FIELD OF INVENTION
The present invention relates to a method of improving high-voltage insulation
in power semiconductor diode disposing polyimide-amide coating over mica-foil
insulator.
BACKGROUND OF THE INVENTION
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/OFF 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 (AI2O3) 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.

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'.

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 (IR)=10MA, Test temperatures=25°C, 160°C.
a) Mica-foils with natural finish (No coating).
b) Mica-foils coated with Durimkide-32A polymer film.
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 (!) 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).

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 (Ur) measured at IT =600A and 25°C.
Acceptance criterion: Ur < 1.40V.
c) Thermal resistance (RTH) between p-n junction to the diode-case.
Acceptance criterion: RTH <0.17K/W.
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) < 300V.
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 (0D-ID)/2, as the copper conductor (anode) is at the
center and the steel cup (cathode) runs close to the 00. 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

the mica-foil rings. [The annular lengths of S1 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 flashover 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 flashover 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
20°C to 200oC.
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.

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 ID 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.

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

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 peeiing-off of coated thin-film layer.

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. Ho 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.

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;
arranging to rest the mica-foil ring (2) on a flat circular platform of a holding
fixture (F) allowing the internal diameter of the mica-foil ring (2) passing
through a projection (11) of the fixture (F) arresting it from flying away
during rotation; dispensing over the mica foil ring (2) a few drops of polymer
solution spreading and covering up the entire top surface of the mica;
arranging driving the motor (8) to rotate the fixture (F);
controlling the motor speed by variable speed control (9) to adjust the speed
of rotation of the fixture (F) so that a pre selected final coating thickness is
provided when the coated mica-foil ring (2) is taken out and placed on a
clean stainless steel tray wherein all the above steps are repeated for as
many samples as required and
setting the conveyer belt furnace (13) ready to a pre selected curing
temperature wherein the mica-foil rings (12) are loaded on the conveyor belt
(14) and belt movement is switched on when the belt speed is selected to
provide a pre fixed hours of curing and the trays are unloaded after the
curing process when all the steps of the method are repeated for coating the
reverse side of the foils (12).
2. The method as claimed in claim 1 wherein the final coating thickness provided
is 10 microns.
3. The method as claimed in claim 1, wherein the curing temperature of the
mica-foil rings is maintained at 350°C for two hours.

4. 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 3.

ABSTRACT

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