Tenison
I  had this experience long time ago in one of my previous companies and
I had been suffered a while. Fortunately, I figured out the problem came
from the pad pattern design which  the pad spacing was too big and
caused uneven tensil when reflow if a little misalignment occured. After
narrowing down the spacing for the 0402 pad, this problem disappear!
I hope this work for your case!

Fulton Feng
Nortel
>----------
>From:  Tenison Stone[SMTP:[log in to unmask]]
>Sent:  Monday, November 24, 1997 11:40 AM
>To:    [log in to unmask]
>Subject:       [TN] tombstoning
>
>I have having trouble with tombstoning on a large flat with lots of 0402
>parts.
>
>I have investigated pad geometry, stencil aperture geometry, and
>recalibrated placement equipment.
>
>Can anyone help with this problem?
>
>Tenison Stone
>Telex Communications
>
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