Keith Armstrong article

The US Government’s National Highway Traffic Safety Agency (NHTSA) has been wanting to speak to me for some weeks, to discuss the EMI implications of Toyota’s spate of sudden unintended acceleration incidents. They said they wanted to speak to me because they had no-one on their staff with my experience or knowledge of EMI and EMC.

I imagine the fact that I’ve been presenting IEEE EMC Symposium papers on EMC and Functional Safety since 2001, including one addressed to the auto industry at a symposium in Detroit in 2008, played a part in their decision.

They also said they wanted to speak to my colleague, Dr Antony Anderson (www.antony-anderson.com), a forensic electrical engineer, because they had no-one with his knowledge or experience either.

I eventually spoke for over an hour with two of their senior officers, on February 2nd, while I was in Florida attending a Sudden Unintended Acceleration case as an Expert Witness against the Ford Motor Company (my first ever appearance in court, anywhere, for any reason!).

Unfortunately, a confidentiality agreement prevents me from describing what we discussed.

Armstrong paper

Lead-free soldering

Its really just another cause of intermittent or fixed short-or-open circuits in electronic PCBs and modules - but one that would not have been any problem until a few years ago, and so could have caught Toyota by surprise.

John R Barnes has created a monumentally huge library of references to the problems of lead-free soldering, especially tin whiskering, see www.dbicorporation.com/rohsbib.htm. Prepare to be totally overwhelmed!

Removing lead from solder has the following effects:

9.1 Tin whiskers

These will grow out of soldered joints and can contact other conductors, causing short-circuits between PCB copper traces and the pins of connectors. They are often no longer than 0.5mm (about 1/50th of an inch) but can grow to 1mm (about 1/24th of an inch) or longer, especially in damp conditions.

Even at 1/50th of an inch they can short between the pins on a modern integrated circuit (IC). And the process of removing the PCB for inspection can brush them off, so you never find them.

And if you didn't accidentally brush them off, they are so thin they are very hard to see - you need a powerful microscope. They are as fine as the finest spider-web threads, yet can carry sufficient current to short-out the electronics. You won’t see them unless you are looking for them.

Being so thin, they can wave around in the breeze and/or due to shocks, vibration and acceleration, causing intermittent short-circuits.

The iNEMI organisation has published guidelines (www.inemi.org) on how to ensure that tin whiskers don’t grow too long, but I don’t know to what extent these are followed by suppliers of electronics to the car industry in general, or Toyota in particular.

9.2 Brittle solder joints

Solder joints made using lead-free solder are less resilient, more brittle, more prone to cracking, especially in areas with large temperature excursions and vibration (e.g. motor vehicles), and especially under large devices such as BGAs – resulting in more open-circuits on PCBs.

These cracks may be invisible without X-Ray inspection, and will often be intermittent.

E.g. when the PCB is mounted in its box and mounted on the vehicle, it is bent slightly and that opens the connection or makes it subject to vibration. But when the PCB is taken out for inspection, it tests just fine.

9.3 Tin Pest

Lead-free tin tends to revert to a different crystalline structure, which means it becomes just grey dust and falls off or is blown away by a breeze. Obviously creates an open-circuit solder joint, and one that is easily spotted if it occurs.

9.4 Higher soldering temperatures can weaken components

Lead-free soldering has to use higher temperatures, and this increases the stresses on the components that are mounted on the board during the soldering process.

Another issue is that the range of temperatures for lead-free soldering is not only higher than for traditional solder, but it is also narrower. Some PCB-assemblers’ soldering equipment may not be able to maintain the temperature within the required bands at all times, causing an increased number of bad joints and/or heat-damaged components.

So the actual components (especially ICs) may test OK when newly-assembled, but fail or behave incorrectly when in use, due to internal changes that require an electron microscope to see.