It is hard to believe that in July we will celebrate the 9th anniversary of the advent of RoHS. So the timing seemed right when I was recently asked to speak at the Boston SMTA Chapter on The Status of Lead-Free 2015: A Perspective.
An overview of the entire 75-minute presentation would be a bit long, so I am going to discuss three of the “questions” that I covered.
- Q: We are now almost nine years into RoHS’s ban on lead in solder. How has lead-free assembly worked out?
A: Something over $7 trillion of electronics have been produced since RoHS came into force, with no major reliability problems. One senior person, whose company has sold hundreds of millions of lead-free devices since 2001, reports no change in field reliability. The challenge that implementing lead-free assembly placed on the industry should not be minimized, however. Tens of billions of dollars were spent in the conversion. In addition, failure modes have occurred that were not common in tin-lead assembly, such as the head-in-pillow and graping defects. But assemblers have worked hard with their suppliers to make lead-free assembly close to a non-issue. Some people ask how I can say that lead-free assembly is close to a non-issue. My office is across the hall from some folks that purchase millions of dollars of electronics a year for Dartmouth. Several years ago, I asked them how they feel that electronics perform since the switch to lead-free. They answered by saying “What is lead-free?” If people that buy millions of dollars of electronics have not even heard of lead-free it can’t be a big issue.
- Q: In light of sourcing difficulties, is there an industry consensus regarding lead-free conversion for military, medical, aerospace etc. assemblers that will continue to be exempt?
A: The main issue is getting components with tin-lead leads, especially BGA balls. Many assemblers are reballing BGAs, which has become a mature technology, although with an added cost. As years go by and there becomes more confidence in medium to long term lead-free reliability, some exemptees may switch to lead-free. However, I think mission critical applications with 40-year reliability requirements must be extremely cautious to make the switch. There may be subtle reliability issues that may show up in 40 years, that are not found in accelerated testing. One concern is aging. Even at room temperature, solders are at over 50% of their melting temperature on the absolute scale (300K/573K = 0.52). So aging can occur at room temperature. Some research suggests that lead-free alloys may be more affected by aging than tin-lead alloys.
- Q: It has been said that you claim that lead-free assembly has some advantages. Can this be true?
A: Guilty as charged. Lead-free solder does not flow and spread as well as tin-lead solder. This property can result in poor hole fill in wave soldering and some other assembly challenges. However, this poor wetting and spreading means that pads can be spaced closer on a PWB without the concern of shorting as seen in the image below. Your mobile phone would likely be bigger if assembled with tin-lead solder.Lead-free solder does not flow as well as tin-lead solder. Hence, closer pad spacings are possible.
Photo courtesy of Vahid Goudarzi.