In gathering information on the status of lead-free soldering, some helpful friends pointed out two great sources of information: NASA and the US Navy.
NASA sponsored an impressive lead-free reliability investigation: “Lead-Free Solder Testing for High Reliability Project 1.” This project is finished and the reports are online. A follow-on project, NASA DOD Lead-Free Electronics Project 2, is underway.
Regarding -20°C to +80°C thermal cycling, NASA concluded:
Under the conditions of this test, Sn3.9Ag0.6Cu (SAC) and Sn3.4Ag1.0Cu3.3Bi (SACB) were always more reliable than eutectic SnPb regardless of component type (CLCC, TSOP, BGA or TQFP).
It has been shown that conditions that highly stress the solder joints by maximizing the CTE difference between the PWB and the component will favor SnPb over SAC6. Conversely, conditions that minimize the stress put on the solder joints (e.g., compliant components such as BGA’s and/or a thermal cycle with a small delta T) will favor SAC over SnPb. The current test falls into the latter category and we can say with some confidence that the lead-free alloys tested will outperform eutectic SnPb under field conditions that are even less stressful than the -20 to +80°C thermal cycle test conditions.
For -55°C to +125°C thermal cycling, the conclusions were more cautious, likely because the data were mixed:
The feasibility of using Pbfree solder alloys in place of SnPb solder alloys for new product designs was demonstrated under thermal cycle test conditions. Additional investigation and characterization of Pbfree solder alloys will be required as a segment of a Pbfree solder alloy implementation plan. The application/introduction of Pb-free soldering processes for legacy product designs is not recommended without extensive materials characterization and product design review.
These results seem to be consistent with what others report: namely, lead-free assembly produces good thermal cycle results for commercial-type thermal cycling, but the results are mixed for harsh environment thermal cycling.
More to follow.