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70 PCB007 MAGAZINE I FEBRUARY 2020 on the thickness of the gold layer; the mini- mum allowable thickness has been reduced, and a new parameter for the maximum gold thickness was introduced. If the gold thickness is too low, the deposit may not remain intact once in use. This would result in corrosion and cause weak solder joints and board failure. With the critical nature of the gold thickness, the specification focuses on three factors in order to: 1. Control the ENIG plating to produce a normal and reliable distribution of thick- ness for both nickel and gold deposits. 2. Determine the accuracy of the instrument used to measure deposit thickness. 3. Reliably yield consistent and uniform deposit characteristics. In addition, the IPC-4552A gives details on corrosion identification, with measurements required for pass and fail criteria. The update to the specification required extensive testing by the IPC and associated organizations, and the specification now yields a high-quality fin- ish. However, the stringent measurement con- trols imposed can be difficult to meet in a pro- duction environment without the use of an XRF instrument to precisely control thickness. IPC-4552A also refers to XRF calibration methods required to meet the statistical re- quirement of the specification. To use XRF, the instrument must meet specified performance requirements, including stringent criteria to measure the gold thickness. There are two ma- jor classes of XRF detectors suitable for plating analysis: proportional counters and high-reso- lution semiconductor detectors, such as a sili- con drift detector (SDD). Both may be used to conform to IPC-4552A. It is incumbent on the production facility to evaluate their existing equipment, and decide which detector tech- nology is most appropriate for their process, as this will have an impact on analysis time and control tolerance. High-resolution detectors are best because they can differentiate gold peaks from copper and bromine. Avoid Oxidization on Your PCBs With the Immersion Silver Standard: IPC-4553A Immersion silver's main function focuses on RoHS compliant finishes which protect the underlying copper from oxidation over its life- time. This thin deposit's main function is to serve as a solderability preservative, by dissi- pating into the solder to leave a clean copper surface for soldering. The IPC-4553A specification details the pa- rameters for the immersion silver surface fin- ish in a production environment that results in a reproducible and stable sol- der joint. The first immersion sil- ver specification, IPC-4553, was issued in 2005 and transitioned to an update in 2009 where the supply of the thin deposit was re- duced. The thick version became the industry norm as captured in IPC-4553A. The revised specification out- lines an upper and lower limit to the immersion silver thickness deposit to improve quality control and field reliability of the compo- nent. The regulation is designed to provide a reliable surface fin- ish for a shelf life of 12 months per IPC J-STD-003. It focuses on preventing a thickness that is too Figure 2: Loading boards into XRF for plating thickness measurement.

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