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SMT & Inspection | July 05, 2005

What's industry's best lead free solder?

Imagine how much easier the transition to lead free would be if manufacturers didn't have to research which solder to choose. Now, thanks to the IPC Solder Products Value Council's (SPVC's) lead free testing program, that research has been done.
For three years, the IPC SPVC invested the equivalent of one million dollars in volunteer time, equipment and supplies to come up with the answer: "Final Report - Round Robin Testing and Analysis of Lead Free Solder Pastes with Alloys of Tin, Silver and Copper."

"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," says Roger Savage, president, Kester, and chair of the IPC SPVC.

The final report has a long title but answers two key questions. What alloys will most likely be used as tin lead solder replacement? What tests can accurately determine the differences (if any) in the properties of the most likely candidates? The Council determined the majority of potential "standard" replacement alloys are composed of tin, silver and copper (commonly called SAC) alloys and analyzed the three most commonly used lead free alloys:

* 96.5/3.0/0.5 tin/silver copper
* 95.5/3.8/0.7 tin/silver/copper
* 95.5/4.0/0.5 tin/silver/copper


Test methods used in the research included DSC melt analysis; wetting balance; area of spread; visual inspection of solder joints; X-ray analysis of solder joint voids; temperature cycle testing; thermal shock testing and metallurgical analysis.

The data contained in the 50-page final report supports the Council's recommendation of 96.5/3.0/0.5 tin/silver/copper (SAC305) as the alloy of choice for lead free applications. The final report also includes summary findings on the effect of voiding on solder joint reliability. Although the research project was not designed as a head to head comparison of lead free versus tin lead solder, because tin lead solder was used as a control, the report does highlight solder joint performance based on package type.

In addition to the final report, 18 appendices will be included as separate files on CD. These appendices, with nearly 60 megabytes of data, cover such topics as alloy characterization, down select data, an assembly processing parameter summary, test vehicle description, assembly process data, void data, metallographic analysis of 500 cycles up to and including 6000 cycles, and results of thermal shock and temperature cycling.

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