Failure analysis for bad wetting on HASL PCB

Hot air solder level (HASL) surface finish is one of the popular options in printed circuit board (PCB) surface finishing technology. This kind of finish is popular because of its long shelf life and low cost compared with other PCB finishes. But one of the disadvantages of HASL finish is its low planarity. This uneven pad surface is sometimes creating coplanarity problems in reflow soldering surface mount components and eventually end in poor solderability. In this case, badwetting (dewetting and nonwetting) was found on HASL PCB after wave soldering. SEM&EDS analysis was performed on badwetting pads surface and cross section respectively; it can be inferred that thinner tin coating had almost alloyed with copper from pads completely. The results of solderability test showed that because of SnCu alloy, the solderability of HASL pads had degraded. According to the HASL process, this kind of problem can be overcome by optimization of the HASL process.