> We had some boards in with LARGE areas of tin/lead, unmelted, with
> soldermask applied to them. This lot had peeling of the mask from the
> unmelted(PrimarySide) as well as secondary side. What are some specific
> additives that can be called out to help this problem?
> Also, how is Bow and Twist measured? What does the percentage indicate?
> Would appreciate any help. Thanks everyone-


I have used two methods to correct this problem.  Which method you use depends
on your process and your electrical requirements.
1)  Hatch any large solid areas using a trace no wider than 50 mils with a
        spacing of 30 to 50 mils.  This decreases the amount of copper and
        the soldermask will be less likely to peel or crack when the tin
        lead reflows.

2)  Change the process from tin-lead plating to SMOBC HAL (Solder Mask Over
        Bare Copper - Hot Air Level).  The solder mask will be on top of copper
        which has a much higher melting point than tin-lead.  The HAL will
        put tin-lead on your pads to aid in soldering and protect the copper
        from oxidation.

If you talk to your board fabricator they can recommend the correct callouts
and settings for both of the above.  They can most likely give you samples of
each method so that you can see the finished results.

=============================================================
Clint Cote                      GE Fanuc Automation
PCB Designer                    PO Box 8106, Mail stop D-40
Phone:  (804) 978-5946          Charlottesville, VA 22906
e-mail: [log in to unmask]

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