Implementation of a conformal solder mask system

The systems engineering approach was used to implement conformal solder mask in the manufacture of printed wiring boards (PWBs) at the AT&T Microelectronics Richmond Plant. Existing solder mask had a planar surface causing low soldering process yields on PWBs using Surface Mount Technology (SMT). SMT reduced product cost by allowing 50% more components per unit area of a PWB. Conformal solder mask processing reduced the mask thickness applied, and allowed future SMT PWBs to be manufactured.

Coating methods to apply solder mask were evaluated. Two methods, curtain coating and electrostatic spray (ESS) f were chosen since no method satisfied all operational requirements. Ciba Geigy's Probimer process was implemented to meet short term revenue and capacity requirements. Probimer was an industry standard but had performance limitations. ESS met all requirements but required additional development to select a solder mask material and obtain customer approval.