[GreenKeys] Citric acid as soldering flux

[Jones, Douglas W]

From: steve shumaker <shumaker at att.net> -- Thursday, February 6, 2025 8:41 PM

> Doug,  is the writeup  re citric acid online?

An Investigation of the Chemistry of Citric Acid in Military Soldering Applications
-- https://apps.dtic.mil/sti/pdfs/ADA295042.pdf

Also look at the following patents issued to Hughs Aircraft.
Note that they have expired, so the recipes are in the public domain.

Water-Soluble Soldering Flux, 5,085,365, Feb 4, 1992
-- https://patentimages.storage.googleapis.com/fa/85/87/fbd184ebc059cb/US5085365.pdf
Water-Soluble Soldering Flux, 5,452,840, Sept 26, 1995
-- https://patentimages.storage.googleapis.com/76/26/88/3e60c57d72c70e/US5452840.pdf

The second patent contains a real recipe:
40% by weight of citric acid to water gives the best results
Commercially produced citric acid powder works just fine
Corn starch and gelatin work as thickening agents
Clothes dies may be added as colorants so you can see the flux.

The recipe doesn't say how to thicken, but I assume you boil the thickener into the water until it has the right consistency -- the classic test is to take a spoonfull, blow on it until cool, an test the thickness.  Once it's right, add the citric acid and just enough colorant to give it color.  A little die goes a long way.  Commercially produced citric acid would include what you can buy at a supermarket.

                Doug