Study of Pb-free HASL PCB surface appearance impact on 2nd level interconnect solderability

Different surface appearances were detected on printed circuit board (PCB) pads with a lead-free (Pb-free) Hot Air Solder Leveled (HASL) surface finish although all were processed in the same solder machine at the same time. The appearance of these HASL finished pads could be categorized into two main groups: shiny surface and dull surface. A useful feature of the HASL finish that distinguishes it from other PCB finishes is that its appearance provides a good indication of its solderability. Surface appearance is, therefore, a primary quality criterion with a smooth shiny finish the target. Experiments were carried out to identify the root causes of this difference in solder surface appearance and its impact on the solderability of these PCB pads in downstream processes such as surface mount technology (SMT) and wave soldering. Counter measures to achieve uniform shininess across the PCB panel are proposed.