White Residue: Good or Bad?, Part II -- So, is white residue good or bad? The answer is, well, "it depends.".

At issue: Does white residue foreshadow corrosive invisible residues below or in flux? Typically, based on my investigations, flux residue is a good insulator. Many of today's low-solids fluxes, pastes and cored solder can generate a white residue if the flux is partially cleaned and exposed to a small amount of water. This residue is not corrosive, but may be conductive if not completely heat activated. Such residues can be moisture absorbing and conductive-creating electrical leakage and stray voltage failures on sensitive circuits.

What Causes White Residue?

In many cases, white residue is primarily caused by a reaction between moisture and flux residue. I have also seen dry-film solder mask absorb cleaning chemistries and water, causing a white, hazy surface. Figure 1 shows a typical hand-soldered application with a low-residue flux that should be left on the assembly without cleaning. The area around the solder joint is shiny compared to the finish of the standard laminate. Cosmetic issues may motivate some assemblers to remove the evidence of flux residue with a standard benchtop cleaner or with alcohol and a brush followed by a water rinse.

White residue is a common reaction that occurs after rinsing a partially cleaned flux residue with water. If the residue is from a water-soluble flux, this material sometimes contains activators that are very conductive, moisture absorbing and corrosive. Such residues must be removed if they are water-soluble flux residues. Low-solids fluxes do not need to be removed, but the bare board and components must be clean and not contain surface residues that are corrosive or conductive, such as hot air solder leveling (HASL) flux and plating bath residues.

A Closer Look

Tools such as ion chromatography (IC) have allowed me to evaluate residue and determine if residues visible and invisible must be removed. I was once presented with a project to determine the nature of a heavy white residue on the bottom (wave solder) side of an assembly and determine if the residue could easily be removed. A corner of the board was analyzed to determine the nature of the material and if any corrosive residues were present. The data showed that the area with the white residue, on a HASL finished board, was only a low-solids flux that should not have been cleaned.

This unit was exposed to the resistivity of solvent extract (ROSE) process control tester, which uses a 75 percent isopropyl alcohol and 25 percent deionized water solution, then air dried before being put back into inventory. The residue shows a heavy spray-fluxing pattern from the wave solder process typical of such material. After the first extraction, the board was then cleaned in a specific saponifier-at a 5 percent concentration, in 60 degrees C wash and rinse water, in a batch cleaning system-then analyzed (in a different area) by IC. Data shows that cleaning reduced the visible flux residue and corrosive residues from the incoming HASL board residues (Table 1). After cleaning, the board was free of visible and ionic residue. Ionic residues are corrosive, while no clean white flux residues are benign and pose no risk of electromigration.

Terry Munson is with Contamination Studies Laboratory (CSL), Kokomo, IN;[phone];e-mail: Residuguru @aol.com; Web page: www.residues.com.

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