All Printed Circuit Boards (PCB’s) have copper finishes on their surface. If they are left unprotected then the copper will oxidize and deteriorate, there are various protective finishes available. In this chapter, we will introduce the most popular surface finish in PCB production: HASL.
HASL means: Hot air solder leveling. It is the predominant surface finish used in the industry. The process consists of immersing circuit boards in a tin/lead alloy and the excess solder is then removed by ‘air knives’, which blow hot air across the surface of the board. For the circuit board manufacturer, the HASL process is expensive and maintenance intensive. Not to mention the increased health and safety hazards. For the printed circuit assembly (PCBA) process, HASL has many advantages. It is the cheapest PCB available and the surface finish remains solder able through multiple reflow, wash and storage cycles.
HASL has been working well for many years but with the advent of ‘greener’, more environmentally friendly processes, its days are numbered. In addition to the lead free movement, the increased complexity of boards and finer pitches has exposed many limitations with the HASL finish. If lead-free is not a concern, HASL is a very cost effective, reliable surface finish utilized in the manufacturing of lower technology PCBs. The HASL process can add stress to high layer circuit boards which can cause long-term reliability issues. This added stress, along with uneven solder height on dense SMT or BGA pads, are good reasons to replace HASL. Inevitably, tighter design criteria, advancing technologies, and/or environmental legislation will force the replacement of HASL.
Pro’s: Lowest cost PCB’s, remains solderable through whole manufacturing process, no negative effect at ICT
Con’s: Uses lead process which is currently restricted and eventually eliminated by 2007, for fine lead pitches (<0.64mm) can lead to solder bridging and thickness issues, unevenness of finish causes co planarity problems in assembly process.
What does it look like? PCBA’s test points and via holes will have solder covering.