Four reasons to use polycapillary optics for coatings analysis

As electronics and electronic components continue to get smaller and more complex, metal finishes on these components need to be plated on smaller features - as thinner layers - and controlled to tighter tolerances. If finishes are too thin, the product won’t meet performance specifications and could fail prematurely, risking warranty claims and reputation damage. If platings are too thick, materials and money are wasted, and there are possible issues with mechanical fit that could result in costly scrap or rework.

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

The newer approach to measuring fine features is to use a polycapillary optic. This is a focusing optic made up of arrays of small glass tubes that are curved and tapered.  X-rays are guided through the tubes by reflection, similar to the way light is guided in fiber optic technology.  The polycapillary optic is matched to a micro-spot X-ray tube to collect more of the tube 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 analysers have several advantages:

1  Smaller Features Measurement

Polycapillary optics have a beam sizes 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 mechanical collimators.

2  Thinner Coatings Measurement

By focussing more X-ray tube output onto the sample, an XRF analyser fitted with a polycapillary optic  can measure nanometer-scale coatings.

3  Increased Testing Throughput with Higher Confidence

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

4  Easier Conformity to Specifications

The use of XRF in determining  and controlling finish thickness is allowed by performance specifications IPC-4556 for ENIG (electroless nickel / immersion gold) and ENEPIG for (electroless nickel/electroless palladium/immersion gold) analysis these tests must demonstrate performance levels within a defined tolerance.  Using a capillary optic makes it easier to achieve this performance level.

The FT150 series of coatings analysers from Hitachi High-Tech Analytical Science, combines a polycapillary optic with a high-resolution Vortex® detector, a high-precision stage, high-definition camera and clever software for the ultimate analysis of ultra-fine coatings on ultra-fine features. 

Hitachi High-Tech has 40 years of coatings analysis expertise, having developed over 1,000 applications with a range of products that includes: microspot, benchtop and handheld XRF instruments, as well as benchtop and handheld electromagnetic gauges.   

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Date: 14 February 2018

Author: Matt Kreiner, Benchtop XRF Product Manager

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