyond a maximum of 50 microns. b) the coating shall have good adhesion and shall withstand the applied bad 300-400 Kg without any peel-off. c) the coating shall withstand continuous opening temperature in the range 20oC to 200°C. d) the coating shall have dielectric strength better than mica. e) The coating material shall essentially be a liquid with viscosity suitable for easy dispensing and the curing temperature shall be much higher than the maximum operating temperature of 200°C. 7 Based, on the technical requirements listed above polyimide-amide polymer solution by name durimide-32A, selected as coating agent having the following properties. a) Final coating thickness of thin-film = 2-11 microns b) Tensile strength (at break) of coated film =184 MPa [Mica: 175MPa] c) Thermal decomposition temperature =494°C d) Dielectric strength of coated film =324 V/um[Mica: 120-200 V/um] e) Kinematics viscosity at 25°C = 530-850 cS f) Cure temperature and duration = 350 °C for 2 hours. An apparatus [Apparatus-1 shown in Fig-3] for uniform and thin coating devised comprising, a) Holding fixture: this fixture is made up of aluminum. It has a flat circular platform to rest the (2) mica-foil ring. It also has a small projection (11) matching the 10 of the mica-foil ring. b) Motor: The motor is for driving and rotating the fixture (8). c) Variable sped control: This is an electronic circuitry that controls the motor speed and helps to adjust the speed of rotation of the fixture (9). d) Dispenser: This is used to dispense a few drops of coating solution on mica surface (10). In curing the coated film: A conveyor belt furnace [Apparatus-2 shown in Fig-4] was employed for curing the film at 350°C for duration of 2-hours. The speed of the belt is adjusted to achieve the 2 -hours duration. 8 The process of application and curing is illustrated as under. a) The mica-foil ring is placed on the holding fixture of Apparatus-1, the arrangement holds the foil firmly without scope for flying away during rotation. b) A few drops of polymer solution are dispensed over the mica foil ring. The quantity of drops shall be so chosen as to spread and cover up the entire top surface of the mica. c) Activate the motor and rotate the fixture at a selected speed that provides a final coating thickness of ~ 10 microns. d) Take out the coated mica-foil ring and place it on a clean stainless-steel tray. e) Repeat the steps (a) to (d) for as many samples as required (12). f) Set the conveyor belt furnace (13) [Apparatus-2] ready for the curing temperature of 350°C g) Load the trays with the mica-foil rings (12) on the conveyor belt (14) and switch-ON the belt movement. The belt speed shall be selected to provide 2- hours of curing. h) Unload the trays after the curing process. i) Repeat steps (a) to (h) for similar coating on the reverse side of the foils. As a part of the invention comparative study of coated and uncoated mica-foil of diodes were conducted through following verifications of the data. Uncoated mica-foil rings were subjected to high-voltage of required magnitude (up to 3200V peak of half-wave rectified pulses.) A copper shaft that is guided by the ID of the mica ring is used as one polarity and a circular ring of copper matching to OD of the mica ring is used as other polarity. Flashover arcs were observed on many samples of uncoated foils at various voltage values beyond 9 2200V. These arcs were observed to have multiple landings on mica surface during the traverse between the two polarities. Similar experiment repeated with coated mica-foil rings showed no evidence of any visible flashover arcs. Verification of this invention has been performed on a stud- type, pig-tailed power semiconductor diode of rating 250A/ 2800V. The sample values of reverse blocking voltage (UR) measured at a fixed leakage current (IR) value of 10 mA and test temperatures of 25°C and 160°C are as shown in Table-1 for both the cases (a) without Durimide-32A coating and (b) with Durimide-32A coating. It is evident from the data on the diodes fabricated without coating that the surface-arcing does not allow the characteristics to reach the UR value measured directly on the chip prior to encapsulation. This is because the arcing establishes a parallel current path and the total leakage current exceeds the measuring current of 10mA by several folds. The data on diodes with coated mica-foils shows that arcing is completely arrested by the coating. With mica-foil coating there was no significant change observed in the results of the other tests mentioned earlier. This observation establishes that the coating has not adversely affected the other characteristics of the diode. Analyses of cut-opened samples of diodes show the mica-foil rings in intact condition without any peeling-off of coated thin-film layer. 10 As part of the manufacturing procedure, all the diode components including the semiconductor chips and mica-foil rings are subjected to heating cycle at 200°C in rough vacuum (0.3 mbar) for a soak-period of 12 hours. It is only after this heating procedure that the chips are assembled with the assembly components and encapsulated within the ceramic-to-metal housing. No degradation of any kind was observed on the coated mica-foils after this heating procedure. The completely fabricated diodes are further tested up to temperature of 160°C. 11 WE CLAIM 1 A method to improve high voltage insulation of power semiconductor diode by applying a polyimide-amide polymer solution coating on mica- foil ring surface of diode comprising a) polyimide-amide polymer solution (Durimide - 32A) as coating agent. b) Aluminum holding fixture having a flat circular platform to rest the mica-foil ring with a small projection matching the internal diameter of the mica-foil ring. c) A driving motor to rotate the fixture d) A variable speed controller of the driving motor. e) A dispenser to dispense a few drops of coating solution on mica surface. f) conveyor belt furnace for curing at 350°C Characterized by the thickness of the coating not exceeding 50 microns, adhesions of the said coating providing withstand load of 300-400 kg without any peel off, the coating to withstand continuous operating temperature in the range of 20°C to 200°C 2 The method as claimed in claim (1) wherein the holding fixture holding the mica-foil firmly not allowing the foil to fly away during rotation 3 The method as claimed in claims (1) and (2) wherein the dispensing process of polymer solution characterized by the quantity of drops chosen to spread and cover up the entire top surface of the mica foil 12 4 the method as claimed in claims 1,2 and 3 wherein the rotation of the driving motor at selected speed to provide a coating thickness of 10 microns. 5 The method as claimed in claims 1,2,3,and 4 wherein the curing process of the coated mica foil through conveyor belt furnace at 350°C for 2 hrs. 6 The method of polyimide-amide coating on mica-foil ring of a diode improving high voltage insulation and preventing any flash over or gracing over the surface of the diode, as substantially described in claims 1 to 5. Dated this 28th day of September 2007 The present invention provides a method for improving the reverse blocking I-V characteristic of stud-type power semi conductor diode through appropriate selection of a polyimide-amide polymer coating on the surface of mica insulator that is otherwise susceptible to arcing at higher levels of voltage. The invention also describes the method of coating and the curing procedure. The thin-film coating helps to completely eliminate surface flashover on the dielectric surface thereby providing improved high-voltage insulation in the internal structure of the diode. The processing yield and the long -term product reliability of the diode are improved. The method has been adapted in industrial manufacturing process.

