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66 DESIGN007 MAGAZINE I JANUARY 2023 Lead-free HASL is considered "RoHS" com- pliant (< 0.1% BW of finish, for Pb, Hg or Cd), while SnPb (tin/lead) HASL is not. HASL is available in two versions: a tin/lead alloy and a lead-free option. HASL is a vari- able thickness metal alloy coating, typically 25 to 2000 µin [0.65 to 50 µm] thick, designed to create a solderable surface and to maintain solderability over an extended period. Using specialized equipment, panels with exposed copper (pads and/or traces) are inserted into a vat of molten solder. A series of compressed hot-air knives then remove excess solder and "level" the surface before the solder solidifies and cools. HASL is the most prevalent and most solderable finish. Boards that have been HASL leveled will have a bright, silvery pad coloration. Solder thickness and uniformity on SMD pads is a function of pad size, pad orienta- tion during processing, and HASL equipment. As pad size increases, thickness and unifor- mity decrease. is is caused by the high sur- face tension of solder and the airflow over pads that remove excess solder. Small pads (0.025" pitch SMD) will have a more uniform solder coating than large pads (0.050" x 0.050"). Large pads may be partially covered with a mound of solder while the rest of the pad is thin eutec- tic solder. Small pads may have the same con- dition depending on pad orientation during processing. Pad orientation also contributes to solder thickness and uniformity. Fine-pitch SMD pads perpendicular to the direction of process- ing will have a uniform solder coating covering the entire pad. Fine-pitch SMD pads that are parallel to the direction of processing will have the leading half reduced to the thin eutectic solder with a mound of solder on the trailing half (eutectic solder is the optimum metal alloy to maximize solderability). Angled process- ing exposes both pad directions to similar air blasts resulting in more consistent pad thick- ness for similar pad sizes. 45-degree angle pro-