Choosing the Right Coatings Analyser for Circuit Boards

Demand for communications equipment, electronic wearables, the Internet of Things, plus the increasing dependence on electronics in the automotive and other industries, is driving rapid growth in the circuit board market.  Companies are looking for ways to improve quality, while keeping up with growth in electronic components and ever increasing quality expectations. 

Using the right analyser is an important part of this process. X-ray fluorescence (XRF) is a widely used coatings technique for measuring coating thickness and composition because it’s non-destructive, fast and straightforward.  XRF instruments come in many forms with features that determine how appropriate they are for your application.

 


Feature size

To measure coatings on small features, XRF coatings instruments use one of two ways to reduce the beam size of the X-ray tube to fractions of a millimetre (microns):  Traditionally used is a mechanical collimator, where a metal block with a small hole drilled through it is placed in front of the tube, allowing only the X-rays aligned with the hole to pass through and reach the sample.  Newer technology uses a polycapillary optic, which is a focusing optic made up of arrays of curved and tapered, small glass tubes.  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 in order to collect more of the tube output, focusing it onto smaller areas, with flux that is orders of magnitude greater than that of a mechanically collimated system. 

Polycapillary optics typically include only one spot size, designed to measure very small areas.  This is able to measure larger features as well, but it may be necessary to average the results from multiple measurement locations, or to perform a scan to get a representative result for the entire feature.  An instrument with multiple mechanical collimators may provide greater flexibility, since the collimator shape and size can be optimised for the feature to be measured.  Also, larger areas are analysed more quickly with a large collimator.

Since electronics and electronic components continue to get smaller and more complex, so surface finishes will need to be plated on smaller features. Although you may not need polycapillary optics today, it may be a good choice to help future-proof your purchase.

Complexity of your application

In any XRF instrument, an X-ray source is used to fluoresce elements present in a sample.  During fluorescence, each element emits a series of X-rays having characteristic energy.  With an energy dispersive XRF instrument, the detector simultaneously sees all of the elements that are excited by the X-ray tube.  The detector is one of two kinds: a gas-filled proportional counter, or a solid state semiconductor.  The most common solid state detectors are called PIN diode or silicon drift detectors (SDD).

One of the key differences between these detectors is the resolution - or the ability to clearly distinguish elements with similar energies.  When nickel and copper are present in a sample, as with ENIG or ENEPIG, a proportional counter does not fully resolve the nickel and copper peaks.  The instrument relies on mathematical deconvolution or sequential analysis, using additional hardware which can increase the measurement time.  A solid state detector can fully resolve these peaks, measuring both elements simultaneously for faster analysis.  It is possible to conform to IPC-4552A and IPC-4556 with either kind of detector.  For surface finishes where the elements are easy to resolve (eg. immersion silver, immersion tin or HASL), the resolution of the solid state detector does not offer any advantage.

In addition to the elements present in the sample, coating thickness and beam size are also considerations. Solid state detectors have an advantage. When measuring very thin coatings, SSDs have an advantage, as the background noise for these detectors tends to be lower than that of proportional counters.  If your plating is less than ~2 µin (0.05 µm), it is worth considering a solid state detector.  However, when measuring very small areas with a mechanical collimator, a proportional counter may offer better precision and measurement time owing to the larger active area. 

Functionality

XRF coatings instruments come in many shapes and sizes, even including handheld versions designed for measuring larger features and assemblies.  Your decision will be influenced by three considerations: chamber design, sample stage and advanced usability features. 

The chamber size and instrument footprint can be chosen to match the size of your boards.  Small boards can be handled easily by any size chamber, but larger boards are easier to handle and are better supported with bigger chambers with larger stages.  Some chambers are fully enclosed, which prevents the board from being moved accidentally during analysis.  Other chambers are slotted, which allows for easier sample loading.  With slotted chambers it is important that the base or stage is large enough to support the board so that it doesn’t bow, impacting the results.

Sample stages can either be fixed or motorised and programmable.  A fixed stage is sufficient for spot-checking one or a few locations on a board where the measurement area is easy to locate.  With a fixed stage, it is up to the user to manually position the sample, but boards with complex designs and very small features are better handled on a motorised stage which offers better precision for making fine adjustments.  Motorised stages also offer the ability to program a series of measurements at different locations across the sample, freeing up the user for additional tasks.

In addition to features commonly found in XRF instruments, there are also advanced features that can improve productivity and make is easier to operate the instrument.  One of these is a wide view camera.  In addition to the narrow field of view used to finely position the sample for analysis, some instruments offer a wider angle to show more of the sample.  This makes it easier and faster to move from one position to another and ensure you are measuring the right location.  Another feature is pattern recognition, where the software is taught to look at the sample image and pinpoint the exact, pre-defined measurement location.  This saves the user a lot of time in set up.

The FT, X-Strata and MAXXI series of coatings analysers from Hitachi High-Tech Analytical Science, offer a comprehensive range of beam sizes, detectors and configurations.  As many applications can be carried out by any of these instruments, the ultimate choice is the one that balances functionality, performance and cost for circuit boards you produce now, and those you plan to be producing in the future.  Our coatings experts are committed to providing you with the right solution.

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, plus benchtop and handheld electromagnetic gauges.  

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Date: 20 March 2018

Author: Matt Kreiner, Benchtop XRF Product Manager

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