Alternative surface finishes – options and environmental considerations

Printed circuit boards (PCBs) requiring component attachment, whether leaded or surface mount technology, must have the exposed copper land areas coated with a protective finish. This protective coating must not inhibit solderability and at the same time must act as a barrier for preventing the copper from oxidizing and the inevitable assembly problems that would ensue for the end-user. Globally, the predominant surface finish in the PCB industry is hot air solder levelling (HASL). Driven by the adoption of solder mask over bare copper, HASL was developed as a reliable method of applying solder to the copper surfaces after solder mask. During HASL, a thin layer of solder is deposited onto the exposed copper by passing the boards through a hot, molten wave (or pot) of solder and subsequently blowing the excess solder from the boards using high velocity hot air. This process has been increasingly under scrutiny due to environmental and safety issues (hazardous waste, lead exposure, etc.), technological limitations (fine-pitch device assembly) and equipment maintenance cost. This paper reviews the major alternative surface finishes being currently deployed and additionally seeks to give an overall assessment of the broader environmental aspects of such finishes.