Four Reasons to Choose Polycapillary Optics for XRF Coatings Analysis


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As electronics and electronic components continue to shrink and increase in complexity, metal finishes on these components need to be plated onto smaller features—as thinner layers—with tighter controlled tolerances. If finishes are too thin, the product won’t meet performance specifications and could fail prematurely, risking warranty claims, safety and damage to reputation. If platings are too thick, the cost of plating material increases and money is wasted, plus there are possible issues with mechanical fit of the plated components that could result in costly scrap or rework.

X-ray fluorescence (XRF) is a widely used technique for measuring coating thickness and material composition because it’s non-destructive, fast and straightforward to use. To measure coatings on small features, traditional XRF instruments use a mechanical collimator to reduce the beam size of the X-ray tube down to fractions of a millimeter. This is achieved through the instrument by placing a metal block, with a small hole drilled through it, in front of the X-ray tube, allowing only the X-rays aligned with the hole to pass through and reach the sample. Using this method, a vast majority of the X-ray tube output cannot be used for analysis, as it’s stopped by the collimator block. 

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Figure 1: A comparison of the collimator and capillary methods.

Today’s approach to address the need to measure fine features is to use a polycapillary optic. This type of focusing optic is comprised of arrays generated from thousands of small and hollow glass tubes which are curved and tapered. Using this method XRF can easily accommodate a wide variety of geometries relative to the complexity and miniaturization of components which typically need to be plated within the electronics industry. Within a polycapillary optic, the X-rays are guided through the tubes using reflection, which is very similar to the way light is guided in fiber-optic technology. The polycapillary optic is paired to a micro-spot X-ray tube to collect more of the tube’s output. This focuses it onto smaller areas with flux that is orders of magnitude greater than that of a mechanically collimated system. Polycapillary optics in XRF coatings analyzers have several advantages:

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Figure 2: Chart detailing how a polycapillary optic functions.

1.  Smaller Feature Measurement

Polycapillary optics have a beam size of less than 20 µm, making it possible to measure ultra-fine features on microelectronics, advanced circuit boards, connectors, lead frames and wafers. This allows measurement of areas that can’t be achieved with the traditional mechanical collimators.

2.  Thinner Coatings Measurement

By focusing more X-ray tube output onto the sample, an XRF analyzer fitted with a polycapillary optic can measure nanometer-scale coatings and measure thicker coatings with better precision.

3.  Increased Testing Throughput with Higher Confidence

A greater intensity generated by the optic results in higher count rates. In XRF, higher count rates translate into improved precision and faster results. This allows for more measurements to be taken in any given time period and higher confidence in the results, leading to better quality control and tighter production.

4.  Easier Conformity to Specifications

XRF plays an essential role in determining and controlling finish thickness to meet with performance specifications IPC-4552A for ENIG (electroless nickel immersion gold) and IPC-4556 for ENEPIG (electroless nickel electroless palladium immersion gold). In using XRF to meet the required specifications, analyzers must demonstrate performance levels within a defined tolerance. Using a capillary optic makes it easier to achieve this performance level and allows operations to run as close as possible to the minimum control limits, saving money on materials and chemicals. With recent advances in software, it is now possible to simultaneously measure the thickness and composition of electroless nickel coatings under gold and palladium, assuming these layers are thin enough to allow X-ray transmission. 

Combining a polycapillary optic with a high-resolution detector such as a silicon drift detector (SDD), plus a high-precision stage, high-definition camera and clever software provides the ultimate analysis of ultra-fine coatings on ultra-fine features.

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