Hot Air Solder Leveling (HASL) has always been the main staple of PCB surface finishes. In the late 1980s, 60/40 tin-lead reflow started to phase out of processes and was replaced with Hot Air Solder Leveling. HASL finish, the long running reliable surface finish, is still used today in military, aerospace, medical, and other applications.
The Evolution of Hot Air Solder Leveling
The long history of HASL representing an excellent solderable finish for printed circuit boards has kept it alive in the global PCB market, both domestic and offshore, even though it contains lead (Pb).
We recognized since the beginning of EU RoHS in mid-2006 that HASL was here to stay. Longevity and reliability combined with the trust of customers meant finding a solution to keeping the process alive, which meant trying to remove the lead was going to be a necessity.
Given that PCBs are in everything from appliances to toys, the safety concern was obvious. Once the health risks from prolonged exposure to lead for children and adults were established, getting lead out of products became the focal point for the electronic manufacturer.
Originally it was thought lead would be gone from everything we touched.
Now, we've come to the realization that lead may never be totally gone from all products, but choosing a surface finish that works best for you and your process while keeping the environment safe is ideal.
What About Lead Free HASL Finish?
The Lead Free Version (LFH) of HASL PCB finish 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, demand has increased, as well as 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, both surface as well as appearance improve to a shiny, much flatter, and smoother even coat. However, 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 finish (Electroless Nickel Immersion Gold), OSP finish (Organic Solderability Preservative), and even immersion tin and silver all took the lead in manufacturing while Pb Free HASL finish 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 once not-so-popular surface finish has consistently been gaining popularity amongst customers, making manufacturers give it a second look as a main staple up against immersion silver, OSP, and immersion tin.