Our expert clarifies his stance on brush cleaning.

It has been brought to my attention (“The Big Brush Off Revisited,” April 2008) that I was unclear in a December 2007 case study on localized brush cleaning.1 To recap, I am not suggesting localized cleaning is good or bad, but if you clean, you must rinse. If you solder or repair with a no-clean flux and don’t clean, you must ensure the flux (flux pen, bottle or core wire) is completely heat-activated, guaranteeing it becomes a benign residue, but not until then.

During repair, rework and hand solder, flux is used to prepare the soldering surface. Now, let us address what happens after surface prep and with flux after soldering.

Issues that need to be addressed during repair, rework, selective and hand soldering beyond soldering:

1. A soldering operator must understand more flux does not improve soldering (although sufficient flux does).

2. Two flux choices are available: cored wire, and cored wire with liquid flux in a pen or bottle to assist in wetting.

3. When using flux to prepare the surface, remember flux has a low surface tension and will travel everywhere – to nearby components, or the opposite side of the board through small vias.

4. A general rule is no-clean fluxes need to be heat-activated to become benign residues. The flux supplier identifies the activation temperature. Also, just because the flux evaporates at room temperature doesn’t mean it is completely benign.

5. When the only area that gets heat is the repaired or hand solder joint, the remaining no-clean flux around and under nearby components is moisture-absorbing and conductive. Dendrites can grow with no-clean flux residues and 1.5V difference (Figure 1).


6. Some of you are saying, We brush clean only because test pads at ICT have a problem with the no-clean flux. Remember to rinse effectively in these and surrounding areas. This requires localized testing and is not a visual inspection issue.

7. This residue can be dealt with in one of three ways:

a. Do nothing and expect residues to become benign because of magic. We see this a lot and are surprised when clients bring us these failures.

b. Localized brush cleaning/can of cleaning solvent. When cleaning with a solvent in a localized area, if you do not rinse the contamination, you are simply relocating it to nearby areas. Brush clean, then rinse/flush in a controlled fashion, using a solvent rinse or a steam approach. If you cleaned an area, then rinse the area as well. Remember, low-standoff parts hold a lot of fluid by strong capillary forces.

c. Secondary heating of the nearby areas to transform the no-clean flux from a conductive moisture-absorbing flux to a benign residue. This can be done in a controlled cure oven, or even with a heat gun in a stationary fixture and thermocouple, to avoid thermal damage.

8. Localized cleaning should be part of a process qualification, as should hand soldering and rework, but that’s a separate case study.

How to close this little discussion? When cleaning with any localized method, rinse effectively and use a localized analysis method to assess effectiveness.

References

1. Terry Munson, "How Effective is Localized Brush Cleaning?" Circuits Assembly, December 2007.

Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. This column appears monthly.

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