One of the biggest challenges facing PCB designers is not understanding the cost drivers in the PCB manufacturing process. This article is the latest in a series that will discuss these cost drivers (from the PCB manufacturer's perspective) and the design decisions that will impact product reliability.

Final Finishes

Final finishes provide a surface for the component assembler to solder, wire bond, or conductively attach a component pad or lead to a pad, hole, or area of a PCB. The other use for a final finish is to provide a known contact resistance and life cycle for connectors, keys, or switches. The primary purpose of a final finish is to create electrical and thermal continuity with a surface of the PCB.

HASL (Hot Air Solder Leveling)

The next finish to discuss in this series is HASL. Choosing a surface finish means weighing the pros and cons associated with each, typically a combination of application, cost, and the properties of the finish. For example, Lead-free HASL is considered “RoHS” compliant (< 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 variable thickness metal alloy coating, typically 25 to 2000 µ in [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 orientation during processing, and HASL equipment. As pad size increases, thickness and uniformity decrease. This is caused by the high surface 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 eutectic solder. Small pads may have the same condition depending on pad orientation during processing.

Pad orientation also contributes to solder thickness and uniformity. Fine-pitch SMD pads perpendicular to the direction of processing 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 processing exposes both pad directions to similar air blasts resulting in more consistent pad thickness for similar pad sizes. 45-degree angle processing minimizes the pad orientation difference. HASL is not used for fine pitch components. This is because it increases fabrication and assembly rework for boards that have fine-pitch SMT devices with lead centers below 0.020" and presents coplanarity issues.

Pros and Cons of HASL

Pros

• Economical and widely available
• Excellent solderability
• Can be reworked at the fabricator
• Presents a very flat solder deposit

Cons

• Limited assembly reflow cycles, as the HASL process introduces a thermal shock before it gets to assembly
• Not suited for fine pitch (due to uneven surface height) or high aspect through-holes
• Standard SnPb HASL is not RoHS compliant
• Lead-free HASL requires different assembly soldering parameters

Understanding the cost drivers in PCB fabrication and early engagement between the designer and the fabricator are crucial elements that lead to cost-effective design success. Following your fabricator’s DFM guidelines is the first place to start.