Field of invention
The invention relates to sealing joint of diodes, in general. The present invention
describes two methods of soldering to achieve hermetic sealing of stud-type
power semiconductor diode that is especially used in rotating applications.
Background & Prior Art of the Invention:
A semiconductor diode is hermetically sealed to prevent exposure of the silicon
diode chip to the open environment that contains moisture, various gases,
chemical fumes and dust particles. The sealing is essential to sustain the
functionally critical surface finish of the chip in its original form so that the failure
rate of diode is minimized and the long-term reliability is ensured.
A stud-type diode is sealed by way of a welding process that joins the threaded
(stud) copper base with nickel-ferrous housing that are the respective bottom
and top portions of the diode. Depending upon the joining materials, either
projection welding or cold welding is adopted for diodes used in stationary

as well as low-speed rotating applications. Welding joint is not recommended for
high-speed rotating applications, such as brushless exciter of a turbo-generator,
where the diode experiences high level of centrifugal force. In such cases,
special mechanical constructions are employed, whereby the foot of the top
component is embedded into a cavity of the bottom component by mechanical
folding and pressing operations. Hermetic sealing is then achieved either by
soldering the joint or by applying a synthetic material such as epoxide resin over
the joint. As the sealing is on the external surface of the joint, it affects the
aesthetics of the diode due to variations in finish, overflows and drippings. The
sealing processing, therefore, becomes critical in respect of visual quality apart
from the leak-rate that is measured using helium mass spectrometer method.
This gives rise to rework and repair operations.
In the conventional method, (Glove-box), soldering is carried out on the external
surface of the diode. The solder is applied on the externally visible joint that is
prepared using the mechanical folding and pressing operations. A 0l-mm solder-
wire with an alloy-composition of lead: tin = 60:40 is used in the process. Fig-2
shows the joints and the external position of solder.
The set-up used for soldering is a glove-box designed specially for this process.
It has the following features:
a) A heater plate that is placed within a sealed chamber.

b) Chamber has provisions for entry and exit of nitrogen gas. (Nitrogen is
used to prevent thermal oxidation of the copper-components of the diode
during the heating process)
c) Solder wire feeder.
d) Rotating arrangement for the heater plate.
e) Gloved holes for the operators to use their hands for loading and
unloading the diodes during the operation.
The soldering operation has the following steps:
a) Heater is switched-ON. Chamber is filled with nitrogen.
b) Temperature is set to 200°C.
c) Diode is placed on heater-plate. The plate is set to rotation.
d) Solder-wire is fed close to the joint.
e) Solder melts and settles over the circular periphery of folded joint.
f) Soldered diode is unloaded from the chamber.
This conventional method has following limitations:
a) Slow process and low throughput
b) Aesthetic depends on solder quality, since soldering is on the external
surface of diode.
c) Piece-to-piece variation in solder finish.

d) Fatigue to the operator
e) Large consumption of nitrogen gas
Objects Of The Invention:
The object of the invention is to minimize the criticality of soldering process
in respect of both the visual finish and the leak rate.
The invention has another object to avoid the application of solder on the
external surface of the diode thereby making the visual quality of the diode
independent of the soldering process and eliminating the need for rework and
repair operations.
Yet another object of the invention is to carry out soldering without thermal
oxidation of copper components of the diode.
Still another object of the invention is to improve upon the absolute
magnitude of the leak rate.
The invention has further object to develop and establish methods to
overcome the limitations of soldering on the external surface of the diode.
A still further, object of the invention is to make the soldering process simple
and adaptable to industrial manufacturing.

Description Of The Invention:
Physical construction of diode:
The invention is related to a stud-type, pig-tail diode of rating 250A/2000V
that is especially designed for use in AC-to-DC bridge rectifier that provides
field excitation to the brushless exciter of 250MW and 500MW turbo-
generators. The diode rectifier that is mounted on the same shaft of the
generator experiences intense centrifugal acceleration by way of high-speed
rotation (3000-3600 rpm). The rotating requirement demands a distinctly
different mechanical construction for the diode as compared to static
applications specially in respect of the sealing-joint of the diode. The joint of
a static diode is fabricated by fusing the surfaces of the two joining
components using conventional welding process. In contrast, the joint of a
rotating diode is prepared by folding and pressing the circular periphery of
one component over the circular foot of the other component. A Fe-Ni (Iron-
Nikel) housing say (part-1) and copper-base assembly say (part-2) that are
the two joining parts of the diode. The copper-base assembly contains the
semiconductor diode chip with its two electrical terminals (anode, cathode)
drawn out.
Hermetic sealing of joint:
The hermetic sealing is achieved by soldering the joint between the two joining
parts of the diode. The soldering process has the following basic requirements:

