Surface finishes Electroless Nickel/immersion gold

The substrate surfaces of choice are usually one of the following electroless nickel/immersion gold (ENIG), immersion silver (ImAg), and organic solderability preservative (OSP). Two other finishes, hot air solder leveling (HASL) and immersion tin, are also in use, but HASL has declined in volume and immersion tin has not reached significant volumes due to concerns over reliability. 

Other surface finish-delated defects Immersion silver (Imm-Ag) sometimes exhibits hn-ear arrays of microvoids at the intermetalhc interface which detracts from solder joint strength. Electroless nickel/immersion gold (ENIG) sometimes results in brittle fracture if the plating chemistries are not maintained properly. 

This chapter reviews Uteratme on the microstructure of solder joints and the interactions of solders with substrates and related solder joint reliability issues. The substrates here are limited to Cu, Ni-coated Cu, electroless nickel/ immersion gold (ENIG), and hot air solder leveled (HASL) Sn-Pb. The solders are mainly Sn, eutectic or near eutectic Sn-Ag, and Sn-Ag-Cu alloys. Specific reliability issues discussed here include black pads of ENIG, gold embrittlement, compatibility of Pb-free solders with Pb-containing surface finish, and Kirkendall voids. 

N. Biunno, A Root Cause Failure Mechanism for Solder Joint Integrity of Electroless Nickel/Immersion Gold Surface Finishes, Proceedings of the IPC Printed Circuit Expo, Paper S18-5, March 14-18, 1999... 

There are a number of other surface finishes used in the industry, such as Electroless Nickel Electroless Palladium Immersion Gold (NiPdAu), Immersion Silver, Immersion Gold, Immersion Tin, OSP and Electrolytic Nickel Gold. There are reliability and process trade offs with each surface finish. That is why it is recommended that strain/strain rate characterization and thermal cycling be performed for each set of surface finish before it is selected for the specific end-use conditions in which it will be used. The industry test methods used to evaluate different surface finishes are outlined in detail in the next chapter. 

Other surface finishes include reflowed tin-lead, electrolytic nickel/electrolytic gold, antitarnish and reflux, electroless palladium, electroless gold, direct immersion gold, and solid solder deposit. 

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