BY PHIL GARROU AND RON GEDNEY
With electronics components quickly making their way into landfills around the world, many countries are looking for ways to curb the amount of materials entering the environment. Ironically, the most obvious solution to reducing lead in electronics causes another problem in terms of reliability — tin whiskers.
In 2002, the EU enacted legislation known as the Restriction of certain Hazardous Substances (RoHS) and the Waste Electrical and Electronic Equipment (WEEE) Directives. With 2006 set as the deadline for equipment suppliers to eliminate the use of most lead containing components from their products, the search is on for reliable, cost-effective alternatives. Currently, lead (Pb) is typically mixed with tin (Sn) when applied as a coating or finish for terminals in electronic equipment. The simplest manufacturing substitute is pure or nearly pure tin, which has the distinct economic advantage of allowing manufacturers to use the same plating equipment they used for the older tin-lead terminal coatings.
Problem solved, you might think. Unfortunately, pure or almost pure tin tends to grow whiskers — crystalline filaments — that can grow several millimeters long and bridge adjacent terminals, causing system failure. The filaments can grow until they touch another conductor and cause a short, or can break lose and cause a short where they settle. They also can cause transient short circuits from metal vapor arcing in a low-pressure environment. Or, after breaking loose, they can impede the movement of mechanical parts. In other words, if whiskers grow to critical lengths in service, they could cause electrical shorts, disruption of moving parts, and/or degraded RF/high-speed performance.
The industries with the greatest concern for reliability, most notably the aerospace industry, are avoiding components with pure tin finishes. Consumer products have a less urgent need for long-term reliability and place a greater emphasis on cost control. Many other users will have variable levels of risk tolerance when it comes to reliability.
To date, the industry has not been able to develop an accelerated test that predicts the potential for tin whisker growth. The National Electronic Manufacturing Initiative (NEMI) has proposed three tests to the JEDEC Solid State Technology Association to encourage the industry to consolidate test conditions and build a database. NEMI testing to date shows that there are many variables that affect whisker growth, making the mechanism very difficult to model. In one experiment, two manufacturers with supposedly identical processes showed up to a 10:1 difference in whisker growth. Also, whiskers have an unpredictable dormancy period, typically 1,500 to 3,000 hours, making it all but impossible to create a safe burn-in period.
The discovery of the tin whiskers phenomenon is not new. Tin whiskers have been investigated by scientists and researchers for more than 5 decades. And tin isn't the only material known to cause whiskers. Zinc and cadmium also form whiskers. But despite decades of studies, researchers do not know what all of the causative factors are, or how they combine to cause whisker growth.
In 2001, NEMI formed a project to attempt to model the tin whisker phenomenon, building on available research and applying new tools. A primary cause seems to be compressive stress in the tin film, but that is itself the product of several different factors, including plating chemistry, diffusion of other metals into the tin finish from its substrate, mechanical pressure from forming the terminals, damage to the surface of the plating, and variations in thermal expansion due to different substrates. “Matte” tin finishes are preferred over “bright” tin finishes, which appear to be especially prone to whiskers. An obvious strategy is to avoid parts with 100% tin finishes, but that is becoming harder to do as coating compositions change, and it will add to cost. Another strategy is to use nonconductive barriers, such as epoxies, to block whisker growth.
The pressure for progress on mitigation strategies sparked the 2002 formation of a NEMI Tin Whisker User Group. The group is composed of 11 large manufacturers of electronic assemblies. In May 2004, the group proposed tin whisker test and mitigation requirements for acceptance of devices with tin finishes. NEMI also held a tin whisker workshop at this year's Electronic Components Technology Conference (co-sponsored by the IEEE CPMT Society).
Once more data, and perhaps technical understanding of whisker formation becomes available about this enigmatic phenomenon, solutions that satisfy both environmental and reliability needs can then be implemented. Until then, the research and dialogue continues.
PHIL GARROU, Ph.D, is the current IEEE CPMT Society president and a program consultant for the MCNC R&D Institute, and may be contacted at (919) 248-9261; e-mail: [email protected] RON GEDNEY, staff consultant, NEMI, may be contacted 70 Lake Fairgreen Circle, New Smyrna Beach, FL 32168; (386) 478-1204; e-mail: [email protected]