The lead-free debate continues.
When the European Union adopted the Restriction of Hazardous Substances Directive (RoHS) in 2003, it had the best of intentions: to reduce health risks to consumers and manufacturing employees by eliminating hazardous materials in electronic components. There have been encouraging developments toward finding suitable lead-free solder alloys for use in assembling circuit board components, but critics say the EU has introduced a new risk because these new alloys may not be as reliable as tin-lead solder.
Why the difference of opinion about the reliability of the solder joints of lead-free components? Werner Engelmaier, president of Engelmaier Associates, L.C., an electronics hardware reliability consulting firm based in Ormond Beach, Fla., says part of the problem is that lead-free solder alloys are so new compared to tin-lead, which has been used for decades. Another part is the conflicting results obtained in diverse accelerated tests.
Much of what is known about lead-free solder reliability has been established through accelerated testing, which Engelmaier says doesn’t tell as complete a story as field usage. “Nobody really has any knowledge about the reliability of the lead-free solders in long-term usage because they are so new and there's no history. There are no established acceleration models from which acceleration factors can be established,” he says.
One known problem with lead-free solders is their susceptibility to failure under high stresses or under sudden shock. These issues manifest themselves differently during accelerated testing than can be expected in actual operational environments. One of the reasons are high modulii of elasticity for SAC solders; stiffer materials transmit higher stress levels for the same thermal expansion mismatch than do SnPb solders. Another is the higher Solidus temperature of SAC solders with consequences to the delta-T’s, the room temperature and to the PCB resin glass transition temperature are larger. The results have been solder joint interfacial separations as well as PCB pad cratering, Engelmaier says.
Dr. Gregory Munie, conference director for IPC Printed Circuits Expo, APEX and the Designers Summit, has worked closely with the IPC Solder Product Value Council on lead-free reliability testing. Munie says that while lead-free solders do indeed have a tendency to fracture under intense stress, they perform well in everyday situations. “It appears from my perspective that right now, based on published studies I have seen, for long-term, non-stressed, everyday operations, leaded and unleaded SAC [tin-silver-copper] alloys perform about the same in reliability.” However, the EU excluded high-reliability applications like aerospace and the military from the RoHS rules.
Since lead-free alloys also have a much higher melting point than tin-lead solders, higher temperatures are needed to properly wet lead-free solders. This can damage standard circuit boards and components, making the question of reliability more complex than just the solder joint reliability. “The biggest problem we have with lead-free soldering is not the solder joint reliability, but the printed circuit board's reliability,” Engelmaier says. “The printed circuit boards that we use are simply incapable of surviving the temperatures that they now have to see in order to get the SAC soldering done.”
Engelmaier says comparisons between SAC alloys and tin-lead, using the same accelerated tests, also are misleading. “In side-by-side accelerated tests, it is an apple/orange comparison because lead-free solders creep so much slower,” he says.
But does that translate to a need for a better test? “Maybe and maybe not,” Munie says. “And it's probably worthwhile as an industry to take a look at that and continue to explore it and find out if it really does mean that we need a different protocol for testing.”
Such questions could pose significant hurdles for the industry as it incorporates lead-free solder. But as Jean-Paul Clech, president of the Montclair, N.J.-based Electronics Packaging Solutions International, a reliability consulting firm, points out, “We are still at the beginning of the learning curve.” He notes that other new technologies such as ball grid arrays were doubted at first too, but are now widely used. It’s just a matter of time before the industry fully understands lead-free solder alloys and the new processes to use them in assembly. “Reliability has to be built into the design,” he says. “The new alloys have potential and solder joints can be made just as reliable, provided the designers and assemblers do their homework.”
Want to learn more? Lead-free reliability is a focus of a number of courses at IPC Printed Circuits Expo, APEX and the Designers Summit. In addition, the free forum “Lead-Free Reliability―Where Are We Now?,” will take place on April 2. Find out more at www.GoIPCShows.org.
