From: "Alex Yeckley" Subject: TECH: Watch out for this! (Flux) Date: Saturday, December 06, 2003 3:09 PM I don't usually post tech tips anymore because most good tips can be 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 ---- ayeckley@elektronforge.com www.elektronforge.com