Don,
It would be nice to think of this as an isolated incidence, but it happens
waaaaay to often. Could you possibly share the pictures of the "whiskers"
and dendrites you saw to go with this story?
Apparently, a number of my colleagues ran around the Apex meetings signing
me up for all kinds of handbook writing, and this story would be a good one
to use with pictures, properly sanitized of course. Would also be good
material for the Process Effects Handbook.
Doug Pauls
Donald Kyle
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For some of you, the following will be a little humorous and to others
scary.
Last month one of our CMs called me and asked if I knew of an engineer (who
will remain anonymous to protect the guilty) that works in my building. I
said yes, why do you ask? Well, he says, she sent me some pictures of three
capacitors that had what looked like whiskers. She was claiming that these
whiskers caused a short on her board. (Of course implying that the whiskers
had something to do with the board assembly.)
Looking at the pictures I was amazed at the growth of whiskers on the
capacitors. It looked like something from a 1950s sci-fi movie. So I asked
if an analysis of the board had been done. Yes, she replied; it is
contaminated with chlorides. Well, looking at the SEM report there was
something very strange. Not only was there chlorine on the board, there was
a witch's brew (no pun intended) of elements present.
Na (sodium), Mg (magnesium), Al (aluminum), Si (silicon), P (phosphorus), S
(sulfur), Cl (chlorine), Ti (titanium), Sn (tin), N (nitrogen), O (oxygen),
Zn (zinc), Pb (lead)
After reading the report, I needed to see the board for my self. Wow, the
first thing that I saw was a visible dendrite TREE that was about an inch
and a half long (~38 mm for the rest of the world) headed for ground.
Under the microscope I found evidence of oxidation and etching of the
solder joints and leads on some I.C.s and passives. The board was a
prototype and as such there had been many wires and components soldered on
and off since the board was received three months ago. There was evidence
of flux on the board that was dried but the Alpha 711 flux that we use
couldn't etch away copper.
I needed to figure out what was this contamination and how it occurred. I
had noticed a few weeks ago that a tech was using a grinder in the lab.
This would probably account for the aluminum, silicon, titanium and
magnesium. And maybe the sodium was from someone's snack. But the chlorine
and zinc were puzzling.
I was back at my desk when one of the techs came by with an almost empty
bottle of Eutecsol 682 flux to show me. Yes, what's your point? He said
that this bottle of flux was full last year and that the engineer that
bought it had never used any. Really, what is it used for? He said, it's
for soldering to stainless steels. Wow!
I looked up the MSDS and found that the flux contained the following.
Ethylene Glycol
Zinc Chloride
Hydrochloric Acid
Ammonium Chloride
Well now, that explains it. Although no one would admit to it, it seems
that someone ran out of the Alpha 711 flux and finding the Eutecsol 682
discovered that it cleaned the solder pads much better and faster. Not
wanting to settle for the slow reacting Alpha 711, Mister-Nobody filled the
empty 711 flux bottles with Eutecsol 682.
I was thinking, had the whiskers not grown across and shorted the
capacitors the failure mode might have been the disappearance of the
circuit board. (Heee)
Donald Kyle C.I.D.+
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