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72 PCB007 MAGAZINE I FEBRUARY 2020 oped for reliable contact performance, includ- ing visual references, adhesion, solderability, cleanliness, and electrolytic corrosion. The fo- cus of the document is on the specific thick- ness range for the nickel, palladium, and gold layers. It's essential that the palladium layer be thick enough to impede nickel diffusion to the gold surface, helping to prevent hyper- corrosion of the electroless nickel deposit. (Hyper-corrosion results in unreliable solder joints). Yet, if the palladium layer is too thick, the solder joint can become brittle and may eventually fail. The gold layer is required to protect the palladium layer from contamina- tion that may adversely affect wire bonding and soldering and must be above the specified thickness. Adherence to the specification helps PCB manufacturers deliver products that meet the IPC Category 3 life of at least 12 months. Measurement of the Layer Thickness IPC-4556 states that XRF methodology is required to determine the layer thickness. Through extensive testing using XRF in the development of the specification, the IPC has set out a detailed set of measurement crite- ria, including equipment set-up, measurement protocols, and calibration advice. Those using XRF instruments must be aware of a number of factors affecting the results if they are to as- sure accurate and reliable thickness measure- ments. These include the following. Sample Size Plating thickness varies with the plating ar- ea with smaller features resulting in a thicker plating layer. Therefore, it is essential that the pad size used for measurement is consistent for both calibration and production readings. Calibration Standards The IPC recommends the use of nationally traceable calibration standards consistent with thicknesses to those measured on the devic- es in production. A gauge R&R, or equivalent statistical methodology, should be carried out. Furthermore, calibration standards should be checked frequently. XRF Instrument Software Many XRF instruments come with back- ground correction software, which is designed to eliminate background scatter from the sub- strate that can give incorrect readings. This feature may need activating, and the user will need to determine how to do this, if applicable. Detector Type The detector must be capable of measur- ing tri-level thin coatings. Solid-state detectors (SSD) provide better resolution than propor- tional counter systems; however, there may be a trade-off in longer measurement times for the SSD, depending on the age and capability of the XRF instrument. The IPC-4556 guidelines help to ensure good quality and predictable, repeatable shelf life of the ENEPIG surface finish. However, careful consideration and understanding of the XRF instrumentation and associated software, as well as using the right calibration procedure, are all essential to ensure accurate layer thick- ness measurements with XRF. Next Steps in Meeting the Outlined IPC Specifications Hitachi High-Tech Analytical Science is a member of IPC, and we support customers who conform to IPC standards to achieve qual- ity and reliability within PCB manufacturing. We have created a complimentary IPC guide for manufacturers, which can help you to conform to compliance with IPC-4552A, IPC- 4553A, IPC-4554, and IPC-4556 specifications which you can find here. PCB007 Matt Kreiner is Hitachi High-Tech analytical science's product business development manager for their coat- ings analysis product line. In his current role, he focuses globally on the company's coatings customers across numerous industries to find new solutions to the challenges they face in their everyday work environment. He has over 15 years of experience working with XRF technology. Kreiner resides in Chicago and holds a B.S. in chemical engineering from Northwestern University.

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