TECH: Watch out for this! (Flux)
Alex Yeckley
found using Google. I figured this was worth sharing however:
I was recently working on a Battlezone AUX board which would
report an "L" error (sometimes "T") after about 10 seconds of heating
up. Freeze spray had no effect on the error. The interconnect joints
were fine. I reflowed them anyhow, and was pleasantly surprised to
see how well the solder flowed onto the pins, leaving an abnormally
shiny appearance. Hmmm.
Now, I've had some instances on Space Invaders sound boards where
the conductivity of the residual flux from previous repairs would screw
up the sounds, but I've never seen what I eventually found with the BZ.
It turns out that the conductivity of whatever residual flux was left over
from a previous reflow was 71 ohms over 0.156". That's really conductive,
and it was the cause of the problem. I've never seen anything that conductive
that didn't have metal particles in it. It wasn't solder paste, it was fairly
clear, slightly waxy in appearance. This stuff was really strange. Even
a pretty agressive flux remover didn't have much effect, so I don't think
the stuff on there was traditional flux. It smelled like pine when it was
heated ("primitive" fluxes are derived from pine trees, so that kind of makes
sense). Must've been some sort of plumber's flux.
The lesson: always deflux a board before you start working on it, because
you never know what some Communist has done before you. Sigh...
Alex
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ayec...@elektronforge.com
www.elektronforge.com