Take care not to leave moisture-absorbing conductive residues.

Cleaning flux residues after reflow and wave soldering is not the norm today. Cleaning has been left to high-reliability manufacturers to make water- or solvent-based systems work. However, much of the industry has chosen not to clean after soldering, instead opting for low-solids fluxes designed to be left on and benign when properly heat-activated. Most companies have qualified and worked through the issues to make these assembly techniques work; in our estimation, around 70% of assemblers do not clean. But what happens when a no-clean assembly process is cleaned? Here we discuss the results of three different cleaning approaches on a no-clean flux with paste and reflow, liquid spray flux and wave-soldered assemblies.

Condition 1. Cleaning a mixed technology no-clean flux with only water in a new inline cleaner with high-pressure sprays and closed-loop DI systems.

Condition 2. Cleaning a mixed technology no-clean flux with saponifier added to the wash section at 10% using high-pressure sprays.

Condition 3. Cleaning a mixed technology no-clean flux with saponifier, followed by DI water steam (manual) and low-pressure sprays.

Some no-clean fluxes are more water-cleanable than others, but this is typically in open areas; the fluxes do not solubilize or are not rinsed well in areas of entrapment (below components and in vias). White residue pockets after water cleaning and drying indicate that flux has not been completely removed and has reacted with flux on the assembly surface. Even completely dry, this white residue can absorb moisture in the field. This can cause leakage and stray voltage problems.

Our analytical approach to evaluate the differences in the conditions listed above is a localized extraction system and ion chromatography analysis, along with elevated humidity testing with an electrical biased state applied to the assemblies.

• Condition 1 assemblies failed electrically when exposed to high humidity and powered within 48 hrs. at 85% RH at 65°C.
• Condition 2 assemblies failed electrically when exposed to the high humidity and powered within 72 hrs. at 85% RH at 65°C.
• Condition 3 assemblies passed 500 hrs. at 85% RH and 65°C.

Table 1 shows the ionic differences between the different process groups. The level of chloride, bromide, nitrate and sodium for all three assemblies is well below our recommended limits. The failed Condition 1 is very high for WOA (Figure 1). The failed Condition 2 is high in WOA and ammonia (from the saponifier) and the residue is conductive and corrosive (Figure 2). The good Condition 3 assembly showed great electrical performance for 500 hrs. of humidity exposure (Figure 3). The use of a saponifier, steam rinsing with DI water, followed by DI water rinsing did a good job of solubilizing the flux. Cleaning a no-clean flux with water or just soap can leave moisture-absorbing conductive residues in areas of entrapment, if not properly removed.


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

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