Phosphor bronze substrates contain tin. This substrate tin will add to the tin fluorescence from the coating, causing tin composition and thickness measurement to be high. In addition, the copper K-beta line can inflate the tin measurement if the coating is less than 150µin. thick. The cobalt filter should be employed to absorb copper fluorescence if the coating is less than 150 µin. thick. Figure 14 shows the fluorescence from tin and lead on phosphor bronze substrates.

This can be further complicated if a nickel intermetallic barrier is present between the coating and substrate. This nickel will absorb substrate tin fluorescence. As the nickel increases in thickness, more substrate tin fluorescence will be absorbed.

Normalization, base correction, or other suitable adjustment must be made to compensate for quantities of substrate tin which vary due to type of alloy and lot-to-lot composition tolerance. A test can be performed to determine if a nickel barrier is eliminating the effects of tin from the substrate. Measure a nickel covered area with the tin-lead measurement application (calibration). If a reading other than zero is recorded, the instrument must be normalized or base corrected to an area with a representative quantity of nickel coating.

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