Jim,
We also have experienced similar problems. As the board goes through the
wave, the solder joints heat at slightly different rates depending on
design considerations. For example all the pads may go out to vias, but
each trace to the via may have different lengths. Some pads will thus go
to liquidus before others. The cooler pads now have all the stresses
from the part and also are very weak since they are also near liquidus.
These pads then pull away at the weakest area which at this point is
right at the pad / solder joint interface. Once the part cools the
fracture is still present but it is nearly impossible to see and could
even be making physical contact with the pad such that the device passes
In Circuit Test. Obviously this is a serious reliability concern. It can
be resolved by optimizing wave temperature, contact time, preheat, etc.
and by making sure the designs do not promote excessive or differential
conduction of heat to the topside pads.

Robert Furrow
Lucent Technologies

>----------
>From:  Jim Gryga[SMTP:[log in to unmask]]
>Sent:  Friday, March 20, 1998 2:21 PM
>To:    [log in to unmask]
>Subject:       [TN] Secondary reflow during wave
>
>I have a board with a 208 pin QFP top side.  During the wave process the
>QFP lead at its solder joint reaches 178 degrees C.  Eutectic solder
>reflows at 183 degrees C so the joints should not be reaching molten state
>and visual observation at least confirms that.  What is an acceptable high
>end temperature for these joints to see before the joint is degraded to the
>point of being a reliability risk?  Are there any empiracle tests that  can
>be used to determine the degradation of the joint if any is taking place?
>
>Jim Gryga
>
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