HASL process has the inability to consistently control the amount of solder coating applied to each pad and trace. The HASL process results in dome shaped pads of various heights. With finer and finer pitch surface mount devices, flat, planar surfaces become necessary to achieve high speed, high yield SMD placement and reflow. This trend is driving the need for an alternative finish to HASL technology.

The thermal shock experienced by the pcb during the HASL process places strain and stress on the board. This strain or stress can lead to separation of the construction materials in a plated through-hole, causing a myriad of problems such as an electrical open. The thermal shock could also cause non-linear expansion of the PCB, causing board warpage and problems with component placement.

Extremely aggressive fluxes are used in the HASL process which contain high ionic levels and also contain high levels of hygroscopic polyglycols. If these contaminants are not properly cleaned away, they will reduce the surface insulation resistance of the PCB.

I assume you are inquiring about Pb-free HASL.

The biggest concerns are damage to the board and uniformity of thickness. In addition, certain designs, such as fine-pitch BGAs and CSPs and thick PCBs, can not be HASL’d.