BANNOCKBURN, IL, Nov. 3 - A group of leading vendors of solder today released white paper findings that "no significant difference" exists between a trio of heavily researched lead-free alloys. The lead-free pastes were found to result in less bridging but more voiding than a tin-lead eutectic control, the authors wrote.

In a 16-page report, the IPC Solder Products Value Council 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.

The SVPC is made up of 17 solder vendors who aim to achieve worldwide consensus on a standard lead-free alloy. The paper details the second of a three-phase, $1 million program testing tin-silver-copper alloys.

Five solder manufacturers submitted paste samples for the anonymous testing, which was conducted by an independent party. The pastes studied: • 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.

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 notes.

The final report is due in early 2005 and will include results and analysis of long-term and thermal-shock reliability testing.

"The SPVC is moving forward to complete this very detailed reliability testing program, in what we hope will be an alloy that industry can use with confidence," said Peter Palmer, SPVC chair and vice president of global marketing at Cookson Electronics.