Richard,
your asking a multi million dollar question that in my own opinion no one can give you the right answer since no one really knows. hence I think the lack of answers to your question....thought there would be more people willing to propose a thought or two. there was a question on the SMART Group forum in the uk about voiding in a BGA within an RF system. Basically most people are saying it's not much of a problem since the current flows over the surface and not through the joint. Now I'm just a lowly process guy so I don't know about all that but from a mechanical point of view the void can be a good thing as it'll act as a stress reliever and also inhibit crack propogation in the system. I couldn't tell you if the void would heat up more than the rest of the joint as it's a test again that still need's to be done, if you can loan me an thermal imaging camera I could tell you right enough.
JohnW
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