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SolderTip #48: The Cleanliness of Wire and Terminal Connections

Question:  What are the specifics on wire and terminal connection cleanliness requirements and how do they compare to PCB cleanliness?

Answer:  Cleaning wire and terminal connections is somewhat different than cleaning PCBs along with cleanliness and measurement testing techniques.

Boards and assemblies are cleaned of flux residues for a variety of reasons, dendritic growth on the surface of the laminate, ability of the conformal coating to adhere to the laminate material and not have mealing failures, ease of handling after the soldering operations and basically to remove any residue flux residues which may react with the operational environment where the product is to be used.

Cables are a different animal when cleanliness and cleaning is concern. All documents which I have read, do not recommend the use of water soluble fluxes for the tinning and soldering of wires, cable and connectors and the reasoning being the ability of the flux, through capillary action, to wick beneath the wire insulation where the flux residues will remain without the ability of being cleaned away. As called for in J-STD-001, the fluxes have to be qualified to verify the residual residues are not going to be detrimental to the functional operation of the product. If the flux is not qualified, that is a defective condition for both Class 2 and class 3 products.

Hence the fluxes used for soldering cables and wires should be a flux, which can remain on the product and not need to be removed. If the flux needs to be removed it should be easily removed with its appropriate cleaning agent. Keep in mind however that the flux beneath the insulation will remain and although it is not visible is potentially corrosive and detrimental to the functionality of the product. Now for the requirements of whether or not the flux residues should be removed or cleaned is based upon the visual appearance of the flux. Depending upon the flux type used, flux is usually removed so there are no visible film residues evident on the solder connections. It also makes it easier to inspect the solder joints if the flux residues are cleaned off.

The bottom line is that fluxes are acidic in nature and this acidic product is susceptible to the absorption of moisture, which will allow it to remain active and eat away at the metallic conductors of the cable and wires. So one needs to make sure the fluxes used are rendered benign during the soldering operation and can be left behind or they have to be removed.