Having been involved as a solder mask supplier for about 20 years and I am
very familiar with your problem. In fact, your problem is the biggest single
reason why most PWB designs are now built with SMOBC (solder mask over bare
copper). Since copper does not melt during wave soldering, the solder mask
maintains its adhesion to the circuitry during wave soldering and you don't
have circuits fusing together, solder mask blistering, etc.

The melting point of eutectic tin/lead solder (63%Sn/37%Pb) is 361-363 deg F.
The solder temperature in wave soldering lines is almost always above.450 deg
F (usually closer to 500 deg F). Therefore, when you pass a fused Sn/Pb board
coated with solder mask over the wave, the Sn/Pb circuitry melts, the bond
between the mask and the circuitry is broken, the molten Sn/Pb moves around,
the solder mask blisters, etc. In short, a mess.

The best solution is to convince your customer of the many benefits of SMOBC
boards and hope he will go along with it. Another approach would be to go to
other non melting type circuitry (such as Ni/Au);  however, there is a big
cost penalty here.

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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