Pb-free Impact on Fabrication
Different surface materials with a higher temperature cycle range such as FR406 or FR408 are required for Pb-free assembly. Also, PCB surface finishes are different and depending on cost and board application, immersion silver or gold or OSP can be used to withstand the higher temperatures when the board goes through the reflow oven. Two considerations involved in preparing a bare board for Pb-free assembly are: board thickness and surface finish. If the PCB is thicker than the regular 62 mils, then a specific amount of solder and flux is needed. If it is even thicker than that, or for instance, 93 mil thick, then the board will require more flux activity to soak the pad. Plus, it will require more flux. Contact time, bump speed, and peak temperature must be increased. Lastly, a nitrogen reflow instead of regular hot air reflow may be considered to make boards more cosmetically pleasing.
As for surface finishes, hot-air solder leveling or HASL is used for eutectic boards, but is not conducive to Pb-free assembly. For Pb-free assembly, there are such PCB surface finishes as electroless nickel immersion gold or ENIG, immersion silver, organic solderability protectants (OSP), and a special Pb-free brand of HASL mostly used in ASIA to do Pb-free assembly.
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.