This month’s analysis looks at x-ray fluorescence.

Ed.: This continues a four-part series on typical analysis techniques and their pros and cons in regard to understanding electronics residues.

The x-ray fluorescence (XRF) principle is depicted in Figure 1. An inner shell electron is excited by an incident photon in the x-ray region. During the de-excitation process, an electron moves from a higher energy level to fill the vacancy. The energy difference between the two shells appears as a z-ray, emitted by the atom. The z-ray spectrum acquired during the above process reveals a number of characteristic peaks. The energy of the peaks leads to the identification of the elements present in the sample (qualitative analysis), while the peak intensity provides the relevant or absolute elemental concentration (semi-quantitative or quantitative analysis). A typical XRF spectroscopy arrangement includes a source of primary radiation (usually a radioisotope or an x-ray tube) and equipment for detecting the secondary x-rays.¹


Applications and limitations. XRF is a quick, nondestructive analysis technique to determine the elemental makeup of a material or mass. Because it uses x-ray to release the electron shell of the materials, it can see and measure the thicknesses of plated lead, but has difficulty with thin-film metals and organic and ionic materials on the surface. However, XRF will see ionic residues in thick concentrations, but is unable to quantify these surface films. Figure 2 shows a dendrite growth caused by partially heat-activated, no-clean flux (succinic, maliec and glutaric acids) and fabrication HASL flux (chloride), but the XRF is not able to determine the contaminants, just the metals and flame retardant (organo bromide).


References

1. Wikipedia.

Terry Munson is with Foresite Inc. (residues.com); tm_foresite@residues.com. This column appears monthly.