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38 The PCB Magazine • February 2015 and chlorine, may negatively impact the useful life of the deposit. The impact can range from a slight discoloration of the deposit to the pads turning completely black. Proper packaging is a requirement for achieving a 12-month shelf life. The Immersion Silver IPC-4553 Specification In 2005, there were two distinct types of commercialized immersion silver with differ- ent thickness recommendations, referred to by the committee as "thin" and "thick." Each re- quired its own thickness specification. This cre- ated much confusion as the terms were poorly defined. The initial IPC-4553 Immersion Silver Speci- fication specified two thicknesses and stated the following for thickness of deposit: • Thin Silver: 0.05 µm(2µin) minimum at -2σ from process mean as measured on a pad of area 2.25² µm (3600² mils). Typical value 0.07 µm (3µin) to 0.1 2µm (5µin) • Thick Silver: 0.12 µm (5µin) minimum at -4σ from process mean as measured on a pad of area 2.25² µm (3600² mils). Typical value of 0.2 µm (8µin) to 0.3 µm (12µin). In 2009, the immersion silver specification was revised. At this time the lower thickness supplier has discontinued his product and the industry was left with a common thickness from multiple suppliers. This revised specifica- tion only had one thickness specified (elimi- nating reference to thin and thick). The revised specification now includes an upper thickness limit. Typical values were recommended within the specified limits. The pad size for taking the thickness measurement was also specified. The IPC-4553 Rev A Immersion Silver Speci- fication stated the following for thickness of deposit: • The immersion silver thickness shall be 0.12 µin [5 µin] minimum to 0.4 µm [16 µin] maxi- mum at ± 4σ from process mean. Typical value between 0.2 µm [8 µin] to 0.3 µm [12 µin] as measured on a pad of area 2.25 mm² or 1.5 mm X 1.5 mm [approximately 0.0036 in² or 0.060 in X 0.060 in] or equivalent. Immersion Tin (ISn) IPC- 4554, 2007; amended 2012 The immersion tin (ISn) is a metallic finish deposited by a chemical displacement reaction that is applied directly to the basis metal of the printed board, which is copper. The immersion tin is primarily used as a solderable surface for attachment of components. It may also be used when press-fit connections are employed and for zero insertion force (ZIF) edge connectors. The immersion tin finish protects the underlying cop - per from oxidation over the intended shelf life (storage of greater than six months) of this finish. Immersion Tin IPC-4554 Specification For immersion tin, the committee specified a lower limit for thickness. The relatively thick val- ue of 1 micron (40µin) was chosen to ensure that enough virgin tin would be available at the sur- face for soldering after extended storage. It is well understood that tin forms an intermetallic (IMC) layer with the underlying copper, and that this layer continues to grow in thickness over time. The immersion tin thickness will be: • µm (40 µin) minimum at -4σ from process mean as measured on a pad of area 2.25² µm (3600² mils) or equivalent. Typical value of 1.15 µm (46 µin) to 1.3 µm (52µin). The Immersion Tin Specification IPC-4554 was amended in 2011. The amendment ad- dressed solderability testing and specified the allowed stress testing conditions for the deposit and the type of fluxes to be used for both tin/ lead and LF testing. Electroless Nickel/Electroless Palladium/ Immersion Gold (ENEPIG) IPC- 4556, 2013 ENEPIG is a coplanar tertiary layered surface finish plated over copper as the basis metal. EN- EPIG consists of an electroless nickel base layer over which is plated an electroless palladium barrier layer followed by a deposit of a thin immersion gold as the final outer layer. It is a multifunctional surface finish, applicable to sol- dering and to gold, aluminum and copper wire bonding. It is also suitable as the mating surface IPC PLATING SUB-COMMITTEE 4-14: SURFACE FINISH SPECIFICATIONS continues feature