a) Solder finish should be uniform and should ensure a leak rate of not more
than 3 x 10-8 std-cc/sec.
b) Soldering process should not affect the aesthetic appearance of the diode.
c) Copper-components of the diode should not get oxidized during the
process.
Brief description of the accompanying drawings:
The invention will now be described with help of the accompanying drawings
which depict an exemplary embodiments of the invention. However, there can be
several other embodiments, all of which are deemed covered by the description.
Fig-l(a) shows the outline diagram of the stud-type power semiconductor diode
used in rotating electrical machinery. Fig-l(b) shows a schematic
diagram of the two joining parts of the diode.
Fig-2 shows a schematic diagram of conventional method of external soldering.
Fig-3 shows the solder positions in the external and internal soldering methods.
Fig-4 shows a schematic diagram of the invented method of internal soldering.
Fig-5 shows a schematic diagram of oven-heating method of internal soldering.
Fig-6 shows schematic time-cycles of temperature, gas, vacuum and water
cooling in oven-heating method.
Fig-7 shows a schematic diagram of induction heating method of internal
soldering.

Table-1 shows the summary of induction soldering trials and the results.
Table-2 compares various features of the conventional and invented methods.
Detailed Description Of The Invention:
• Improved soldering methods (present invention):
Two methods of soldering were conceptualized and implemented in industrial
manufacturing. The invented methods overcome the limitations of the
conventional glove-box method.
1) New method-1: Oven-heating
2) New method-2: Induction heating
In the invented methods, the application of solder is shifted from external
surface to internal surface of the diode. This is achieved by reversing the
sequence of folding and soldering operations. In the invented methods, soldering
is carried out prior to the folding and pressing operations. The solder lies
between the foot of the housing and the folded surface. Fig-3 shows the
positions of solder in conventional method viz-a-viz invented method.
Method-1 employs electrical heaters over which the diodes are placed for
soldering. The diode gets heated up by way of thermal conduction through

physical contact with the heater, as in the case of conventional method. In
contrast, method-2 adopts non-contact heating based on principles of magnetic
induction and eddy-current formation in the vicinity of solder-joint.
Arrangement of solders in the invented methods:
The present invention discloses an arrangement of placing the solder material in
the diode. The same arrangement is adopted for both the new methods. Fig-4
shows the arrangement of solder material and the sequential operations of
soldering and folding. The invention employs solder-rings blanked out of 0.35-
mm thick lead-tin (60:40) alloy.
Fig-l(a) shows the outline diagram of the diode, Fig-l(b) shows housing (part-1)
and copper-base assembly (part-2) that are the two joining parts of the diode.
The invented arrangement is detailed as below:
a) Housing (part-1) is positioned in the cavity of the copper-base (part-2).
b) A thin coating of hydrogen-based liquid (hydrazine) is applied all over the
circular foot of the housing and the inner walls of the cavity, using a thin
brush. During the soldering process, the hydrogen cleans up the oxide
layers present on the coated surfaces, by way of reacting with oxygen and
forming water vapour. The cleaning operation facilitates better wetting of
the solder on the surfaces.

c) 3-5 pieces of soldering ring are stacked on over the other and the stack is
placed inside the cavity. The number of pieces needs to be carefully
selected. Lesser quantity does not ensure hermetic sealing, while
excessive quantity distorts the shape of housing during the subsequent
folding and pressing operations.
d) A graphite jig in the ring form is placed over the soldering rings. As
graphite does not get wetted with solder, it is easily removable from the
diode after the soldering operation.
e) A copper weight is placed over the graphite jig. The weight helps to gently
press the solder rings upon the foot of the housing to ensure concurrent
and uniform melting of the solder all around the joint between the two
joining components, part-1 and part-2.
• Oven-heating method of solder (New method-1):
This method employs 'batch-processing', in contract to the 'individual piece-by-
piece processing' of conventional method. A batch of diodes is loaded at a time
in a heating-oven that is specially designed for this purpose. Fig-5 shows the
schematic diagram of the oven-heating method of soldering.

