There is more here than meets the eye!

Dr. Bruce Scruggs, Product Manager Micro-XRF, EDAX

EDAX has introduced a product line of coating thickness measurement instruments based on XRF spectrometry.  These units were designed to measure coatings on samples varying in size from small parts to spools of metal sheet stock a mile long.  The markets for these products are generally in the areas of Quality Control/Quality Assurance and Process Control.

Recently, I received a simple, small electrical component, i.e. some type of solder contact or lug, and was asked to verify the coating thicknesses on the sample and check whether it was in specification or not.  It seemed like a simple enough task and I wasn’t expecting to learn anything special.

Figure 1: Electrical contact lug

Figure 1: Electrical contact lug

I was given the following specifications:
• Sn / Ni / Al (substrate)
• Sn thickness:  5 µm +/- 1 µm
• Ni thickness:  2 µm +/- 1 µm
• eyelet is coated; tail is uncoated

I made some measurements on the eyelet and the tail and these were consistent with the eyelet being coated with Sn and Ni and the tail section being an uncoated Al alloy.  There were some irregularities that I was not expecting.  I found trace Ga in the Al alloy.  I thought that was rather odd because I don’t see Ga that often.  I also found strong peak intensities for Zn and Cu which were completely inconsistent with the weak peaks found in the Al alloy.  A “standardless” modeling quantification analysis of the Al alloy indicated Zn and Cu at 40 ppm and Ga at 110 ppm.  Googling “Gallium in Aluminum alloys” produced numerous hits explaining that Ga is a trace element in bauxite, the raw material used to produce Al metal.  Hence, Ga is a trace impurity in Al alloys.  Incidentally, the following week, I saw trace Ga in every Al alloy I measured for another project.

Since the Zn and Cu peak intensities found in the measurement of the eyelet were much stronger than the base alloy, this means the Zn and Cu had to be in the Sn/Ni coatings.  After completing all the spectral measurements on the eyelet, I had to resort to polishing an edge on the eyelet and evaluating the Sn and Ni layers in cross-section using SEM-EDS to evaluate the content of the Sn and Ni layers.  The Sn and Ni layers were smeared because the polishing was done very quickly without embedding the sample in epoxy.  But, SEM-EDS clearly showed the Zn and Cu originating from the Ni layer and not the Sn layer.  So, now we had a layer system of Sn / Ni(ZnCu) / Al alloy.  It wasn’t clear to me whether the Zn and Cu represented a quality problem or not.

Figure 2: SEM image of the cross section of the edge of the eyelet. Th Sn and Ni layers can be seen from left to right

Figure 2: SEM image of the cross section of the edge of the eyelet. The Sn and Ni layers can be seen from left to right

Now we come to the actual measurement of the coating thickness.  Since, Sn and Ni foils are commercially available for coating calibration, I decided to use stackable Sn and Ni foils, i.e. 2.06 um Sn on 1.04 um Ni, (sourced from Calmetrics Inc, Holbrook, NY  USA) on an Al substrate to calibrate the coating model.  I also used pure Zn and Cu “infinites”, i.e. samples with a thickness such that further increase in thickness provides no increase in signal, to give the coating quantification model a point of reference for these other two elements not in my Ni foil standard.

I built a coating quantification model based on the Sn(K), Ni(K), Zn(K) and Cu(K) lines and another based on the Sn(L) lines as opposed to the Sn(K) lines.  The Sn(K) lines, being more energetic , allow you to measure thicker layers while the Sn(L) lines are more sensitive to layer variations for thinner layers.  Both coating quantification models were calibrated with the same standard.  But, to my surprise, measurements off the same point on the sample using these two different coating models didn’t agree!  This is often a question that our customers ask, “Why are the results not the same if I use a different line series?”

Table 1: Initial coating thickness measurements on the eyelet.

Table 1: Initial coating thickness measurements on the eyelet.

I pondered this result for a while and then remembered that X-rays are penetrating.  This is why this is an effective means of non-destructively measuring coatings.  After measuring the overall thickness of the part, i.e. 0.8 mm, and doing a few quick calculations, I realized that the Al alloy substrate is not thick enough to stop Sn(K) X-rays.  The website I like to use for these types of calculations is: http://henke.lbl.gov/optical_constants/filter2.html.

0.8 mm of Al only absorbs about 30% of the Sn(K) X-rays at 25.2 keV and this sample happens to be coated on BOTH sides of the substrate.  (The absorption for Sn(L) at 3.4 keV and Ni(K) at 7.5 keV happen to be essentially 100%.)  So, the measurement is seeing the Sn(K) from the top surface as well as the opposite surface coating while the measurement is only seeing the Sn(L) and Ni(K) from the top surface.  I thought it would be interesting to make the measurement again at the same spot after polishing off the coating on the opposing side of the part.

Table 2: Coating measurement at nominally same position as in Table 1 after removing the coating on the opposite side of the part.

Table 2: Coating measurement at nominally same position as in Table 1 after removing the coating on the opposite side of the part.

Now the Sn (and Ni) layers agree to within better than 10%. In this case, the result for the Ni layer also changes because, given the same Ni intensity in each case, the quantitative X-ray modeling will predict that the Ni layer thickness must decrease as the Sn layer thickness decreases. You can also see that the Sn layer is well out of specification and there is about 10 wt% Zn in the Ni layer.  I still don’t know if that’s a quality problem or not.  But, I was definitely impressed with how much I learned from just measuring this simple electrical part.

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