Documents:

01347-kol-2007-abstract.pdf

01347-kol-2007-claims.pdf

01347-kol-2007-correspondence others.pdf

01347-kol-2007-description complete.pdf

01347-kol-2007-drawings.pdf

01347-kol-2007-form 1.pdf

01347-kol-2007-form 2.pdf

01347-kol-2007-form 3.pdf

01347-kol-2007-gpa.pdf

1347-KOL-2007-(19-12-2012)-ABSTRACT.pdf

1347-KOL-2007-(19-12-2012)-CLAIMS.pdf

1347-KOL-2007-(19-12-2012)-CORRESPONDENCE.pdf

1347-KOL-2007-(19-12-2012)-DESCRIPTION (COMPLETE).pdf

1347-KOL-2007-(19-12-2012)-DRAWINGS.pdf

1347-KOL-2007-(19-12-2012)-FORM 1.pdf

1347-KOL-2007-(19-12-2012)-FORM 2.pdf

1347-KOL-2007-(19-12-2012)-FORM-13.pdf

1347-KOL-2007-(19-12-2012)-OTHERS.pdf

1347-KOL-2007-(19-12-2012)-PA.pdf

1347-KOL-2007-CORRESPONDENCE 1.1.pdf

1347-kol-2007-form 18.pdf

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