Hi Todd,

I have a feeling you will get a lot of responses but here is my two cents
worth..

45% voiding is not acceptable.  On 1mm pitch BGAs using HASLs boards in N2
ovens, 0-5% voiding is the norm for our production line.  Even in our worst
configurations, we never see 45% voiding.

My guess is that you are not holding your solderpaste/BGA ball solder long
enough at temp to allow the volatiles to clear the molten mass.  Your whole
process adds up to only 4.5 minutes which seems too fast considering your
results.   What you did not say was how long is the time above 208 C
(probably not much more than a few seconds).  I'd start there and try to
extend the reflow temp (25 degrees above eutectic) time duration somewhere
around 30 seconds.
Out of curiosity I'd like to know what type of finish is on the pwb (NiAu I
suspect)?

Smaller corner balls:
If your print was OK then all of the balls should have the same volume, but
not necessarily the same height.  You don't say how you determined the
corner balls to be smaller but one can envision how stress in the PBGA
package can cause the corners to be shorter, yet fatter due to thermal
expansion.  I've made samples where the corners actually have equal solder
volume but less height.


Hope this helps and please post what actually the solution turns out to be.

Phil
-------
At 03:56 PM 4/23/99 +0000, you wrote:
>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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