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88 The PCB Magazine • July 2017 acidic, and some acidity will be neutralized by drag-in of alkaline residues. Depending upon the concentration of drag-in, the alkaline solu- tion will have an almost negligible neutraliza- tion of the microetch. This is true because the acidity of the microetch is self-generated by the spontaneous decomposition of the persulfate, thus the decrease in acidity is, to some extent, automatically replenished. When examining an acid copper plating bath, however, we know that this bath is ex- tremely sensitive to contaminants, and can be ruined by the same level of contaminants that would have no effect on a microetch. We can conclude that a level of contaminants from an alkaline cleaner that one process solution can tolerate may not be tolerable for a different pro- cess solution. Manageable levels of contami- nation would need to be determined for each specific process. In both cases discussed above, effective rins- ing will prolong bath life of process solutions down the production line. An effective rinsing system that efficiently removes enough process solution can decrease chemical costs, lower re- ject rates, and ease the burden on waste treat- ment systems. There are several myths associated with rinsing performance that deserve discussion. A common myth is that alkaline solutions are more difficult to rinse than acid solutions. Con- sequently, we often see rinses following alka- line cleaners using hot water, and/or extremely long exposure times. Let's examine these myths further. These observations likely started due to the difficulty of removing the slick feeling that an alkaline solution gives when in contact with the skin. The slick feeling is never rinsed away, because the slickness is due to the fixed oils on your skin being converted to a soap by the al- kalinity of the solution. What you wind up do- ing is converting this "soap" to hard water scum when rinsing your hands in water, or to a fatty acid, when rinsing in an acidic solution. Since these oils are not found on copper, soldermask, laminate, or other materials, rinsing alkaline so- lutions from printed circuit boards is no more difficult than rinsing acidic solutions. Alkaline cleaners can also get blamed for poor rinsing because of the presence of foam- generating surfactants in the formulations. The surfactants used in many alkaline cleaners gen- erate large amounts of foam even at incredibly low concentrations. When a small amount of surfactant is dragged into a rinse tank, it can create a stable foam, even at concentrations that would not affect subsequent process steps. The cause of the foam, however, is not because the surfactants are not rinsed inadequately from the work, but because the presence of these surfac- tants in extremely minute amounts can cause substantial foam generation. The perception that an alkaline cleaner would rinse poorly was further supported by the formulation chemistry. Some of the ear- ly alkaline cleaners were formulated using sili- cates. Calcium present in tap water was inter- acting with the silicates in these cleaners form- ing calcium silicate, which precipitated out of solution, and onto the work. Unfortunately, even extensive rinsing would not always help THE CRITICAL IMPORTANCE OF RINSING, PART 1 Figure 1: Significant foam creation due to drag-in of the cleaner to a downstream process tank.