These surface finishes are important to Pb-free board fabrication because conductivity of immersion silver and immersion gold is considerably higher compared to tin lead used for eutectic soldering. These finishes withstand higher temperatures, and there is less likelihood of the pads being peeled off from the board surface when it is exposed multiple times to higher temperature cycles.
These surface finishes have several tradeoffs. Those are shelf life, cost, reflow cycles, and solder joint flatness. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost five to 10 percent extra, depending on the amount of exposed surfaces. Therefore, it is best from a cost/performance point of view to select a finish that can be cost justified in a particular end product.
As for shelf life, HASL has about an 18 months, but OSP has only six months. Immersion silver has a shelf life ranging from 12 to 16 months, while immersion gold is the most durable at 24 months.
OSP cannot undergo more than two to three reflow cycles. If the need for rework continues, then the SMT pads on the OSP finish begins peeling off. Thus, OSP is not the best finish for rework. On the other hand, immersion silver or gold can undergo six to eight reflow cycles.
Using immersion gold or immersion silver comes with an added advantage, which results in a flatter PCB surface finish. Consequently, the flatter PCB surface finish is considerably more conducive to a perfect PCB assembly compared to HASL finish.