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FEBRUARY 2020 I PCB007 MAGAZINE 71 low, so that the copper will not oxidize during the soldering process, causing the joint to fail in production. If the deposit is too thick, the solder joint may eventually weaken and fail in the field. As well as finish thickness specifications, the IPC-4553A gives parameters for porosity, adhesion, cleanliness, electrolytic corrosion, chemical resistance, and high-frequency sig- nal loss. Because silver is an active substance and tarnishes when combined with sulfur, packaging and storage guidelines are given to minimize contact of the silver surface with the environment. The IPC-4553A specification gives maximum and minimum silver layer thicknesses for a specific pad size (60 x 60 mils). This is impor- tant because the thickness of the deposit var- ies with the coated area. Measurement of the layer thickness is carried out with XRF equip- ment. However, it's extremely important that the equipment is set up correctly for immer- sion silver thickness measurements. The speci- fication gives detailed guidelines on how to do this; however, regular and rigorous calibration of the XRF equipment is important. Manufac- turers must use a calibration standard of sil- ver over copper with a deposited thickness and pad size of an order of magnitude of the actual production values. Ensure a Long Shelf Life of Immersion Tin Surface Finishes With IPC-4554 Using XRF The 2007 IPC specification IPC-4554 is ap- plicable to the production of immersion tin as a surface finish for printed circuit boards. It re- lates specifically to the solderability of the fin- ish for reliability and reproducibility and tack- les the difficulty in extending shelf life for over six months for this surface finish type. The immersion tin finish is a single layer of tin that is deposited directly over the copper surface of the printed board. The function of the layer is twofold: to provide a protective surface to prevent oxidation of the copper and create a reliable solderable surface. Tin may also be used for press-fit connections, and also as the interface for zero insertion force (ZIF) edge connectors. However, the specification from IPC focuses solely on tin as a surface for soldered joints. Because copper and tin have a strong affin- ity for each other over time, the two metal lay- ers will diffuse across the copper-tin boundary. Eventually, this will compromise the integrity of the tin over the copper, and the oxidation of the underlying surface will occur, having a negative effect on solderability and shelf-life. Delaying this inevitable diffusion is key to ex- tending the shelf life of the component. Fundamentally, the thickness of the tin de- posit determines the longevity of the part. Therefore, the aim of IPC-4554 is to provide a standard tin thickness for manufacturers to achieve a reliably solderable finish for all sur- face-mount and through-hole assembly appli- cations. Adherence to the specification should result in components with a Category 3 durabil- ity per J-STD-003 for a shelf life of six months. While XRF instruments are used to mea- sure the coating thickness, older XRF equip- ment may struggle to differentiate between the tin and other elements present in the coating, thereby resulting in inaccurate readings. The IPC-4554 specification helps manufacturers to ensure they're carrying out accurate measure- ments by giving detailed XRF instrumentation calibration instructions, including the use of XRF standards specifically for this type of mea- surement. A full discussion of the use of foils over polyester (to prevent the impact of the dif- fusion distorting the results) is given, with rec- ommendations on how to incorporate this into production-scenario XRF calibration. Conforming to IPC-4556 With XRF IPC-4556 is the specification for the electro- less nickel/electroless palladium/immersion gold (ENEPIG) surface finish for printed circuit boards. Released in January 2013, the specifica- tion presents the detailed guidelines for achiev- ing a reliable PCB surface finish for optimal shelf-life, solderability, and wire bonding for gold, copper, and aluminum wire applications. The specification covers a range of PCB sur- face finish parameters that have been devel-