Features of the heating oven:
a) provisions for purging the oven chamber with nitrogen gas throughout the
process to prevent oxidation of the copper components due to heating.
b) Loading trays have built-in heater coils to enable conductive heat-transfer
to the objects placed on the trays. In conventional heating ovens, heaters
are placed in the walls of the oven and convective heat-transfer takes
places from the heaters to the objects placed inside oven. Such ovens are
not suitable for this method, since the nitrogen gas flowing continuously
into the oven weakens the convective heat-transfer.
c) Loading trays have built-in water-cooling coils for flow of chilled water at
the end of soldering, since rapid cooling is desired to protect the molten
solder from prolonged thermal exposure and also to reduce overall cycle
time of process.
d) Temperature programmer for automatic programming of the sequential
process steps.
e) Rotary vacuum pump for the initial evacuation cycles.
Steps Of The Soldering Process By Oven-heating Method:
a) A batch of diodes is prepared with the arrangement of solder rings,
graphite jigs and copper weights.

b) Diodes are placed directly on the loading trays of the oven and the oven
door is closed.
c) Temperature programmer is activated and it performs the following
activities. Fig-6 shows the schematic time-cycles of temperature, gas,
vacuum and chilled water.

1) Oven chamber is evacuated to a vacuum level of 0.01 mbar (typical
duration=5 minutes).
2) Oven chamber is purged with nitrogen gas till the pressure in the
chamber becomes positive (typical duration= 5 minutes).
3) The evacuation and purge cycles are repeated two times.
4) Heaters in the loading trays are switching-ON and nitrogen gas
flow through the chamber is continued.
5) Temperature is ramped up to a level that is sufficient to completely
melt the solder alloy. Melting point of solder alloy = 190°C and
typical tray temperature required to achieve the melting point is
275 °C. Typical ramp-up duration is 30 minutes. Melting stars at
the end of ramp-up.
6) The temperature is held at the same level (275°C) for 5 minutes to
ensure complete melting of solder.
7) Tray heaters are switched-OFF, chilled-water flow into the trays is
initiated and nitrogen gas flow is continued.

8) Oven cools down in a typical duration of 60-90 minutes.
9) Oven door is opened and the diode are unloaded from the trays.
10)Copper weights and graphite jigs are removed from the diodes.
• Induction-Heating Method of Soldering (New
Method-2):
This second method of present invention employs electromagnetic induction for
heating, whereby a high-frequency current flowing in a coil produces changing
magnetic flux that induces eddy current loops in the object placed within the coil.
The object gets heated up by the eddy current. The duration of soldering
process of this method is too short to oxidize the copper components of the
diode and hence, nitrogen gas is not required. This feature eliminates the need
for a closed chamber, in contrast to both the conventional glove-box method and
the invented oven-heating method. The process set-up becomes relatively
simpler, as the soldering is carried out in open ambience. Fig-7 shows the
schematic diagram of induction-method of soldering. The set-up a high-
frequency power supply (induction heater) (7-a), provided with an inductor-coil
(7-b). The sequential steps of this method are as follows:
a) Diode is assembled with solder rings, graphite jig and copper weight as in
the case of new method-1.

b) Diode is then placed on the platform of diode-loading jig (7-c). The jig is
then moved up and down to position the diode within the inductor coil.
The solder rings are positioned at the same plane of the inductor coil.
c) Induction heater is set at the required output power, frequency and
process time.
d) Induction heater is switched ON.
e) Soldering rings melt due to eddy current flow induced in the vicinity of the
rings by the magnetic fields established due to highOfrequency current
flowing through the inductor coil.
f) At the end of the preset process time, the melting of solder rings is
complete and the induction heater is switched-OFF automatically.
g) Diode-loading jig is operated to move the soldered diode out of the
inductor coil and the diode is unloaded from the jig.
• Verification Of Hermetic Sealing Of Oven-heating Method:
A batch of 30-Nos of diodes was assembled with solder rings, graphite jigs and
copper weights. The joining surfaces were smeared with hydrazine liquid prior to
the placement of solder rings. The diodes were placed on the loading trays of the
heating oven and the oven door was closed. The temperature programmer was
activated and soldering process was allowed to continue as per the programmed

