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When an intermetallic forms a distinct interfacial layer or otherwise high localized concentration, it is a rigid structural element, prone to cracking and brittle fracture failure on flexing. Therefore, even though the solder joint is composed of a ductile Sn-Pb composition, its overall strength and endurance is dominated by the brittle intermetallic composition and its thickness and structure within the solder joint. As component lead pitch decreases, so does the amount of solder-per-joint contribution from the solder land metallurgy. Au concentration becomes increasingly difficult to control because its plated thickness cannot be easily regulated to accommodate the reduced surface area and lower solder volumes that accompany finer-pitch component requirements.After all, Au deposit thickness is dependent on its deposition grain size and plated porosity. The porosity must be overcome in order for the Au to serve its function in preventing oxidation of the underlying nickel on a Ni-plated substrate. Another pitfall of Cu-Au or Cu-Au-Sn systems is that of Kirkendall void formation. Pores in the solder joint or at copper-gold interfaces occur through solid-state diffusion. Copper, which has a relatively high solubility and solid-state mobility in gold, diffuses into the gold. Transported by grain boundary diffusion at temperatures below 150 C and by bulk diffusion above that temperature, Au-Sn regions result along with atomic vacancies. When lattice vacancies are in profusion, voids can be observed, and this material depletion weakens the solder joint. Several references are available on this phenomenon.25,26,27,28 Kirkendall voids should in no way be confused with voiding that results from entrapped gases, liquids, or residues from solder paste in the surface-mount process or from fluxes or from PWB laminate materials in wave soldering. Although there is still much controversy on the subject of Au concentrations and effects on the solder joint, there is no doubt that this metal system is being widely exploited successfully in electronics manufacturing. It is, however, prudent to ensure that the Au levels are kept to a minimum via adequate vendor management and implementation of good process control methods for solder deposition. Note that because gold is a non-oxidizing material and therefore does not deplete the flux, more flux is available to work on the other components of the metallurgical system namely, the underlying nickel (which must be kept from oxidizing), the solder, and the component lead. The Ni-Sn intermetallic is more brittle than that of Cu-Sn, and this surface finish is generally not recommended for applications where the product may experience high mechanical stresses such as shock or vibration. It has been stated that in the Ni/Au system, the soldering actually occurs on the Ni surface and the Au serves as a protective coating. Another intermetallic of this system, NiSn3, has been cited as a problem in long-term solderability degradation.29 This IMC rapidly develops a platelet structure at low temperatures, with the greatest growth rate in the vicinity of 100 to 140 C. When intermetallic platelets grow large enough, they may penetrate the surface of the overlying tin, oxidize, and become a material difficult to flux and solder. The presence of elemental Pb was determined to retard NiSn3 IMC formation. Ni/Au surface finish is in use with Pb-free soldering, but there is currently a watch in the industry as sporadic brittle fracture has been observed on occasion. The exact nature of this fracture mechanism is yet to be understood and it is not certain that Ni/Au surface finish is at fault or just coincidental with some failures. 45.6.4 Electroless Nickel/Immersion Gold (ENIG) As the name implies, ENIG is not an electroplated surface finish. Still comprised of Ni with an overcoat of Au, ENIG is a lower-cost alternative than electrolytic Ni/Au. When the plating process is kept in control, it gives an excellent, solderable, and reliable surface finish. On the other hand, it is a finish that has caused many well-known reliability problems. Terms such as black-pad, black line nickel, and mud cracks describe some of the evidence of failed ENIG-related problems. Although these phenomena are most commonly observed on BGA packages plated with ENIG for package-to-ball attachment,30 the failure has also been seen on solder joints on PWBs plated with an ENIG surface finish.
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