Flux residue

Hi Sean,

No-clean flux does leave a residue. Under the best conditions, it is invisible. There are many factors that can cause the residue to become visible such as an inadequate oven profile (or lack of proper preheat on a wave). Also, if wave soldering, the flux application method effects the amount of visible and invisible residues on an assembly. Lead-free processes, with their higher reflow temperatures also contribute to residues as the flux polymerizes (crosslinks) during the reflow process and no longer encapsulates metal salts. There are also environmental factors that can cause residue to appear such as moisture. Your concern is based on the fact that you can see the residue. In fact, the residue was there before it became visible. Is the residue only cosmetic or can it lead to more serious issues? The answer can be yes to both. Flux residue (visible or invisible), when combined with moisture (such as humidity) can create a conductive path allowing metal to flow between an anode and a cathode, leading to dendritic growth or electrical leakage (creepage). If this is a concern, the solution is to remove the flux residue. Be aware that the residue you see does not represent all of the assembly’s residue. All no-clean processes leave residues behind. The challenge is to determine if the residue to harmless or harmful.

Spot removal is not recommended as you are only removing the visible residues. If you have a concern that the residues may be harmful, a defluxing process may be in order.

As ironic as it sounds, more of our customers remove no-clean fluxes in their defluxing systems than any other type of flux.

Mike Konrad Aqueous Technologies konrad@aqueoustech.com

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Dear Sean

Contrary to believe MOST formulators of No Clean fluxes or No Residue fluxes use organic acides that are in fact weak and benign. IF the residue was left on the board and you had a condensing atmosphere then you may see some issue with falling SIR values, but generally not electromigration. UNLESS the resiue left was a Halide containing residue (not No Clean) then you may see electromigration or much poorer SIR values. So the residue appears some time after. Is the PCB visibly wet after wave? Do you see a waxy/greasy residue as it could be that we are seeing the porous resist problem that has occurred for years were it releases the flux some days after the process. More information or picture would be usefull Cheers Greg BLT Circuit Services Ltd

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