Key failure modes of solder joints on HASL PCBs and root cause analysis

Over the last several decades, HASL (hot air solder level) coating has been one of the most robust finishes. During the HASL process, the copper surfaces of bare boards are first cleaned and coated with flux, the boards continue to be dumped into a lead or lead-free solder pot and finally the extra solder is blew out. As the intermetallic has already been formed in the HASL process, the HASL pads are easily soldered well because during the soldering process it is only need to reflow the solder. However, the HASL finishes still have several disadvantages such as lack of flatness on the pads and poor quality HASL finishes. Based on the authors' experience of long-term solder joints failure analysis, two kinds of key failure modes for HASL solder joints were introduced in this paper. One was poor wetting (non-wetting or de-wetting) of the solder to the HASL surface finish on PCB pads during soldering process, and the other one was solder joint crack, causing an electrical open. At the same time, several typical solder joint failure cases were used to show how the two kinds of failure modes occurred. Metallography microscope and SEM were used to reveal details of HASL pads surfaces and the fracture in the solder joints. The compositions of HASL pads surfaces were determined by EDX analysis. Additionally the failure mechanisms of both failure modes were investigated. The root cause for poor wetting is the poor quality HASL finishes which showed poor solderability. Exposed copper-tin IMC at the surfaces of HASL coating oxidized and was very difficult to wet during reflow. For solder joint crack, the reason is because the strength of solder joints was not enough to withstand the overstress. Finally, in order to prevent the two kinds of key failure modes, several suggestions were given.