Below are the questions asked during the event, along with their respective answers.
Q: If a pad is compressed between two surfaces and there is expansion/compression due to thermal cycles, will the pad stretch and stay wetted to both surfaces?
A: Our GR and XR gap fillers have some elasticty and enough tack to ensure they can maintian good thermla contact during thermal expansion.
Q: For non-curing gap fillers, do you suggest them for automotive applications where there is vibration? or would that make the material migrate?
A: If the gap is less than 1.0mm it is worth evaluation. In gap that are larger there is some risk. FP does not recommend it.
Q: If you’re gapping at a 20-30% compression between say a BGA board component and the heat sink, sometimes the compression “cuts-through the pad material at the overhang of the gap filler material. Is this detrimental to the application?
A: Any tears in the material , especially along the endges will have no impact thermally
Q: How do you recommend simulating the gap fillers? Is there a document available for that? Thanks.
A: Unfortunately, we don’t have any document to support any modeling. We have done compression testing on pads. Your customers have used this for modeling stress and compression. If you are modeling thermally you can use the apparent thermal conductivity or the reciprocal of the thermal resistance normalized for thickness.
Q: Pads have fiberglass or other material in them that helps them hold together. The putty is somewhere in between – I assume that it will stick to itself better than a silicone grease, but it will likely be more prone to pump out than conventional pads?
Depending on how cohesive the material is, when things go back to their normal temperature the material may all return back to where it was or it may separate away. This is mostly an issue with thermal interface materials like grease that can pump out with thermal cycling or power cycling (because the stuff doesn’t really stick to itself all that well).
The bigger issue is thermal excursions. As things change temperature, any CTE mismatch will cause things to move and the board to warp somewhat depending on how balanced the design is. As the thermal interface material gets stressed, it squishes out.
A: Putties are less prone to pump out than greases which are very low viscosity. We test all our Gap fillers in heat shock, high temp, low temp and high humidity.
Q: Is there any finite element software able to evaluate the compression behavior before the experiment?
A: No, not at this time.
Q: Do you have a guideline for maximum gap that can be filled with form in place?
A: No more than 1mm
Q: If a putty has almost 100% compression set, then should I be concerned about the material staying in the interface? For example, what if the gap increases in size slightly during thermal cycling?
A: Material has tack and will adhere to the surface. It is enough to maintain contact during thermal expansion
Q: What are some design guidelines for maximum stress (or strain) on FR4 PCBAs?
A: Unfortunately we do not have any data specfic to the PCB.