Figure 15 shows that characteristic x-rays exit a flat sample in a cone roughly 30° above the surface:

Characteristic x-rays do not exit the sample below 30° because they must travel through a distance of material b which is great enough to completely absorb the x-rays. Distance a is not so large that the x-rays will be absorbed. Figure 16 shows how curved surfaces reduce distance b and increase the count rates seen by the detector:

As the count rates increase, the measurement will increase relative to it's actual value. This can happen on small features like gate arrays where the solder has domed during hot air leveling. The effect can be eliminated by proper sample positioning and collimator selection.

First, place all samples so that the axis of the curve is perpendicular to the axis of the detector and position the measuring area back from the edge of the feature which is toward the detector. This will present a relatively flat surface on the detector side which will provide an absorption effect as shown in Figure 2. It is also important to select a collimator according to Figure 17:

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