Water Soluble Solder - Conductive Flux Residue

Hi folks,

I spent half the day yesterday trying to figure out why I could measure an impedance between unconnected pins on my custom PCBs.

Turns out it was because the flux residue from the SparkFun lead-free, water soluble solder is actually conductive! Who knew. I was measuring impedances as low as 500 kΩ, but generally around the 1-5 MΩ range, which is more than enough for significant voltage drops to occur between pins.

I think it could be helpful to add a warning or recommended instructions to the following two products to advise users to thoroughly clean off the flux residue with either deionized water, isopropyl alcohol, or even tap water + hair dryer.

https://www.sparkfun.com/products/10243

https://www.sparkfun.com/products/10242

Cheers,

Adam

You’re supposed to clean the flux off every time you solder unless it’s no-clean flux. The water soluble stuff just means you don’t need any chemicals to clean the flux.

This is normal. All fluxes are conductive. They are all pretty highly conductive when first applied, and less so after being used. What you have there is a rosin-based flux, which most electrical fluxes are rosin-based (or synthetic “resin” with similar characteristics.) Flux works by dissolving metal oxides with acid, and for something to be acidic, it must create H+ ions and negative ions, and ions are conductive.

Using ‘no clean’ types might help in this manner, but in general it is a good practice to always clean flux after application with alcohol.

Have a great day!

Thanks for the info, Russel!

I only recently made the switch to SparkFun’s water-soluble solder. I much prefer it over the rosin-core leaded solders I’ve used in the past, though I do notice that it is quite a bit messier and can leave behind more flux residue. I hadn’t realized just how conductive the residue could be, as I had never encountered this issue before. For those new to soldering, I still think it could be helpful to link to your through-hole soldering tutorial on the solder product page, as it has a section on cleaning the residue (which I probably should have read) https://learn.sparkfun.com/tutorials/ho … dering/all

Cheers,

Adam

Hi,

To provide an update, I have found that using deionized (DI) water to clean the water-soluble flux residue works infinitely better than isopropyl alcohol, which tends to leave a sticky residue over the PCB. I then use my hot-air rework station to make sure any remaining DI water that’s on the PCB is properly evaporated.

Works like a charm!

Cheers,

Adam