BANNOCKBURN, IL – A group of leading vendors of solder today released its final results from a lengthy investigation on lead-free alloys, finding an alloy of Sn96.5Ag3.0Cu0.5 to be the best candidate to replace tin-lead solder.

The three-year study also baselined a series of test methods for determining lead-free solder performance.

The results were released today under the title “Final Report – Round Robin Testing and Analysis of Lead Free Solder Pastes with Alloys of Tin, Silver and Copper.”

The report is the third released by the Solder Products Value Council, a group of solder suppliers organized under IPC. Last November, the group published findings that “no significant difference” exists between a trio of heavily researched lead-free alloys.

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 tin-silver-copper alloys.

The majority of potential replacement alloys are composed of tin, silver and copper (commonly called SAC) alloys, the group found. Those alloys include Sn96.5Ag3.0Cu0.5, Sn95.5Ag3.8Cu0.7 and Sn95.5Ag4.0Cu0.5.

The lead-free pastes were previously found to result in less bridging but more voiding than a tin-lead eutectic control, the authors wrote in the November report.

The study looked at several test methods including DSC melt analysis, wetting balance, area of spread, visual inspection, x-ray void analysis, temperature cycle testing, thermal shock and metallurgical analysis.

The final report also includes summary findings on the effect of voiding on solder joint reliability.

Also released in the new report: findings on alloy characterization, down selection, an assembly processing parameter summary, assembly processing, voiding, metallographic analysis, and results of thermal shock and temperature cycling.

An executive summary can be downloaded at ipc.org.

“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,” said Roger Savage, president, Kester, in a statement.

The 16-page report released in November announced results of a down-select process of six tin-silver-copper solder pastes and subsequent PCB assembly comparing three of the alloys and a tin-lead solder paste.

In that phase, five solder manufacturers submitted SAC alloy samples for anonymous testing, which was conducted by an independent party.

Engent AAT,of Norcross, GA, tested one alloy per vendor using tin-lead eutectic paste as a control. Testing was performed on FR-4 boards using surface finishes of immersion silver, OSP, electroless nickel/immersion gold (ENIG) and bare copper and components with tin and tin-silver-copper finishes. Tests performed included: reproducibility of printed paste volume, solder paste wetting and spread, interconnect voiding.

Next, two major EMS companies, Flextronics and Solectron, built a total of 200 assemblies each, using the three SAC alloys and a tin-lead control. The companies then studied the three pastes for testability, rework and repair, handling and storage, printing, reflow and cleaning. There was “no significant difference” between the lead-free solders, they found. However, in general the tin-lead paste outperformed the lead-free alloys.

According to the report, “Submitted samples ... showed that, in general, the lead-free pastes were not as well-defined or as repeatable as tin-lead solder paste nor was their slump performance as good as tin-lead solder paste. This was most likely due to the use of a generic print process that was not optimized for each material.

The lead-free pastes also resulted in less bridging but more voiding than the control paste, the paper noted.

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