pre-heating evacuation and nitrogen-purging cycles, ramp-up (30min) and dwell
(275°C/5 min) cycles of temperature, chilled water cooling cycle (90min). A
nitrogen flow of lOliters per minute was maintained throughout the process. The
entire process was completed in 150minutes. The diodes were unloaded from the
oven. The soldering finish was visually inspected and was found to be uniformly
melted in all diode pieces.
The sealed joints were tested for leak using a helium mass spectrometer after
the diodes were exposed to pressurized helium in a closed chamber for 12 hours.
The spectrometer readings were observed to have the leak rates in the range
between lxl0-8 std-cc/sec that is acceptable as compared to the acceptance
criterion of <3xl0"8 std-cc/sec.
• Verification Of Hermetic Sealing Of Induction-heating Method:
Samples of diodes were assembled with solder rings, graphite jigs and copper
weights. The joining surface were smeared with hydrazine liquid prior to the
placement of solder rings. The samples were soldered using induction heating
method. Table-1 shows experimental trials that were conducted to optimize the
input parameters for better solder finish. Higher frequency of the order of 400
kHz yielded slow and steady rate of melting resulting in uniform solder melt with
the process duration of 25 seconds per piece. In contrast, lower frequency of the

order of 15 kHz offered rapid and nearly instantaneous melting (2-3 seconds)
accompanied with disturbed solder finish.
The experimental observations are in good agreement with theoretical concepts
of induction heating. With higher frequency of current passing through the
inductor coil, eddy current are induced only in the portions of the job that are
closer to the coil. In the present case, the solder rings are in close proximity to
the coil and the controlled melting at 400 kHz is attributed to located heating in
the vicinity of solder rings. At 15 kHz, penetration of induction is relative deeper
and a larger volume of the job gets heated up causing rapid increase of
temperature that melts the solder instantaneously. Better process control is,
thus, achieved at higher frequencies.
The trials conducted at 170 kHz using an inductor coil placed to the diode (diode
periphery = 046-mm) resulted in excessive heating as indicated by the thermal
oxide layer formed on the copper surface. The problem of oxidation in such a
case was resolved by placing the diode on a heat-sink arrangement that is
cooled by continuous flow of water.

The joints soldered at 40 kHz, 170 kHz and 400 kHz were tested for leak using
helium mass spectrometer after the diodes were exposed to pressurized helium
in a closed chamber for 12 hours. The spectrometer reading were observed to
have the leak rates in the order of 10"10 std-cc/sec that is much superior to the
leak rates achieved in the conventional glove-box method and the invented
oven-soldering method.
• Comparison Of The Invented And Conventional Methods:
Table-2 compares of the two invented methods with conventional method of
soldering with specific reference to system requirements, solder finish and leak
rate. Both the invented methods that employ soldering on the internal surface of
the joint prior to its formation are superior to the conventional method that
applies solder on the external surface of the pre-fabricated joint. Whereas both
the invented methods are well suited to industrial manufacturing, the induction-
heating method is preferred over the oven-heating method because of simplicity
of system requirements and improved leak rate from 10"8 std-cc/sec to 10"10 std-
cc/sec.
• Relevant Feature related to the invention are summarized
below:-

1) Two methods of hermetic sealing of stud-type power semiconductor diode
used in rotating electrical machinery, by applying solder on the internal
surface of the joint instead of external surface.
2) Invention of the solder in the form of annular rings blanked out of thin foil
of 0.35-mm thick solder-alloy of Lead : Tin=60:40 with a melting point of
189°C employed in both the methods.
3) Invention of a solder arrangement that is applicable to both the methods
comprising of:

a) a stack of 3-5 pieces of solder rings, arranged one over the other and
placed in the cavity of the copper-base (bottom portion) of the diode
wherein the housing (top portion) is already positioned.
b) A jig that is purposely made up of graphite to make it removable after the
soldering process and, this graphite jig placed over the stack of solder
rings,
c) A weight in the form of a copper ring, placed over the graphite jig to
apply a definite downward pressure upon the solder rings so that the
melted solder settles down firmly and wets the entire circular periphery of
the joint uniformly.

