Pb alloy reflow temperatures and component integrity

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What does the reflow profile look like , the question I had... - Aug 22, 2000 by Mike Naddra

If the thermal mass of the products is small, a tent-shaped... - Aug 22, 2000 by Dr. Ning-Cheng Lee

Mike Naddra

#6312

Pb alloy reflow temperatures and component integrity | 22 August, 2000

What does the reflow profile look like , the question I had was surrounding the higher reflow temperature and the max temperature that lowest thermal mass components will see. Some other questions that I had were surrounding the components , what of the sn/pb flash on the terninatios of the components manufactured today, how does the leaching of the lead into the joint affect the intermetallic bond. And finally about the boards, do you recommend an OSP, whit tin or will we be sticking with HASL, and if we continue with HASL is the solder leveling accomplished with the pb free solder or with sn/pb solder.

Thanks, Mike

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Dr. Ning-Cheng Lee

#6313

Re: Pb alloy reflow temperatures and component integrity | 22 August, 2000

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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