Limitations of Refractive Index Print E-mail
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Written by Dr. Harald Wack   
Wednesday, 01 April 2009 17:54

Process DoctorContamination can artificially decrease readings.

Albert Einstein wrote, “To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.”

Let’s consider the great physicist’s observation in the context of cleaning agent bath measuring techniques. Prior to the introduction of modern aqueous cleaning technologies (in the 1990s), users mostly were limited to titration. Measuring the pH-value of a product is one important variable, but in itself does not provide the true nature/state of the cleaning bath in question. The refractive index measuring technique was introduced in North America in the late 1990s, as “modern” aqueous products started to gain traction. It allowed users to combine pH measurements with refractive index measurements and assess the organic and alkaline level of ingredients in their product. Companies even started to introduce automated concentration monitoring systems, based on the refractive index measurement. But let’s take a moment to recount the limitations of the refractive index measurement.

Per Wikipedia:

The refractive index of a medium is a measure of how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, a typical soda-lime glass has a refractive index of 1.5, which means that inside the glass, light travels at 1/1.5 = 0.67 times faster than the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect used in lenses. Second, light partially reflects from surfaces that have a refractive index different from that of their surroundings.

The refractive index, n, of a medium is defined as the ratio of the phase velocity, c, of a wave phenomenon such as light or sound in a reference medium to the phase velocity, vp, in the medium itself (Figure 1).

Fig. 1

Since the refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, to confirm its purity or to measure its concentration. In our industry, the refractive index is used to measure the concentration of a solute in an aqueous solution. A refractometer is the manual instrument used to measure the refractive index. When talking about a solution of sugar, the refractive index can be used to determine the sugar content.

But what happens if the sugar solution were to become contaminated with flux residues? A current study discovered that all cleaning agents used in electronics are affected by the contamination (i.e., flux) they remove. And I mean all of them! In extreme cases, the difference between the perceived concentration and the actual concentration deviates by over 15%! For example, the operator is reading a 12% concentration solution, which seems fairly close to the specified bath concentration of 14%. The operator adds a concentrated cleaning solution to make up the difference. They must be certain the cleaning process is in full compliance, as most Western companies produce highly reliable products. They cannot afford any inaccuracy at this crucial point of the production process. The boards either are about to be shipped to the customer or will be undergoing a conformal coating step.

Remember the refractive index is prone to deviation. Given that the operator was not using a freshly prepared cleaning agent in their cleaning equipment, there is a high possibility that what is thought to be a 12% concentration is actually a 5% one. This means that the cleaning agent might not be able to clean at its full strength, possibly resulting in board failure due to contamination-induced electrochemical migration. But there is no way of knowing, unless the operator sends the contaminated sample to the cleaning agent manufacturer and has more elaborate analytical tests performed, such as GC (gas chromatograph) measurements. A GC is much too expensive for regular bath maintenance purposes. Therefore, alternative methods are indeed required. Further, the contamination also can decrease the operator’s reading artificially. In other words, the actual concentration is 20%, but the contaminated solution gives the perception of 14%; so the result will be significantly higher concentrations and higher chemistry costs. Again, any contamination added to the cleaning agent has a different effect on the refractive index and cumulatively will affect the actual bath concentration accuracy.

We are glad to report that this issue now has been resolved, and new products are once again eliminating problems that once seemed impossible to overcome. Let’s marvel at science and how it allows all of us to become more efficient at what we do.

Harald Wack, Ph.D., is president of Zestron (; This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Wednesday, 01 April 2009 11:49


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