The July 1, 2006 deadline for RoHS implementation in the EU means different things to different companies. Many of the larger OEMs and CMs have been working at selecting materials and developing processes for several years. Within mid-sized companies there is more variation, some have just started, some have not even started figuring that they are exempt or donâ€™t have to worry about the ruling since they donâ€™t believe their products are going to Euorpe (this is equivalent to sticking your head in the sand â€“ RoHS and RoHS legislation is coming to other countries as well).
In either case, companies are now finding that they have to deal with mixed-metal systems. Where we get the most questions is over using Pb-free bumped area array devices with SnPb solder paste and process. Some early work reflowing these devices using SnPb conditions [D. Hillman, et al., â€œThe Impact of Reflowing a Pb free Solder Alloy Using a Tin/Lead Solder Alloy Reflow Profile on Solder Joint Integrity,â€ International Conference on Lead-free Soldering, CMAP, Toronto, Ontario, Canada, May 24-26, 2005, http://www.CMAP.org] showed failures as early as 200 thermal cycles. In his paper Mr. Hillman showed some great images of solder joint microstructure that varied within the solder joint, it wasnâ€™t homogenous as is common for most solder joints.
Jasbir Bath from Solectron presented some additional reliability data [Jasbir Bath, et al., â€œReliability Evaluation of Lead-free SnAgCu PBGA676 Components using Tin-Lead and Lead-free SnAgCu solder paste,â€ Proceedings of 2005 SMTA International, Chicago, IL, http://www.smta.org]. This paper shows that mixed metal systems, even when reflowed at Pb-free temperatures are not as reliable as Pb-free/Pb-free or SnPb/SnPb systems.
Some recent work by John Pan (San Luis Opispo) and Bath [Pan, et al., â€œLead-free Soldering Backward Compatibilityâ€, IPC/JEDEC Pb-free Conference, San Jose, 2006, http://www.ipc.org] calculated the melting point of the mixed metal system. The idea behind this study was to figure out what temperature was needed in order to get complete melting of the solder ball and paste and get complete mixing â€“ without having to go to Pb-free soldering temperatures. For example, a 1.27mm pitch BGA with a bump diameter of 0.028″ printed with a 0.005″ stencil (0.027″ round aperture) melts at 218C whereas a 0.5mm CSP with a bump diameter of 0.010″, printed with a 0.004″ stencil (0.011″ square aperture) melts at 206C.
Bottom line: Mixed SnPb and Pb-free solders is not a good idea. If you have no choice, make sure that you have a high enough temperature at the solder joint to permit complete melting and dissolution in order to get the best reliability. You also need to realize that this â€œbest reliabilityâ€ is not likely to be as good as â€œpureâ€ metal systems.