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SolderTip #38: Clean vs No-Clean Fluxes

Question:  We are using a Kester 186 RMA Flux for our assembly process. Can you tell me if you have to clean an RMA flux and what the issues are if we do not?

Possible Solution:  You don’t have to clean an RMA Flux. The flux hardens and can be left on. The problem most people find with it is that it has a dark residue and when we used to use it on Printed circuit boards, the test folks would get upset because it would foul up their test fixtures and test pins. But for wires, this was and still is the flux to use.

So can this flux be left behind, yes, but it will leave visible residues, which by the way was one of the reasons the low solid content fluxes came about. Customers and users did not want to see any residue after the soldering operation.

Kester 186 is an RMA flux. The 186 came in three versions or solid contents if you will, 18%, 25% and 36%. The flux you have is probably very dark and therefore the alcohol may have evaporated from it. You can thin this flux out with Kester #120 Thinner and measure the specific gravity to 0.0831+/_ .003…