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Modeling force from thermal gap pad material

Modeling force from thermal gap pad material

Modeling force from thermal gap pad material

(OP)
Hi,

I'm currently working on a problem involving a printed circuit board (PCB).  This PCB has a number of discrete chips which are being cooled via a large common heatsink.  To transfer heat from the individual chips to the heatsink, each chip has a piece of gap pad material compressed between it and the heatsink.  The gap pad material can best be described as soft rubbery compound.  It generally acts as a non-linear spring when compressed.

What I'm trying to model is the deflection and stress on the PCB caused by the compression of the gap pad.  I can model this quite simply by looking at the compression at my nominal gap, and work out a force or pressure that I then apply to the top of each chip.
However, this is overly simplistic for two reasons.  The main issue is that as the PCB deflects compression of the gap pad material will drop, and thus the force should drop accordingly.  What I am modeling is a constant force regardless of deflection.

The other issue is that the heatsink itself will also deflect. since there is an equal and opposite force.  As both PCB and heatsink deflect away from each other, the force/pressure will drop even more. From an FEA perspective, I think I need to model a spring in compression; i.e.: a pre-loaded spring between two parts, even if its linear.  Is there any way to model this in Pro-Mechanica?  WF4, if it matters.

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