We are just starting to manufacture a PCB with a .039" fine-line PBGA.
We are currently seeing voids, mainly in the center, that are as much
as 45%. We also intermittently see the corner balls of the PBGA
smaller than the center balls. We are using a 6-mil stencil with a 1:1
aperture size (.019" mil). We are using a convection oven and
reflowing in air. The solder paste is an OA 62SN36PB02AG and the
reflow profile is:
Preheat - 0-150 degrees C in 90 sec
Soak - 150-189 degrees C for 120 sec
Reflow - 189-210 degrees C for 60 sec
Thermocouples were attached to both the corner and center of the PBGA
and they both had the exact same profile with a max temp of 210
Degrees C.

Does anyone have any recommendations for minimizing the amount of
voiding? Is 45% voiding acceptable? What would explain the corner
balls being smaller than the center balls?

Thank you for your input.

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