Tin-lead is commonly measured on copper both for connector and printed circuit board applications, and on kovar and other glass-sealing alloys for leadframe applications. Figure 10 shows tin, lead and copper fluorescence. It is important to note the copper K-beta line which coincides with the tin K line used in measurement. This copper peak is absorbed by tin and tin-lead coatings unless the coatings are less than 150 µin. in depth. Then, this copper K-beta line will add to the tin fluorescence, causing both composition and thickness measurements to read high. This effect can be eliminated by employing the cobalt filter or suitable numerical filtration. Figure 11 shows the effect of the cobalt filter on copper fluorescence .

Bromine fluorescence, as shown in Figure 12, can add to lead fluorescence if the copper plane underneath is sufficiently thin (i.e., less than 700µin.) or unless the measurement is located on a small feature where a layer of copper does not exist in the area surrounding the feature (i.e., on a surface mount pad).

Bromine interference is usually easy to identify. Lead composition measurements will be unusually high. Measurement precision may also deteriorate. Use the same procedure as described for measuring gold and nickel on bromine-laden substrates to diagnose and eliminate bromine interference.

Figure 13 shows that fluorescence from kovar and other glass-sealing alloys does not interfere with tin-lead solder:

Abstract
Accuracy and Precision
Principle of Measurement
Gold and Nickel Measurements on Copper
Gold and Nickel Measurements on Phosphor Bronze
Gold and Nickel Measurements on Kovar and Other Glass-Sealing Alloys
Gold and Nickel Measurements on FR-4 Circuit Board Substrates
Tin-Lead Solder on Copper, FR-4 Circuit Board Substrates, and Kovar
Tin-Lead Solder on Phosphor Bronze
Adjustments for Part Geometry
Measurement Assurance
Conclusion