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.