Hello Suzanne, my best guess at this is the formation of intermetallics
between the solder coating and the copper.  The growth of the intermetallic
layer between the copper and tin/lead surface will be very brittle and will
crack if enough forces are applied. Once the intermetalic cracks then it
will start to flake off leaving behind and appearance of exposed copper.
Under metalurgical evaluation you will probably find tin/copper
intermetallic formations to confirm this effect. 

There is probable no way this board can be made to be solderable again and
it will most likely need to be scrapped.

Good luck,

Leo Lambert



At 08:36 AM 8/13/96 PDT, Nachbor, Suzanne (MN51) wrote:
>
>We have a vendor who asked us to forward this inquiry to technet:
>
>A customer had a board that accidentally got left in an oven for 7 days at 
>200F over the 4th of July shutdown.  This board is a multilayer board with 
>IR  fused solder finish.
>
>The board was then wave soldered and the solder flaked off the pads leaving 
>unsolderable copper surfaces.  Another  board from the extended bake failed 
>all adhesion testing prior to solder.  Boards without the extended bake 
>passed adehsion testing.  The copper surface on the extended bake boards was 
>dark brown (similar to too much heat) to a white haze.
>
>They are trying to explain what happened and have looked for intermetallic 
>and are planning on using auger to look for possible surface contaminants. 
> Has anyone had a similar experience or insight for further evaluation? 
>
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