The small amounts of solder in a vehicle aren’t a major concern for most engineers, but a few specialists are paying close attention to solder. The auto industry’s exemption from the EU's RoHS (Restriction of Hazardous Substances) directive regulations is nearing its end, so lead-based solder won’t be allowed after 2013.
Lead-based solders were banned by RoHS regulations during the 1990s, and most industries have been using lead-free solder for years. But high-reliability markets such as autos and aircraft were granted exemptions because of concerns over long-term reliability.
Many of those questions have been answered for automakers who routinely use lead-free components, but some remain. One of the most vexing is tin whiskers, the name given to tendrils that sometimes grow from lead-free solders.
These metal tendrils don’t occur often, but they can cause short circuits, so their potential impact is high. There’s little understanding about which solder joints are susceptible, and tin whiskers growth can begin years after production, giving them a mysterious status.
“There are a lot of misconceptions about what tin whiskers are, what causes them, and what impact they have," said Sam Platt, Manufacturing Development Engineering Manager at Delphi Electronics and Safety. "The term evokes terror in electronics circles; it’s almost like bird flu or swine flu, something that’s going to wipe out the free world as we know it.”
Platt is among the many auto-industry engineers who have worked within industry groups such as IPC (the Association Connecting Electronic Industries) and iNEMI (International Electronics Manufacturing Initiative) that focus on circuit-board production issues. These trade associations’ technical conferences still have many sessions on the impacts of lead-free solder.
Automakers have already adapted to lead-free solders because many chips and passive components come only in lead-free packages. Delphi has shipped lead-free components since 1995 with very few problems.
“Overall, we found limited damage from tin whiskers. Our position is that the risk associated with tin components is very minimal,” Platt said.
However, he noted that Delphi has used lead-based solder on boards, and this solder covered the lead-free solder that coats most components’ contacts. That means that tin whiskers can still grow from the portion of a junction that is not covered by lead. When lead-based solders are eliminated, the entire connection will be unprotected, making it susceptible to whisker growth.
There are mitigation techniques that will prevent problems that arise with lead-free solder. One of the most popular is to use conformal coating.
“We push hard for purchasing to buy lead-free components that are conformally coated. If tin whiskers occur, the coating will keep them in place, preventing them from moving around and causing problems,” Platt said.
That approach has also been used in other industries such as aerospace that have been granted RoHS exemptions.
“Almost everyone agrees that conformal coating should be one of your mitigation tactics, said Dave Hillman, principal materials and process engineer at avionics supplier Rockwell Collins. "The coating captures and contains whiskers.”