Hello Todd,
I agree with the response on smaller corner balls may be due to warpage of
the PBGA substrate.

The 45% voids in your solder balls need to be verified with a cross section.
I  thought I was seeing 35% voids and found through cross sectioning that
they were really a number of smaller voids which the x-ray could not
resolve.

Jim Kittel

> -----Original Message-----
> From: Todd Ness [SMTP:[log in to unmask]]
> Sent: Friday, April 23, 1999 9:56 AM
> To:   [log in to unmask]
> Subject:      [TN] BGA Voiding Problem
>
> 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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