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Issue #215 June 2008
It's All In The Whiskers
by Steve Ciarcia
Speaking as someone who has been on both sides of the fence and still intimately involved in electronics, let me just say that I certainly find it a lot easier to talk about embedded technology than wade through the muck and mire of compliance and certification that electronics manufacturers deal with today. When I used to own a manufacturing business (sold nine years ago), my one certification/testing nightmare with conductive spray coatings for EMI/RFI shielding convinced me that dealing with government edicts can be a real pain in the tush.
Twenty years ago, electronic manufacturing success simply involved a good product idea, some production samples, and lots of advertising luck. All it took were a few large production orders and yet another flourishing company was born. Unfortunately, for today’s entrepreneur, the global economy has changed both the dynamics of producing a product and the logistics of marketing it. The concept of a single entrepreneur personally soldering boards together for shipment to a customer in the next town and transitioning that U.S.-compliant product to a mass-produced item serving the global economy has gotten a whole lot more complicated. All the other countries have their fingers in the compliance-and-regulation soup now, too.
Presented in February 2003 by the European Union (EU) and mandatory as of July 2006 (China instituted a similar but stricter version in March 2007), suppliers of electronic materials have been making significant changes to their products to comply with the Restriction of Hazardous Substances (RoHS) Directive. The regulation restricts the use of six materials: cadmium (Cd), hexavalent chromium (Cr (VI)), lead (Pb), mercury (Hg), polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE). Basically, you can’t ship products to these countries that include more than a prescribed amount of these substances.
A few minutes on the search engines will tell you all the materials and products affected by these restrictions, but the most significant one affecting the embedded control community has to be lead solder—or, more importantly, lead-free solder. I have a bit of manufacturing experience, so when I say that arbitrarily replacing good old 60-40 tin-lead with lead-free solder has a whole lot of problems, believe it. Certainly, the regulations were conceived for environmental concerns, but there was a fundamental misconception that the electronics industry could easily switch from tin-lead solder chemistry to pure tin (or alloys of tin, silver, and copper) with the only difference between classic solder and lead-free solder being the melting point. Bad idea!
Replacing tin-lead with pure tin appears to have been a big mistake and the environmental motivation may have faulty logic. First of all, avoiding lead because it leaches into our environment may be true for paint and gasoline, but there is little evidence that the lead in solid form on PCBs migrates into the ground the same way. Also, the added resources necessary for increased tin, silver, and copper mining and smelting are a poor trade when compared to the fact that most lead solder is derived from recycled products.
Toxicity issues aside, the real problem with lead-free solder is that it just plain doesn’t work as good as tin-lead solder. Regardless of the multitude of substitute alloys that have been tested so far, current lead-free tin solder chemistry has this nasty propensity to spontaneously grow tin “whiskers” that can become long enough to short adjacent traces. Typically microscopic in size, some whiskers have been almost as long as a centimeter and, at 30 mA current capacity, disastrous to digital circuitry. If you think problems from whisker shorts are irrelevant or inconsequential, ask the Swiss watch company Swatch about the $1 billion they spent in 2006 recalling short-circuited watches. Or research what happens when a nuclear power plant shuts down because a steam pressure monitor shorts out, which is what happened at the Connecticut-based Millstone 2 nuclear power plant in April 2005. Lead-free solder is also considerably more brittle and therefore susceptible to fractures and open connections, too. As a company owner, these are certainly not “features” you want in a “parts and labor” warranted product that is shipped to the other side of the world.
To my knowledge, the U.S. hasn’t specifically outlawed the use of lead solder (a quick check over at www.jameco.com shows a variety of lead and lead-free solders available), but like BMW and Mercedes making cars that meet California emissions and U.S. crash test regulations, the desire to participate in the global market ultimately dictates product ingredients that meet everyone’s regulations. If you’ve bought a car, a TV, or a computer since 2006, regardless of nationality, it most assuredly uses lead-free solder. And, of course, we all got extended warranties because we expect a lower MTBF. ;-)
Seriously, the issues with lead-free solder are both a goal for future chemical research as well as a logical motivation for reassessing a bad regulation. The irony of the situation is that the European Union has already exempted military hardware and critical communications gear from using lead-free solder because they know it affects reliability. Even Swatch has received an exemption that allows it to go back to using lead solder.
The path for small businesses and entrepreneurs is a bit grayer. Certainly, developing a product that becomes a mass-market item is every engineer’s goal, but be careful that you design and manufacture it to meet the intended customer destination. To add insult to injury, if making the PCB isn’t enough aggravation, try shipping it to China. There, even the packing materials have to meet stringent RoHS certification. As for me, I’m just going to stay down in the Circuit Cellar making Sn-Pb soldered prototypes and not even think about all the directives, certifications, and regulations. ;-)
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