If the thermal mass of the products is small, a tent-shaped profile is recommended, with peak temperature around 240C. The hottest spot on board should be less than 250C. However, if the thermal mass of the product is large, such as a large server PCB, the temperature gradient accross the board can be up to more than 30C. In this case, the hottest spot can be no lower than 260C, if 240C is still used as the average peak temperature.
Leaching of Pb from surface finish of lead into Pb-free solder joints will have negligible effect for systems such as SnAg or SnAgCu eutectic alloys. The problem is with Bi containing alloys, such as SnAgBi, SnBi, or SnAgCuBi systems. The ternary SnPbBi eutectic formed has a melting point around 96C, which will result in early failure in temperature cycling test.
As to the Pb-free surface finishes, the well established choices include (1) OSP, (2) Ni/immersion Au, (3) Ag. White tin is receiving more interest at this stage. Lead-free HASL is also one option. However, it is not good for fine-pitch applications, due to the variation in flatness from pad to pad. OSP may have problem with double-sided process, since the OSP may be burnt off during the first reflow.
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