2005 Articles

After three years of trials, a group of leading solder vendors released in late June its final report from its investigation into lead-free alloys. Their verdict: An alloy of Sn96.5Ag3.0Cu0.5 is the best candidate to replace tin-lead solder.

Before the findings, a disclaimer. If there’s a shortcoming with the study, it’s that the only solders tested were SAC alloys – compositions of tin, silver and copper. Those alloys include Sn96.5Ag3.0Cu0.5, Sn95.5Ag3.8Cu0.7 and Sn95.5Ag4.0Cu0.5. However, the researchers, known as the Solder Products Value Council, reasoned that alloys with silver content of 3.0 or lower would perform on par at lower cost to higher silver content solders.

Let’s get to the findings.

• No statistically significant difference was found in alloy performance for melt temperature (DSC), time to zero, maximum force in wetting balance testing and solder spread as determined by area and diameter.
• No difference was found in assembly performance.
• No difference was found in process ability or defect rate (although tin-lead bested the SACs).
• No metallurgical difference was found after assembly and before thermal cycling.
• All three SAC alloys showed similar failure rates for similar packages, and performed consistent with the iNEMI Sn95.5Ag3.9Cu0.6 alloy, based on comparisons of studies.

One interesting byproduct of the study was the finding that based on thermal cycle interconnection failure data “there is no evidence that solder joint voiding has any significant impact on solder joint reliability.”

All in all, not bad. The document, “Final Report – Round Robin Testing and Analysis of Lead Free Solder Pastes with Alloys of Tin, Silver and Copper,” is the third released by the SPVC, a group of solder suppliers organized under IPC. A free executive summary can be downloaded at ipc.org. The full report is going to cost you, but as one of the SVPC members told me, you don’t have to buy it to understand that all the tested solders performed similarly.

Just who was behind this work? The SVPC is made up of 17 solder vendors who aim to achieve worldwide consensus on a standard lead-free alloy. The vendors contributed a reported $1 million in research time toward the three-phase study of SAC alloys.

The findings are consistent with the group’s previous work. In a 16-page report published last November, the group said that lead-free pastes were found to result in less bridging but more voiding than a tin-lead eutectic control.

This time around, it settled on SAC 305 as the optimal replacement due, as the authors wrote, to its “lower cost and equivalent performance” compared with the other SAC alloys.

The unknown variable is long-term field performance. That’s a huge wild card, with debate coming from every side. The latest to weigh in: The Department of Defense, whose policy-setters were holding a joint meeting in Philadelphia in mid-July to develop a cohesive plan.

Still, that shouldn’t undermine the impressive performance of the participants. As Kester president Roger Savage said upon the report’s release, “The final report of the round robin testing and analysis of the lead-free alloys is an excellent example of how companies, although fierce competitors in the marketplace, can come together and work for the benefit of the industry.” We wholeheartedly agree.

Final thoughts. Speaking of new, you may have noticed two additions to Circuits Assembly. Last month, we launched a column on wave soldering by one of the top names in that sector, Gerjan Diepstraten of Vitronics Soltec. And beginning this month the American Competitiveness Institute will author a monthly column on soldering and manufacturing. We are all lucky to have their contributions.

In July, the article “Optimizing Stencil Design For Lead-Free SMT Processing” included text from an outdated draft. The revised article is posted here. Also in July, the Better Manufacturing column showed a graph of projected component use. This was representative of a forecast from research conducted in 2000 and is not representative of the current state. We regret any confusion and apologize for any errors on our part.

I want to personally thank all those who attended the memorial service for our former colleague Ron Daniels in early July. Friends came from as far as Singapore to pay their respects, and the memories shared by Phil Zarrow, Laura Sims, Frances Stewart, Jim Raby and several others were truly moving.

As we went to press, bombs were going off on the streets of London. Our thoughts and prayers are with our friends in England at this tragic time.

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