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April 2015 • The PCB Design Magazine 15 IPC SURFACE FINISH SPECS UPDATE continues feature 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 underly- ing copper 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 rela- tively thick value of 1 micron (40µin) was chosen to ensure that enough virgin tin would be available at the surface for soldering after extended storage. It is well understood that tin forms an interme- tallic (IMC) layer with the underlying copper, and that this layer continues to grow in thickness over time. The immersion tin thick- ness 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 im- mersion gold as the final outer layer. It is a multi- functional surface finish, applicable to soldering and to gold, aluminum and copper wire bonding. It is also suitable as the mating surface for soft membrane and steel dome contacts. Additional applications include use in low insertion force (LIF) and zero insertion force (ZIF) edge connec- tors and for press-fit applications. The electroless palladium layer forms a diffusion bar- rier that impedes nickel diffusion to the gold surface. The immer- sion gold protects the palla- dium layer from reacting with contaminants prior to pro- cessing that might otherwise affect joining processes, such as wire bonding and solder- ing. ENEPIG has a minimum shelf life of 12 months under standard storage conditions. Thickness specifications are set to ensure the ability of the finish to meet all the attribut- ed functionality. The ENEPIG IPC-4556 Specification This is the last specification issued by the committee. The document produced is very com- prehensive and includes a wealth of information from the RR studies that were conducted. The Appendix contains a documentation of these studies, each authored by the princi- pal who conducted the testing. It also includes a section on the proper methods of equipment setup for a reliable measurement of very thin layers of metal deposits. The thickness specification for ENEPIG states: • Nickel: 3 to 6 µm [118.1 to 236.2 µin] at ± 4 sigma (standard deviations) from the mean. • Palladium: 0.05 to 0.15 µm [2 to 12 µin] at ± 4 sigma (standard deviations) from the mean. • Gold: minimum 0.025 µm [1.2 µin] at - 4 sigma (standard deviations) below the mean. No upper limit was set for IG. All measurements to be taken on a nominal pad size of 1.5 mm x 1.5 mm [0.060 in x 0.060 in] or equivalent area. enePIG is a coplanar tertiary layered surface finish plated over copper as the basis metal. enePIG 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. " "