As engineers, we know that quality is found in the rigors of a process. More often than not, this statement rings true.

In the case of HASL, however, there may be surface issues left behind that impact quality of the end product. In other words, the process itself may be hindering reliability. Let’s talk about that.

The commonly used method of HASL (Hot Air Solder Leveling) is designed to provide an inexpensive surface finish to exposed PCB metallization. First, the board is cleaned, and then flux is applied. The flux is intended to remove oxides and is often a glycol solution. Once that process is finished, the board is placed in a bath of molten solder. The excess solder is blown off with air knives, followed by another cleaning to remove flux residues, and the board is ready for the next step.

But what sounds like a solid process can become a problem in and of itself.

First, there may be levels of corrosive residue in the glycol flux itself. This particular flux can be moisture absorbing - which can be detrimental if they aren’t fully removed. To ensure flux is uniformly applied across the board, the full PCB is typically coated with rollers. Keep in mind this action forces the flux into all areas on the board surface. Some of these areas turn out to be places where you may not want moisture to reside. This part of the process can create surface residues that may lead to unintended consequences.

After fluxing, boards are usually dipped and submerged in a bath of molten solder. This exposes the PCBs to a significant thermal excursion—this can uncover potential delamination issues before components are attached, but may also lead to the production of internal layer damage/defects. If the thermal conditions cause flux to burn off, wetting may not be optimal.

Next, air knives are used to blow excess solder off the board. Variability in air knife pressure, flow, angle, etc. can result in poor surface finish quality (too thin, too thick, non-planar), or reduced diameter, or plugging of plated through holes.

Finally, the boards are washed to remove excess flux residues after the solder has solidified. Tap water is still commonly used for bare board cleaning. Consider the variability in tap water impurities that could be added to the surface simply by the water source used. This will vary from location to location, and potentially over time as well, depending on the upstream treatment regime.

In essence, HASL is a dirtier process than you might realize. But the process, when utilized with care and precision, can still lead to cleanliness and reliability with the inherent advantages of a low cost, reworkable surface finish with excellent shelf life.

Once again, we see that quality is found in the rigors of a process.