CHOOSING A SURFACE FINISH FOR FINE-PITCH COMPONENTS
Welcome to our new blog series based on our experience with PCB manufacture and assembly, courtesy of our engineers. Over the years, we’ve seen our fair share of blunders and subsequent losses over the course of the hardware development process. With a bit of understanding, forward-thinking and patience many of these problems can easily be avoided. This series aims to tackle the understanding part with a look at some real-life examples we’ve encountered.
Today we are going to share with you a real case study involving a Seeed Fusion PCBA customer’s design. At the time of placing the order, the customer requested the default HASL surface finish, but listed in their Bill of Materials was a 0.4mm BGA. It’s a small BGA with only four balls in the array, but the combination of 0.4mm pitch BGA and HASL surface finish should ring alarm bells, and here’s why:
The HASL surface finish is essentially a coating of solder that protects the copper pads from oxidation. As its name suggests, the Hot Air Solder Level technique has the boards dipped in a bath of molten solder and then uses a hot air blade to remove as much of the excess solder as possible. But as anyone who has soldered a joint before knows, solder behaves very much like water in its molten state, in that it tends to form droplets and welcomingly coalesces with other liquified solder droplets. It is the high surface tension that results in an uneven surface and rounded edges, a meniscus, instead of covering the surface evenly.
Compare these images of HASL finish on the left and ENIG on the right:
For larger pads and leads, the uneven surface, or poor coplanarity in technical terms, barely has any effect on the solderability of the pads, but as component leads and pads get smaller and closer together, it becomes increasingly more serious and can have a major impact on yield.