An Engineer's Logical Disconnect

This passed by me on an email list and frankly I’m not shocked to see this particular logical disconnect. (Emphasis mine)

I once had a vacation canceled because all 150 of the high tech switching power supplies that had been 100% qualified at 50C for 24 hours were shipped to the integrator where all 150 failed spectacularly near simultaneously after 72 hours at 40C.

The fault was an opto isolator in the current feedback path. It was a an expensive full mil spec part specified for 5000V RMS AC, but carried no DC rating. At 500VDC …

This was a supply design that had been rigorously reviewed and subjected to extreme testing…

That is NOT a rigorous review when you don’t pay attention to the specifications for a part in your design! Sadly I’ve encountered these kinds of problems far too many times in the past 25 years. It seems to be a common problem that engineers will say they’ve made a careful review of a design and when I review the design I find they either failed to read or ignored a components specification. I’ve seen this exact mistake on switch applications where AC vs. DC operational differences are missed or ignored. IME, a more commonly ignored specification is component operating temperature range, it seems many people think that all electric components have no lower limit on operating temperature, RTFM!

RoHS and Tin Whiskers

The latest edition of The Embedded Muse newsletter from The Ganssle Group has a good summary with a few links about this topic.

The EU’s RoHS standards have caused a wholesale retreat from the use of lead in solder. While their intentions are noble, the electronics industry is likely to suffer mightily since alternative solders spontaneously grow tin whiskers that can,¬†and have, create short circuits.

This problem is actually a bit of a blessing for manufacturers with short warranties (< 2 years). It can force more rapid replacement/upgrade cycles yielding more sales dollars.

Back in the early 1990’s there was bill before the US Congress to force removal of lead from electronic solder in the US. The goal of the bill was to reduce lead exposure for children. The EIA and other organizations pointed out to the Congress that lead in children had long ago been traced to other sources, not electronics. The Congress rightly dropped the idea when they realized that lead from electronic circuits was only likely to get into children whose parents allow them to suck on electronic circuit boards. Now the EU has these RoHS regulations and if US manufactures want to sell to EU countries they are supposed to move to lead free solder.

I encourage every embedded systems engineer to head over and sign up for a free subscription to The Embedded Muse newsletter, a great source of embedded systems news and information delivered free to your inbox.

Lead free solder and tin whisker links:
NASA Goddard Tin Whisker Homepage
Roll back the lead-free initiative: 12 ROHS myths
THAT Corporation Links to Lead Free Information