Clearly flux and process set-up are influencing factors, but once all available 
adjustments have been made, we too found that the biggest improvement 
available was to specify matt finish soldermasks. The glossier finishes 
apparently soften slightly at soldering temperatures and allow solder balls to 
adhere to the surface - the rougher surface texture of a matt finish 
discourages the balls from staying. We also found matt finish masks helped 
shift stubborn solder bridges caused by PCB design. The "extra-matte" version 
of the Coates Imagecure resist gave us the best results, but no doubt others 
are equally good.

<http://www.sunchemical.com/products/electronic_materials/imagecure>

<http://www.axoncircuit.com/downloads/Coates.pdf>

Nigel

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