With the advent of lead-free processes, HASL remains a widely used surface finish with lead and lead-free solder options.
In my first column, I will focus on hot-air solder (lead containing and lead-free) leveling (HASL) since this was and continues to be a widely used surface finish. This process came into its own during the early 1980s at a time when reflow solder was still predominate. The emerging HASL process offered a more automated way to protect copper through holes and surface attachment areas. When HASL became popular, surface-mount technology was not yet widely used; so solder planarity was not an absolute prerequisite for the surface finish treatment. HASL was ahead of its time.
The HASL process works very well for attaching components in assembly simply because the bulk of the soldering work is being done in the PCB fabrication process. The HASL process creates the copper-tin intermetallic before the PCB gets to assembly. At assembly, reduced heat and dwell times are required because the copper-tin intermetallic joint has already been formed. The PCB fabricator has already provided the energy required to obtain the bond. The assembler only needs to melt the solder paste into the existing solder layer above the intermetallic layer. With all other surface inishes – OSP, immersion silver, ENIG, and immersion tin – the assembler must create the solder bond to the base metal substrate putting a higher strain on the assembly process.
In general, solder will be 50 to 1000 micro-inches thick across the board. The first 25 to 30 micro-inches of solder are in fact a copper-tin intermetallic layer. This layer will grow with time and heat exposure. It is the additional free solder above the intermetallic layer that provides the long-term protection. Vertical HASL machines offer the benefits of lower costs, smaller footprint and greater flexibility in handling various thicknesses of circuit boards. Horizontal HASL machines offer the advantage of greater throughput, better leveling characteristics, lower dwell times and better material utilization.
The HASL process includes, a micro etch, rinse and dry followed by preheating, fluxing, solder application, hot-air leveling, extensive rinsing and a final drying. Each step is critical in providing a reliable surface finish. As an example, good drying is necessary because the water left behind will not be displaced by the flux, and out gassing in the form of steam will occur when the board contacts the 500°F molten solder. With either vertical or horizontal systems, this usually leads to exposed copper in through holes.
In the case of horizontal systems, pre-heaters are generally used to get the board temperature closer to the solder temperature. This step provides two benefits: less time to get the entire board above solder temperature in the solder pot, reducing exposure time; and second, it helps reduce temperature shock on the board. If preheat temperatures are set too high, solder mask damage can occur, but also soldering will suffer as the residual heat burns off flux components before the board enters the solder pot.
Fluxing is a critical step in the HASL process. Picking the right flux for the shop is critical. High-viscosity fluxes (300 to 450 centipoises) can provide better leveling, leaving thicker, more uniform solder but will not work well for fine-pitch surface-mount pads. The thicker the flux, the lower the ability to wet smaller copper features, especially solder mask defined features. Low viscosity fluxes, 60 to 70 centipoises for horizontal machines and 70 to 150 centipoises for vertical machines, will do a better job of wetting fine copper features but will not offer the solder mask protection or leveling characteristics of a thicker flux. Flux selection also plays a part in machine maintenance. Flux built from poorly chosen raw materials can lead to more solder splatter, carbon build-up, poor ionic cleanliness and foaming in the rinse water.
Solder application and leveling are next in the process. In a vertical system, the board is submerged in a solder pot, and time is allowed for heat to be absorbed. The board is then extracted. Upon extraction, air knives blow hot air at a very low contact angle, nearly perpendicular to travel. The air knives are offset in such a way that one will blow solder flat on surface-mount pads while it pushes solder through the holes. The second knife is then designed to level pads and remove solder that has been blown out of the holes. Vertical machines, by nature, do not provide vigorous solder impingement across the board uniformly, which can result in exposed copper or high copper dissolution. Horizontal machines use rollers to dam solder and transport boards in a linear fashion. This provides for more uniform and even solder impingement across the board. Directly exiting the solder damming rollers, the board then passes through the air knives much like a vertical machine, but with the added benefit of the board laying flat so solder wicks back more readily for more uniform pad coverage. After leveling, boards are allowed to cool momentarily before final wash and dry.
This was a very precursory explanation of the HASL process. Much more could be written about the intricacies of the process and failure mechanisms, but I will leave that for another time. PCD&F
Jim Kenny is global product line manager for Enthone Inc., a business of Cookson Electronics. He can be reached at
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