Hot air solder leveling, also known as HASL, has always been the major part of PCB surface finishes. As an excellent solderable finish for printed circuit boards, it has kept this surface finish alive in the market since late 80s. Given that PCBs are in everything from appliances to toys, it was almost instinct to find an alternative. Exposure of lead to children and overall health in general being a concern, getting lead out of products was the focal point of the electronic manufacturer.
It is obvious that lead may never be totally gone from all products, then the lead free version (LFH) existed. It became the most practice at surface finish next to immersion gold early on. So, why is lead free HASL still considered a hassle?
The chemistry makeup of LFH has changed over the years as well as the applications. Vertical or horizontal applications both initially had the same issue as HASL, a pooling, non-flat finish with a somewhat foggy appearance in areas of the PCB.
Combinations of the LFH gave the finish a bad review. The combination of tin, silver, and copper alloy originally had poor results at the processing level, leaving behind a bumpy uneven coat that was dull and unattractive as well as having a poor performance in assembly. Removing the silver, changing the tin-copper, and tweaking the manufacturing process has allowed for a better smoother surface coating than originally found.
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