In Part 1 of this series on the importance of rinsing, the author presented an overview of the critical aspects of rinsing as it applies to the overall quality of a printed circuit board, with considerable space devoted to water conservation. Thus, we now turn to how one can improve rinsing effectiveness without increasing water consumption and, by default, significant waste treatment costs.
The preferred rinsing method of design engineers is counterflow, or cascade rinsing. Still another theory widely held throughout our industry reasons that, if work is left in a rinse tank for longer periods of time, better rinsing will be the result. First, let’s explore the idea of leaving the circuit boards in the rinse for a longer period.
When a rack of PCBs is immersed in a rinse tank, the residual surface contamination is reduced to a practical minimum within 30 seconds as the solution carried in on the surface of the work disperses into the rinse waters. A typical rinse tank—100 gallons with a water flow of five gallons per minute—would decrease the concentration of the solution contaminants at a rate of only 5% for each minute that it remains in the rinse tank. Leaving the work in any longer would have virtually no effect. This demonstrates that rinsing time in a rinse station is a non-linear relationship with respect to removing contamination from the surface of the parts. Yes, this is counter intuitive. We often think that if two minutes in the rinse removes 50% of the residues, then doubling the time in the rinse will remove 100%! This is not the case. Basically, the law of diminishing returns applies. There are many factors in play here including the type of contaminants to be rinsed, affinity of those contaminants to adhere to the printed circuit board, etc.
It is more desirable to have brief exposure times in many (presumably cascade) rinse tanks, which will result in a better dilution rate of the contaminants, than a long exposure time in just a few rinse tanks.
There have been countless studies performed on rinsing mechanisms and how to improve rinsing efficiency. Most of the published studies related to rinsing are based upon multitudes of calculations of volume of water flow relative to number of rinse tanks in use.
To read the Part 1 of this article click here.
To read the full version of this article which appeared in the October 2017 issue of The PCB Magazine, click here.