IGBT Modules - Technologies, Driver and Application (Second Edition) - page 101

2C
89
Fig. 2.27
Principleof coldwelding
Pressing a suitable contact into a PCB creates a cold welded, gas-proof and
conditionally separable connection. The cold welded connection is the basis of the high
current capacity and reliability of the contact point. The very low contact resistance of
less than 0.1mΩ makes a load current of more than 25A per contact pin possible.
Currents of 150A and more are achieved in IGBT power modules by paralleling
individual contact pins. If tin-plated contact pins are used, PCB with chemical tin
surfaces
19
,
whichmeet the lead-free requirements set out in theRoHSDirective, can be
used.
A press-fit tool must be used to press IGBTmodules into aPCB or press aPCB onto an
IGBT module. In order to be able to press the module into the PCB, a specific force
must be applied for each pin in themodule. For the forked pin i
for example,
the press-in force per pin is between 75N and 81N. The amount of force depends on the
diameter of the hole in the PCB. For example, if there are 30 pins in one module, the
total press-in force requiredwill bebetween 2.25kNand2.43kN.
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Chemical tin surfaces are flat, solderable surfaces with a layer thickness of approximately 0.8 to 1.2µm on
PCB. The name comes from the fact that a chemical process is involved. Chemical tin, while ideal for press-in
contacts, impairs solderability after long periods in storage, due to the formation of an intermetallic phase
between the copper of themetal conductor and the tin. IPC-4554 specifies the chemical tin for use inPCB.
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