PCB manufacturing is divisible into two primary stages: fabrication and assembly. The surface finish process inextricably links them together, ensuring sufficient bonding between the bare board and the components for continuous performance and extended reliability. PCB surface finishes come in many flavors, but HASL is the most prominent for non-consumer devices: as an extension of the solder material (and thus, the solder processes), it is readily integrable for all board materials that are already using the same solder material to bond components to the bare board. This advantage holds for both leaded and non-leaded solders making HASL a highly applicable solder finish that is indispensable for PCB production.
Weighing Lead and Lead-Free Solders for HASL
|+ Eutectic tin-lead alloy offers unparalleled performance
+ Lower processing temperature reduces costs, energy expenditure, and constraints on IC design
+ Highly reliable and well-characterized, an excellent choice for high-Class boards
– Lead is a toxic heavy metal, and its performance benefits do not outweigh environmental pollution concerns for most devices
– Because of industry realignment due to RoHS, many of the advantages over lead-free soldering have been minimized or eliminated
+ Far more environmentally friendly solder alloys
+ Concerns over processing conditions are mostly unfounded; the industry has adapted to other constraints (e.g., higher temperature requirements) with little disruption
+ Performs better in a few areas than lead-containing solders, namely a higher surface tension which produces better pad planarity
– Ultimately, all lead-free solders are chasing tin-lead as the gold standard; material properties involve some tradeoffs from ideal
– While most IC designs and materials have successfully assimilated lead-free processes, some restrictions remain that may require additional manufacturing steps (i.e., manual soldering)
The Logistics of HASL in Manufacturing
The HASL process is relatively straightforward: after the necessary contact time with the molten solder, the panel passes through planar-aligned air knives, which blow away excess solder with air jets at temperatures above the solder’s melting point. After solidification, the panel undergoes a wash to remove leftover flux, followed by a bake to dry. The HASL process is performable with a vertical or horizontal system, depending on the method of solder deposition: a molten solder bath for the vertical alignment or a nozzle/roller configuration that dispenses solder to the top and bottom of the board simultaneously. The horizontal process has a few advantages:
- As an in-line process, it can easily integrate with existing equipment like pre-etch, washing, and drying. Integrating multiple processes within a single in-line system allows for simultaneous pre-heating of the solder, which reduces processing time and provides a corresponding increase in production throughput.
- There is greater uniformity in the solder finish, as the effects of gravity are constant for each side of the board.
- In general, an in-line system reduces the amount of human contact with panels, reducing defects related to mishandling while increasing throughput.
Inspection and Troubleshooting of Common HASL Errors
The foremost advantage of HASL to manufacturers is that solder processes are already so well-characterized and integrated into PCB production that there is little difficulty translating this to a surface finish. There is no need for X-ray fluorescence or destructive cross-section analysis, as a simple visual inspection will suffice. A HASL finish should appear silver and lustrous – just like a solder joint – with no visible evidence of partial or incomplete solder wetting. The presence of an uneven HASL finish typically indicates a few potential process faults:
- Thinned coating – During application, there was insufficient solder to provide complete coverage of the pad surface.
- Incorrect process parameters – Some combination of the solder flux, the solder temperature, or the time the panel underwent the HASL process resulted in a poor finish outcome.
- Excessive solder removal – After the air knives remove the bulk of solder, the residual solder remaining cannot provide complete coverage of the pads after accounting for surface tension.
Process Control for HASL Surface Finishes
A deficient HASL finish exposes the intermetallic to the atmosphere when it usually sits between the solder and copper pad layers. Although the presence of the intermetallic indicates the wettability of the process, atmospheric exposure gravely impacts the solderability of the finish and should be considered grounds for rework. For this reason, HASL process control must ensure both an even coating and a minimum growth of the intermetallic layer to maximize the distance between the surface solder layer. Since there is a tendency for the exposed copper to dissolve into the molten solder, process engineers and technicians will want to monitor the bath for the intermetallic that precipitates out of the solution and falls to the bottom of the pot due to its density. It’s possible to alleviate this condition by dredging the precipitate or slightly increasing the temperature and copper content of the solder bath to prevent panel copper dissolution and intermetallic precipitation, according to Le Chatelier’s principle.
Comparing Lead vs. Lead-Free Solders Finish Outcomes
Today, HASL is one of the more popular surface finishes available, but things did not appear that way in the early wake of the Restriction of Hazardous Substances (RoHS) directive. While HASL is renowned for its wetting ability and overall robustness with tinned solder, concerns arose (and, to a point, are still present) over process alternations due to the differences in lead-free solders’ material properties. These include:
- Board materials accumulate more thermal stress because non-lead HASL requires greater temperatures to reach its melting point.
- Coplanarity issues owing to the lead-free solders’ higher surface tension.
- Paste extrusion during stenciling due to variations in solder coating thickness on pads.
Fortunately, careful surface finish process control can prevent these challenges or any associated defects. Laminate damage is avoidable by limiting panel exposure to a maximum of two HASL passes alongside more heat-tolerant materials. The matter of coating thickness and coplanarity resolves with an adjustment to the total amount of solder dispensed: roughly half that of eutectic tin-lead solder (this also moderates the per-kilogram cost increase of the lead-free solders). Additionally, the primary benefit of the HASL finish is still available with lead-free solders: the intermetallic Cu6Sn5 that forms at the boundary between the copper pad and the tin-containing solder. Not only does this characteristic provide the reliability HASL is well known for, but it also offers exceptional preservation of pad solderability without additional storage or environmental considerations.
Finish Your Design Strong with an Experienced Contract Manufacturer
HASL has drawbacks, but it is a worthy consideration for nearly any assembly as an extension of an already thoroughly well-characterized PCB manufacturing process. Despite some initial trepidation around the required lead-free solder with the RoHS directive, most concerns have fallen by the wayside with improved manufacturing experience and material parameters. However, surface finish selection is an involved process with many criteria to weigh for optimal assembly outcomes. Here at VSE, we’re a team of engineers committed to building electronics for our customers, including a full review and analysis of designs for optimal process suitability. Over forty years alongside our valued manufacturing partners, we’ve produced PCBAs for various life-saving and life-changing applications.