We stumbled on to something that is quite likely well known to any assembler, but we were too busy to notice. We had some boards manufactured (stuffed with components) at an assembly house in Colorado. There were a few issues that required a little re-flow of solder. We heated up the pads, applied some of our solder, but only wound up with a sticky gooey mess and no real solder flow.

Then it dawned on us we were using rosin core solder and the assembler might have not used rosin core solder. Sure enough they used water soluble flux in their soldering technique. We switched to some new solder with water soluble flux and things re-flowed beautifully.

Don’t mix rosin and water soluble fluxes when soldering.