Solder Mask Strategies for Surface Mount Assembly

Over the past few years there has been increasing utilisation of higher density surface mounting on printed wiring boards. As components and pads decrease in size, the topography of the solder mask relative to the conductors becomes an important solderability issue. There exists convincing evidence that thinner, more conformal solder mask geometries improve soldering yields of both stencilled and wave soldered surface mount components. In order to provide the solder mask coverage required for improved assembly performance, the authors critically compared several commercially available solder mask coating technologies. The coating methods were appraised according to both assembly and printed wiring board manufacturing criteria. Within this programme, seven liquid photoimageable solder masks were also evaluated. The materials were rated according to their final cured properties (electrical, mechanical, chemical performance), their manufacturability in the printed wiring board manufacturing process (maximum throughput, major defects, etc.) and their performance in assembly operations (soldering yields, propensity to ‘solder ball’ formation, white residues, scratches, etc.). The information obtained was used to choose a solder mask strategy which would not only improve assembly efficiency but also increase PWB manufacturing yields and flexibility.