In either case, if full mixing of the alloys is ensured by thereflow profile, reliability of the resultant joints is not significantlydegraded. With SnPb balls and SAC solder paste, you'll need to reach full SACsoldering temperatures, assuming that the remainder of the SMT on the board isbeing soldered with the SAC alloy. This is actually a better scenario thantrying to ensure full mixing within the constraints of a cooler SnPb profile.
Because the BGAs areSnPb, there is a concern that the package materials may not be designed towithstand the stress of a SAC profile. The component manufacturer should beable to tell you whether they are or not. These days, many SnPb packages usePb-free compatible packaging materials because they have Pb-free equivalents(same package, different finish or ball alloy).
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 ofthe Lead-Free process will be high enough to melt the Tin/lead alloy and theTin from both alloys will diffuse into each other. Whether or not the diffusionis 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 thetemperature excursion of the components which have tin/lead plated leads. Thequestion to ask is: Have these components been qualified at the highertemperatures typically used for Lead-Free alloys? If the BGA components are notqualified to be exposed to these high temperature other issues will come upwhich will impact the reliability of the product.
The other element to addressis the RoHS issue relative to having lead in the solder joint due to the Sn/Pbsolder balls on the BGA, and these products will not meet the requirements oflead-free directive as they are 37 or 40% by weight of lead.
Bottomline is to check for the RoHS requirements and have the BGA solder jointsmicrosectioned to conduct a metallurgical evaluation of the grain structure ofthe resultant alloy.
Vice President, Technical Director
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.