Tin-Silver-Copper (SAC) alloys with a composition of 95.5 Sn, 3.8 Ag, and 0.7 Cu are widely regarded as the most suitable choice in the near future for Pb-free soldering. The liquidus temperature of SAC alloys is 217 to 220ºC, which is about 34ºC above the melting point of eutectic SnPb. This higher melting range requires peak temperatures to achieve wetting and wicking to be in the range of 235 to 245ºC. Lower peak temperatures can be used with SAC solders such as 229ºC. By adding Cu to Sn/Ag, the melting point is reduced, wetting is improved, and long-term reliability increases. Also, SAC is not as sensitive to Pb contamination as Bismuth-containing alloys and can be used with SnPb-plated components with no major problems.
Tin-Zinc (91Sn 9Zn) and Tin-Bismuth (58Bi 42n) are two attractive alloys due to their respective low melting points of 199ºC and 138ºC. Tin-Zinc is ideal for soldering on to aluminum. It is attractive as a replacement for SnPb because constituent metals are inexpensive. However, Zinc reacts with both acids and bases, and as a result solder paste with this alloy has a very short shelf-life.
The low melting point of Tin-Bismuth makes it attractive since there is considerably less thermal shock to the assembly during reflow and less energy is required. But this is a brittle alloy with poor shear strength; it is also sensitive to Pb contamination.
Materials
Different surface materials with a higher temperature cycle range are required for Pb-free assembly. In this case, higher temperature FR4 material is required such as FR406 or FR408. 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.
There are two considerations involved in preparing a bare board for Pb-free assembly. One is board thickness, the other is the 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.
Surface Finishes
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.
The reason these surface finishes are so important to Pb-free board fabrication is 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 a very less likelihood of the pads being peeled off from the board surface when it is exposed multiple times to higher temperature cycles.
Costs
There are several tradeoffs involved with these surface finishes: shelf life, cost, reflow cycles, and solder joint flatness. Immersion silver and gold are expensive metal alloys. At production levels, these finishes could cost 5% - 10% extra depending on the amount of exposed surfaces. Therefore, it’s 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 is a flatter PCB surface finish. Consequently, the flatter PCB surface finish is considerably more conducive to a perfect PCB assembly than a HASL finish.
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