Precision and Accuracy in a Cleaning Process? Print E-mail
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Written by Michael McCutchen   
Thursday, 02 August 2012 18:08

Refractive index is proven to return all the wrong answers.

At some point during my studies, the terms precision and accuracy were introduced. I recall the illustration in the textbook that differentiated very simply between the two. The image displayed two targets with bull’s-eyes and arrows inserted. The first target demonstrated precision, with all arrows striking the same point of the target, but no arrows reaching the coveted bull’s-eye, thus equating precision to repeatability. The second target demonstrated accuracy by having one arrow striking the bull’s-eye. In this case, no two arrows arrived at the same point on the target, thereby representing some degree of accuracy (poor in this example) and no repeatability or precision.

Whenever new customers approach us with cleaning issues, these images come to mind. Many of these customers are upset that enormous amounts of time and money were invested to specify all parameters of their cleaning process, only to experience issues in production. The frustration that arises is completely understandable. After all, a properly conducted cleaning design of experiment (DoE) determines the necessary conditions required to produce clean boards (accuracy) with repeatability (precision). So what happened? What’s the point of determining these conditions to achieve reproducibly clean boards if this breaks down over time? To answer this question, we need to think about the cleaning process in terms of precision and accuracy.

A typical cleaning process DoE establishes the conditions required to clean a specific number of boards sufficiently and reproducibly. In terms of precision and accuracy, you need to hit the figurative bull’s-eye with every single arrow. The important thing to remember is that a DoE is intended to occur in a “perfect world” situation with fresh chemistry, relatively few test vehicles, low flux loading, etc., but that many unforeseen variables and circumstances can arise in production conditions to affect results. Therefore, it is critical to establish a very wide process window. In the cleaning world, this situation equates to how “forgivable” your process is. In other words, if variability is introduced altering the DoE established conditions, will the cleaning process tolerate this variance and still hit the bull’s-eye every single time? If not, meetings will be called to discuss the term we dare not speak of (white residues). And the fire drill begins!

Success in a DoE is easily achievable given the latest cleaning technologies available, but does this ensure excellent results carry forward from the DoE into real production conditions? Chemistry type and concentration are critical factors that determine the answer to this question. Often issues are directly attributed to the inability of customers to accurately determine chemistry concentration under production conditions due to flux loading and the impact on measurement techniques.
Refractive index is widely used throughout the industry to ensure cleaning agent concentrations are within the specification. No doubt, the popularity of refractive index is directly related to how quickly and easily an operator can obtain a concentration value with relatively low risk of introducing operator-related error. When the wash bath is fresh, it is very accurate, but it is erroneous under production conditions.

Let’s take a moment to review the limitations of the refractive index measurement.

The refractive index predicts or identifies a particular substance by confirming its purity or measuring its concentration by understanding the fundamental physical property of a substance based on the speed and direction of light passing through a medium. Theoretically, this makes it very useful in the cleaning industry to determine wash bath concentration, but what happens if a pure wash solution becomes contaminated with flux residues? Since flux affects the speed and direction of light as it passes through the sample medium, it can introduce unpredictable error, as contamination types and levels change over time, causing operators to incorrectly assume the process concentrations are within the specification determined by the DoE.

To illustrate this, let’s say an operator obtains a 16% concentration reading by refractive index, which is above the specified bath concentration of 10%. The operator adds DI-water to bring it down, then checks a sample and finds 10% concentration has been achieved. Problem solved and the operator walks away happy. What they do not realize is that refractive index measured was not a pure solution, but chemistry, water, flux, etc. As a result, precision and accuracy are suddenly out the window. The process is now well below the 10% concentration needed to reproducibly clean boards. Now the operator’s “arrows” begin to drift away from the bull’s-eye, striking random marks on the target. This is when issues begin to surface. The problem is compounded by the fact that sometimes these issues are not immediately apparent. Rather, they are intermittent and show up in the form of field failures and recalls. Traceability is often difficult because procedures were followed, and documentation indicates that the cleaning process was within the specified concentration by refractive index, which we now know is wrong, and the cycle continues.

Fortunately, extremely accurate manuals and available automated methods of monitoring concentration avoid use of refractive index. It is time to restore precision and accuracy in the cleaning of high-reliability products by shifting the paradigm away from refractive index as a measurement tool. To those who say that refractive index is a technique proven in the industry, I agree. Unfortunately, it is proven wrong.

Michael McCutchen is vice president Americas and South Asia at Zestron (zestron.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Friday, 03 August 2012 12:45
 

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