Flux pens left a white film on my test pads until I switched brands

I had a batch of 20 boards from a client in Dayton that kept failing continuity checks at the same spots. At first I blamed my soldering, but the pads had a chalky residue that wouldn't clean off with alcohol. Turns out the flux pen I was using had a high solids content that baked on when I preheated the board. Swapped to a gel flux with a low residue formula last month and the problem disappeared completely. Has anyone else dealt with a flux brand that behaves fine on one board but leaves film on multilayer ones?

2 comments

2 Comments

white.grant26d ago

Ngl, I ran the exact same setup for years with those cheap no-name pens and never saw a single film, even on 12 layer boards with preheat at 100C. Sounds like you might've had a bad batch or the board finish was more porous than usual, not a blanket flux issue. I've had gel fluxes leave worse gunk in vias than any pen when the solvent flashed off too fast. The chalky stuff you saw could've been baked on activator from the solder paste itself, not the pen. Trying a different brand is fine, but blaming the pen feels like a shortcut when multiple variables changed at once.

torres.elizabeth26d ago

Wait, so the pen was fine until you warmed the board up, like it just needed a little heat to show its true personality? Sounds like the flux equivalent of a mood ring, except instead of changing colors it just ruins your day.