Tony Revier, President, Uyemura USA

PCBs, wafers, and semiconductors all have "final finishes" that are central to their functionality. XRF is the standard technique for determining what does -- and doesn't -- meet specifications.

Plating thickness measurement is the most common function XRF function, but advanced XRF systems like the 7-member Bowman benchtop suite, also perform alloy analysis, and solutions analysis -- a huge advantage. These systems can determine the % composition for each alloying element and its alloy grade number. For solutions analysis, metal ions in plating baths can be quantified for process control.

However XRF is used, its two "opposing ends" -- x-ray emission and detection - determine what an instrument can achieve.

Entry-level XRF systems use pin-hole collimators, which filter x-rays so only those parallel to a specified trajectory pass through. Collimator size determines the size of the irradiation spot projected onto the sample. As collimator size reduces, the resulting x-ray flux irradiating a sample decreases rapidly.

The alternative is polycapillary optics, an assembly of hundreds of thousands of glass channels that collect a large solid angle of X-rays from a diverging source. X-rays pass through the optic by total internal reflection and focus to an extremely small spot. These optics deliver almost 100% of the flux of the x-ray tube to a very small
spot. Elite systems such as the Bowman W Series XRF focus x-ray excitation down to 7.5µm.

XRF instruments use one of two solid-state detectors: a Silicon PIN Diode or a Silicon Drift Detector (SDD). These detectors have many advantages over older gas-filled proportional counter technology, which is notorious for poor resolution, high noise levels, and a requirement to constantly adjust for peak location drift.

The silicon PIN diode provides excellent spectral resolution, so operators can measure thinner deposits and lower element concentrations, and test alloys and heavily layered samples. Silicon PIN detectors are low noise, have excellent resolution, and low detection limits.

SDDs produce higher count rates, and highest spectral resolution -- typically 50% higher than silicon PINs. They have the lowest baseline noise, the best detection limits and the greatest versatility in element testing range. They also allow for the shortest test times, while achieving great repeatability.

SDDs are best able to precisely measure overlapping elements -- nickel, copper, zinc and chrome and iron -- where separation of signals is minimal. SDDs are the detectors of choice for films below 10 µin, such as ENIG, EPIG and ENEPIG. SDDs can also directly measure %P in electroless nickel deposits.