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June 20, 2012

BGAs Mixing Tin-Lead and Lead-Free

We know about the concerns when soldering BGA components with SAC solder balls using tin-lead solder paste. My question is what issues may we have when we solder BGA components that have tin-lead solder balls using SAC solder paste? Will the joints have reliability issues?

W.W.

Experts Comments

In either case, if full mixing of the alloys is ensured by the reflow profile, reliability of the resultant joints is not significantly degraded. With SnPb balls and SAC solder paste, you'll need to reach full SAC soldering temperatures, assuming that the remainder of the SMT on the board is being soldered with the SAC alloy. This is actually a better scenario than trying to ensure full mixing within the constraints of a cooler SnPb profile.

Because the BGAs are SnPb, there is a concern that the package materials may not be designed to withstand the stress of a SAC profile. The component manufacturer should be able to tell you whether they are or not. These days, many SnPb packages use Pb-free compatible packaging materials because they have Pb-free equivalents (same package, different finish or ball alloy).
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
The reflow temperature of the Lead-Free process will be high enough to melt the Tin/lead alloy and the Tin from both alloys will diffuse into each other. Whether or not the diffusion is complete is an issue to address and monitor. This is accomplished with micro-sections to verify the metallurgy of the joint. The other problem is the temperature excursion of the components which have tin/lead plated leads. The question to ask is: Have these components been qualified at the higher temperatures typically used for Lead-Free alloys? If the BGA components are not qualified to be exposed to these high temperature other issues will come up which will impact the reliability of the product.  

The other element to address is the RoHS issue relative to having lead in the solder joint due to the Sn/Pb solder balls on the BGA, and these products will not meet the requirements of lead-free directive as they are 37 or 40% by weight of lead.  

Bottom line is to check for the RoHS requirements and have the BGA solder joints microsectioned to conduct a metallurgical evaluation of the grain structure of the resultant alloy.
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
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