4) Invention of the sequence of processing whereby the soldering process
employing the arrangement mentioned in (3) is carried out prior to the
formation of the joint.
5) The first of the above two methods is oven-heating method of sealing of -
--type semi conduct diode, hermetically with the help soldering that
contains:

a) closed-chamber with provisions for flow of nitrogen gas that helps
prevention of thermal oxidation of copper components of the diode during
the soldering process,
b) loading-trays that are specially designed with built-in heater and cooler
coils to enable rapid heating and rapid cooling of the diodes,
c) temperature-programmer wherein the time-cycles are programmed for
evacuation, venting, heating and water-cooling events.
6) The oven-heating method operates with pre-programmed events following
a sequence of process control features:-
a) 2-3 alternating cycles of evacuation (0.01 mbar) and nitrogen-venting
(positive chamber pressure),
b) 30-minutes ramp-up to 275°C of loading-tray temperature,
c) 5-minutes dwell at 275°C followed by chilled water cooling.
d) Nitrogen flow throughout the heating and cooling cycles.

7) The second of the above two methods is induction-heating method that
contains an inductor-coil carrying high-frequency (40-400 kHz) current with
5-10 kW power and inductively heating up the diode placed within its loop
and a loading jig for diode.
8) The induction-heating method offers improved leak rate (10"10 std-cc/sec)
as compared to the leak rate (10"8 std-cc/sec) achievable using the oven-
heating method mentioned in (5) and (6).
9) The induction-heating method is simpler compared to the oven-heating
method mentioned in (5) and (6) in that the former does not require closed
chamber with nitrogen flow by virtue of its indirect method of localized
heating for a duration as short as a few seconds.

10) The induction-heating method, mentioned in (7), (8) and (9) requires
water-cooling of the diode from the bottom side of the diode in case when
the chosen combination of power and frequency and coil size results in
oxidation of copper base of the diode.
11) Both the invented method as mentioned (1) to (10) are adaptable to
industrial manufacturing.

We Claim:
1) A device for oven-heating method of sealing of stud-type semiconductor
diode, hermetically with the help of soldering, consisting:-
- a closed chamber with provisions for flow of nitrogen gas that helps
prevention of thermal oxidation of copper components of diode during the
soldering process;
- loading trays that are specially designed with built-in heater and cooler coil
to enable rapid heating and rapid cooling of the diodes;
- a temperature-programmer wherein the time-cycles are programmed for
evacuation, venting, heating and water cooling events.
2) A method of oven-heating of sealing of stud-type semiconductor diode,
hermetically with the help of soldering by providing a device, as claimed in
claim 1, through programmed events and following a sequence of process
control features equipped in it, comprising steps of:-
- 2-3 alternating cycles of evacuation (0.01 m bar) and purging with nitrogen-
under positive chamber pressure for 5-6 minutes,
- 28-32 minutes, preferably 30 minutes ramp-up to 275° C of loading tray
temperature;

- 4-6 minutes preferably 5 minutes dwell at 275° C followed by chilled water
cooling; and
- nitrogen flow throughout the heating and cooling cycles;
wherein the soldering through said process control features in the internal
surface of the joint has minimized the criticality of the process like overflows
and non-uniforming of solder and has the advantage of batch processing.
3) A device for induction-heating of sealing of stud-type semiconductor diode,
hermetically with the help of soldering, consisting:-
- a set up for induction heating high frequency power supply source (7-a);
- an inductor coil (7-b), provided for carrying high-frequency current and
inductive heating up the diode placed within the loop;
- a loading jig (7-c) for diode.

4) The device as claimed in claim 3, wherein the inductive coil (7-b) carries
high frequency current in the frequency range of 40-400kHz.
5) The device as claimed in claim 3, wherein the inductive coil has power in
the range 5-10KW.

