Hot Air Solder Leveling (HASL) has always been the main staple of PCB surface finishes. In the late 80s, 60/40 tin-lead reflow started to phase out of processes and was replaced with Hot Air Solder Leveling. HASL, the longtime reliable surface finish is still used today in military, aerospace, medical, and other applications.
The Story of Hot Air Solder Leveling
The long history of HASL being as an excellent solderable finish for printed circuit boards has kept this surface finish alive in the market both domestic and offshore even though it contains lead.
We recognized since the beginning of EU RoHS in mid-2006, that HASL was here to stay. Longevity, reliability, and trusted by customers meant keeping the process alive even while consciously trying to get the lead out was going to be a necessity.
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.
Originally it was thought lead would be gone from everything we touched.
It is obvious that lead may never be totally gone from all products, choosing a surface finish that works best for you and your process while keeping the environment safe is ideal.
What About Lead Free HASL?
The Lead Free version (LFH) of became the most looked at surface finish next to immersion gold early on. So, why is lead free HASL still considered a hassle? Some circuit board manufacturing facilities may need to outsource this process.
PCB with HASL / Lead Free HASL Surface Finish
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.
Pre-trial 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. With this promising development in application has increased the demand, brought more LFH in house and decreased time to customer product delivery.
PB Free HASL Needs To Be Processed Differently
LFH needed to be applied at a hotter temperature. On the first pass the surface is left grainy and dull. Once a second pass is added the surface and appearance improves to a shiny, much flatter, and smoother even coat. However, with the excess heat from two dips in the molten solution leaves copper on the hole walls’, reducing the copper below the acceptable limits per IPC. This forced another process change once again adding a stain to the LFH future.
After a lot of changes to chemistries and processes lead free HASL is now a stable surface application to use on PCBs. It is consistent as its parent HASL using this finish has become hassle free in the PCB manufacturing industry.
LFH Decrease In The PCB Industry
Why does LFH seem to be the least used surface finish in the industry still today?
ENIG (Electroless Nickel Immersion Gold), OSP (Organic Solderability Preservative), and even immersion tin and silver all took the lead in manufacturing while PB Free HASL was still a process headache. As technology develops and continues to do so, real estate on surface becomes tighter and foot prints decrease, it is clear LFH still has to prove itself to be a winner and the go to surface treatment.
Lead Free HASL still has some groundwork to overcome a tarnished reputation However, this not so popular surface finish is consistently gaining popularity amongst customers making manufacturers give it a second look as a main staple up against immersion silver, OSP, and immersion tin.