The L-beta line of gold is used for the top layer measurement. The ability to measure nickel under gold is directly proportional to the thickness of the nickel and inversely proportional to the thickness of the gold. As the gold increases in depth, nickel fluorescence is absorbed. As a result, nickel cannot be measured through gold coatings greater than 80µin. in depth. As the gold approaches 80µin., precision on the nickel measurement will decrease.

This loss of precision may be compensated for by increasing measurement time or by averaging several measurements. As your sample size increases (through either method), the mean of nickel measurements will tend toward the actual thickness. It will also be important to normalize regularly to keep the nickel measurement within tolerance when encountering thick gold.

It is also important to note that the nickel characteristic x-ray is very close to the copper characteristic x-ray from the substrate. Therefore, the copper fluorescence must be removed, either by a cobalt filter or by numerical filtration, in order for the nickel to be measured. Figure 6 shows the effect of the cobalt filter on the measurement.

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