pH Neutral vs. Alkaline Cleaning Agents

Harald Wack, Ph.D., and Umut Tosun, M.S.
ZESTRON America, Manassas, VA, USA

John Neidermann
Speedline Technologies, Camdenton, MO, USA

John Radmann
Trace Laboratories, Hunt Valley, MD, USA

In recent years, significant changes in solder paste formulations and assembly processes have occurred. Post reflow residues of tin-lead and newer lead-free soldering materials are more difficult to remove due to increases in component density, larger component packages, higher lead counts, finer lead spacing and lower standoff distances. While modern aqueous alkaline cleaning agents effectively remove these flux residues, achieving satisfactory results often requires an increase in temperature, exposure time, chemical concentration, and mechanical energy. This often presents a new set of challenges in the area of material compatibility.

The results indicate that since obvious differences in surface and under-component cleanliness levels do exist, the pH-neutral agent clearly outperformed the competitor alkaline cleaning agent A. A board that is perfectly clean on the surface can fail when the spaces underneath the components are examined. The authors conclude that there are several potential reasons for these results.

First, any lack of performance could be related to concentration, i.e. 10% may be too low of an effective concentration to clean these challenging boards properly, especially for the alkaline product. Second, the observed cleanliness issues may be due to the inhibition packages (type and amount)that the cleaning agents contain. Third, cleaning product formulations and mechanisms also play an important role. Finally, the residues found may be a result of inhibitors bonding with the very contamination the cleaning agent was intended to remove thereby preventing the dissolution of the residue.

In order to further quantify and qualify these findings, several additional test vehicles were chosen, reflowed, and cleaned with both solutions. Subsequently, the boards were subjected to SIR and Ion Chromatography analyses.