6) The method of induction-heating for sealing of stud-type semiconductor
hermetically with the help of soldering is carried out providing with the device
as claimed in claim 3, comprising steps of :-
- smearing of hydrazine liquid all over the circular foot of the housing and
inner walls of cavity prior to placement of soldering;
- assembling the diode with solder rings, graphite jig and copper weight;
- placing the diode on the platform of diode-loading jig (7-c) and positioning
of the diode within the inductor coil being carried out by moving the jig (7-c)
up and down;
-setting of output power, frequency and process time of the induction heater;
- switching 'ON' of the induction heater;
- melting of soldering rings due to eddy current flow, in the vicinity of rings;
- melting of solder rings having completed after preset process time;
- the switching 'OFF' of induction heater getting done automatically; and
- diode loading jig (7-c) being operated to move the soldered diode out of the
induction coil and diode is unloaded from the jig (7-c);
wherein said melting of solder rings in the internal surface of the joint
becomes uniform due to localized heating in the vicinity of solder rings for a
short duration say 25 sec/piece at 400kHz, requires no nitrogen for
prevention of thermal oxidation of copper and yields improved

leak- rate (10-10 std cc/sec) compared to oven-heating method (10'8 std
cc/sec).
7) The device as claimed in claim 1, wherein the process of said soldering is
carried out through a solder arrangement which is applicable to both
methods, as claimed in claims 2 and 6, comprising :-
- a stack of 3-5 pieces of solder rings arranged one over the other and placed
in the cavity of the copper-base (bottom portion) of the diode wherein the
housing (top portion) being already positioned;
- a jig, made up of graphite to make it removable after soldering process and
this graphite jig having placed over the stack of solder rings;
- a weight in the form of copper ring having placed over the graphite jig to
apply a definite downward pressure upon the solder rings so that the melted
solder settles down firmly and wets the entire circular periphery of the joint
uniformly.
8) The device as claimed in claim 7, wherein said jig of graphite is provided,
as the graphite does not get wetted with solder and easily removable from
the diode after soldering operation.

9) The methods as claimed in claims 2 and 6, wherein soldering carried out
with the solder rings are in the form of annular rings blanked out of thin foil
of 0.35 mm thick solder-alloy of Lead : Tin= 60 : 40 with a melting point of
189°C employed in both the methods.
10) The method of induction heating as claimed in claims 6 and 7, wherein
said diode having copper base is resulted in oxidation due to chosen
combination of power and frequency, and to avoid the oxidation due to over-
heating, water cooling from bottom side of the diode is required.
11) The method of oven-heating and induction -heating as claimed in claims
2 and 6, wherein hermetic sealing of stud type power semi conductor diodes,
provided in electrical rotating machinery by applying solder uniformly in both
the methods, on the internal surface of the joint instead of external surface.
12) The method as claimed in claim 11, both the methods are adaptable to
industrial manufacturing.
13) The methods as claimed in claims 2 and 6, wherein diodes after soldering
tested for hermetic sealing with helium mass spectrometer having the diodes
been exposed to pressurized helium in a closed chamber for 12 hrs.

14) The method as claimed in claim 13; wherein said testing of diodes for
hermetic sealing, gives results of the leak rates achieved therefrom are 1 x
10-8 and 2 X 10-8 Std-cc/Sec for the oven heating method and 10-10 STd-
cc/Sec for the induction heating method, wherein the values are acceptable
as compared to the acceptance criterion of <3 x 10-8 std- cc/sec.

The present invention provides two methods of hermetic sealing of stud-type
power semiconductor diode that is especially used in rotating applications. Both
the invented methods viz. oven-heating method and induction-heating (by eddy
current) method, employ soldering on the internal surface of the joint between
the bottom and top portions of the diode and soldering is carried out prior to
folding and pressing operation against the conventional method of applying the
solder over the external surface of the joint. The invention includes selection of
the solder material in the form of a ring of specific dimensions blanked out of
thin solder-alloy foil. A arrangement whereby the solder rings are held in position
during the soldering is also invented. The holding arrangement that is applicable
to both the methods is essential to achieve uniform wetting and finish.
Solderings through invented process control features in the internal surface of
the joint has minimized the criticality of the process like over-flows and nonuniform
finish, etc) of the oven heating has the advantage of batch processing,
induction-heating is simpler and offers improved leak rate of 10-10 std-cc/sec as
compared to 10-8 std-cc/sec.