Process Control of ENIG and ENEPIG Surface Finishes with X-Ray Fluorescence

Two very common uses of x-ray fluorescence (XRF) spectrometers are measuring specific coatings used in the manufacturing of printed circuit boards; ENIG and ENEPIG coatings. The ENIG coating is immersion gold plated over electroless nickel on the bare board (Au/ENi/Cu) while the ENEPIG coating has a layer of electroless palladium in between the gold and nickel (Au/Pd/ENi/Cu) because it acts as a diffusion layer and limits nickel diffusion to the gold layer. These coatings have been available for some time and have become popular within the electronics industry because they provide a lead free option with a long shelf life, excellent solder-ability, and an ideal surface for wire bonding.

However, as you may already know, these coatings are relatively expensive options because of the gold, palladium, and nickel involved - so the question we hear is 'what is the best analytical tool for measuring ENIG and ENEPIG coatings?

Why Measure with X-Ray Fluorescence?
As noted above, the metals involved make ENIG and ENEPIG coatings relatively expensive. So, controlling costs is one reason that a suitable x-ray fluorescence analyzer is a critical tool for organizations plating ENIG and ENEPIG...an appropriate approach to process control can limit wasted spend associated with over-plating. Additionally, both coatings call for ultra thin layers of gold (typically 1 - 2 u") so an analytical method with extreme precision can confirm tolerances are where they need to be in order to maximize profits while meeting industry standards.

As of this writing, the industry standard of IPC-4552A covers ENIG while IPC-4556 is for ENEPIG. These factors combined with XRF's ease of use and non-destructive approach make it a common choice for quality control of ENIG and ENEPIG. 

XRF Spectrometer Considerations 
When considering the use of x-ray fluorescence (XRF) for measuring ENIG and ENEPIG coatings, one needs to consider the reasons that they are making the investment. Depending on those reasons, an organization may look to a low cost proportional counter detection system or make the investment into a higher resolution silicon drift detector based analyzer (referred to as SDD). If an organization simply wants to know that they are within a certain, self imposed, range of thickness and ultimate precision isn't critical then a proportional counter based analyzer may be the right option - these analyzers, like the Hitachi FT110A or many used XRF analyzers, provide a lower cost of acquisition and lower replacement component costs when that time comes.

However, proportional counter systems come with some potential negatives like errors from Bromine content (typically resolved with peak de-convolution software), errors resulting from varying phosphorous content in the ENi layer, and they may not meet future industry standards for testing.

So, organizations that are working towards meeting one of the industry standards (IPC-4552, IPC-4556) or simply want the highest resolution detector technology should make a silicon-drift detector (SDD) based system the priority. The improved resolution of this detector technology (see Fig1) results in thinner measurement capabilities with high precision. Additionally, SDD based models like the Hitachi 6000VX and X-Strata920 with SDD can actually measure the Phosphorous content in ENi, adding another layer of process control.

Comparing XRF Detector Performance
As an Authorized Distributor of Hitachi High-Tech, associates of Eastern Applied Research can support your review of systems with either detector styles. The Hitachi XRF line (formerly Seiko XRF) includes the previously mentioned EA6000VX and X-Strata920 along with the FT160 for when SDD is of interest. We also offer the FT110A and used options for proportional counter system options.

Eastern Applied has data available comparing the performance of these systems and can accept samples in to provide custom reports for review. Contact the XRF Sales Team to learn more. 

Summary: As noted above, the organization making the investment needs to look at both their short term and long term testing goals. For a low cost system to check within wider tolerances, a proportional counter XRF can be considered. However, analyzers with SDD technology should be the priority for long-term adherence to industry standards, improved precision on thin films, and lower thickness capabilities. Contact Eastern Applied Research to discuss further.

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