I have a question regarding vacuolar porosity under leads. 
A potential customer has forwarded me some CAD data and will send me some boards for me to inspect using our 5DX X-ray. His concerns are that during reflow, the flux attempts to outgas and stops just short of total outgassing when the joint solidifies. This situation then forms "pockets" in the joint in the lead/pad interconnect. 
My question is this - Has anyone done any studies regarding this phenomena and if so, does this affect the integrity of the joint? My belief has always been that the structure and formation of the heel portion of the joint comprises the actual integrity of the joint and the toe fillet (or lack thereof) does not play into the lifespan (or strength) of the joint. Therefore, the lead/pad interconnect would also not be the main point of contention.
In Alabama, we had some components (QFP48) that had a ground lug on the bottom part of the package that was soldered to a pad directly beneath. The lug was approximately 1mm by 1mm and the pad was approximately 7/10mm by 7/10mm. In this case, the porosity dilemma was evident. We would see, through 3D laminography, that large voids were formed, causing roughly a 40% or so decrease in actual solder interconnect area. Some playing around with the reflow profile displayed better results, but never resolved the issue. 
Any thoughts/comments would be greatly appreciated.

Thanks,


Jason Gregory
Software Specialist 
Sanmina-SCI - Tech Center Austin
15508  Bratton Lane
Austin, Tx. 78728
(512)246-5648
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