Just because the official RoHS compliance deadline has now come and gone doesn’t mean every manufacturer has switched to a Pb-free process. In fact, many haven’t – and won’t – make that switch anytime soon, if ever. RoHS legislation exemptions are many: products for automotive, aerospace, medical. Consequently, SnPb manufacturing is alive and well. Or is it?

Though many manufacturers are still using SnPb, their suppliers are not. PCB and component suppliers have completely transitioned or are quickly migrating their products to Pb-free. Sometimes changes in plating are not identified; boards may have different finishes on the same PCB, and components may be supplied with a diversity of lead finishes such as pure tin, SnBi or AuPdNi. Because SnPb-finished board and component availability is dwindling – and quickly – those firms still using SnPb processes have no choice but to use Pb-free devices within their otherwise SnPb manufacturing operations. Attempting to reflow a Pb-free device with a SnPb optimized profile simply may not work. The result can be incomplete wetting, which causes defects such as voiding and opens. Conversely, trying to force SnPb solder paste through a higher temperature profile isn’t ideal either, as the elevated temperatures exceed the capability of most SnPb pastes; again, incomplete wetting is the result.

With standard leaded devices, the most common issue with mixed metal manufacturing is improper wetting. But much more catastrophic problems exist when using Pb-free bumped array devices within a SnPb process. SnPb solder pastes achieve liquidus at 183°C, but the Pb-free SAC alloys used to bump array devices most often require a temperature of 217°C. If the Pb-free BGA and CSP bumps are processed at lower, SnPb temperatures, incomplete bump melting or collapsing occurs. Ideally, the goal is to promote even distribution of lead from the SnPb paste throughout the solder joint, but if the Pb-free bump doesn’t melt completely, achieving this condition is impossible. Without the lead evenly dispersed within each solder joint, premature joint cracking and early thermal fatigue are the most likely outcomes, yielding uncertain device reliability.¹ To account for this, some manufacturers have attempted to raise reflow temperatures to encourage complete bump melting and the lead distribution required. This method is problematic, too, as the elevated temperatures may cause dewetting on other solder joints on the assembly.

So, what’s the solution? Basically, there are two options: The first, and definitely less-reliable approach, is to re-bump the Pb-free BGA. To do this involves removing all the Pb-free material – including the bumps and the intermetallic – from the device and then re-bumping them with SnPb solder. This method is rife with problems: The package has to go through another intensive heat cycle, which can lead to cracking and delamination within the package and, thus, earlier device failure. And, because any device altering procedures violate the component manufacturer’s warranty, you’re basically out of luck should there be any subsequent problems with the component. As if the reliability concerns weren’t enough, re-bumping a device is also a difficult, expensive and time-consuming process.

The second and better option is to find a solution that will work within an assembler’s current process and enable the use of a single paste for the entire assembly. The temperature has to be raised enough so that the SnPb/Pb-free mixture dissolves the lead into the solder joint and results in a homogeneous mixture and a higher reliability end-product. Some SnPb solder paste materials have been developed to help alleviate the mixed metals dilemma. Because these formulations are comprised of similar raw materials, activators and components used in Pb-free solder pastes, these materials effectively achieve Pb-free performance within a SnPb system. The extra activity and wider process window of the materials overcome challenges caused by the interaction of SnPb solder and Pb-free device finishes. Some of these pastes also offer a high resistance to humidity – another condition that is problematic and exacerbated in a SnPb/Pb-free mixed environment.

Without question, mixed metal manufacturing is here to stay, as the exemption deadline is uncertain, and the availability of SnPb finished PCBs and devices is continually reduced. Using a SnPb solder paste formulated to withstand higher Pb-free processing conditions is the most cost-effective approach to ensuring long-term reliability.

References

1. J. Pan, et al., IPC/Jedec Pb-free Conference, San Jose, 2006.

Renzhe Zhao, Ph.D., is a senior applications engineer at Henkel’s Electronics Group (henkel.com); renzhe.zhao@us.henkel.com.

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