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intermetallic compound. The reaction rate is temperature-dependent. This intermetallic, like most intermetallic compounds, is difficult to solder, requiring a higher process temperature, a more active fluxing agent, or both. The Au-Cu system has poor solderability and the brittleness of this intermetallic results in diminished solder-joint strength. Ni-Au coatings are in widespread use worldwide. Ni coating thickness is generally on the order of 2.54 mm (>100 min). Au is a more difficult metal to plate. It tends to coalesce into islets, resulting in a porous coating. Any pores in the gold overcoat allow oxidation of the underlying Ni, rendering it locally unsolderable. To preclude this, the gold is liberally plated with a targeted minimum thickness greater than about 0.127 mm (5 min) to ensure complete Ni coverage. There are problems with the Ni-Au metallurgical system. First and foremost, it is well known that gold can cause solder joint embrittlement. When gold is introduced to tin as in tinlead solder, it forms alloys with both the Pb and Sn. The Au-Pb alloy has two predominant intermetallic compounds: Au2Pb and AuPb2. Of the two, the latter is of greatest concern. It begins to form at the Au-Pb eutectic (85 wt percent Pb, m.p., 215 C), a temperature that encroaches into the normal reflow solder processing regime. AuPb2 is stable below 254 C and is characterized by brittle, plate-like structures. Regardless of the composition, size, and concentration of the intermetallic grains in the solder joint, they can either strengthen it (forming miniature reinforcing bars in a solder cement ) or they can embrittle it, detracting from the solder s inherent ductility. Gold is non-oxidizing, like platinum and palladium, its noble metal neighbors in the periodic table. It readily alloys with most common solder alloy compositions, especially Sn-Pb and Sn-Ag-Cu. A. Korbelak and R. Duva16 consider gold the most readily solderable material, but Sn-Au intermetallics are of even more serious concern than those of Au-Pb. The former is a characteristically brittle intermetallic that has been widely linked to solder-joint failure. Too much gold in a tin-lead solder imparts a dull, grainy look to the resultant joint the only possible visual clue, although there are other conditions that can result in a similar appearance. There are marked differences in solder-joint integrity when it is loaded with Au. The embrittlement causes a reduction in the joint strength and fatigue life. Although there is much controversy over how much gold is too much, there is universal agreement that Au can be detrimental to the final assembly if its fractional weight-percent is not strictly controlled. The controversy is in part due to the type of product being soldered, its intended use, and its life cycle. If an electronic assembly is subjected routinely to high operating temperatures and/or frequent on-off power cycling, embrittled solder-joint fatigue problems will be evident. Although there is unanimous agreement that gold can be responsible for solder joint embrittlement and reduced solder-joint service lifetime, there is wide disagreement on the exact amount of gold permissible. Reported values generally fall between 2 wt percent and 10 wt percent, with most experts agreeing that less than 2 wt percent is a safe value.17, 18 Ebneter19 determined that Au as a thin, protective coating 0.762 mm to 5.84 mm (30 min to 230 min) does not pose an embrittlement problem. He also determined that the Au thickness required was a function of the plated grain size and porosity. Further, grain size is dependent on the type of plating bath (cyanide or acid) used. Hedrig20 also concluded that >5 percent Au in Sn-40Pb solder was problematic. Foster21 reported a small net increase in solder-joint strength with a 2.5 percent Au addition to the same alloy, but when the addition was as high as 10 percent, there was a marked degradation in solder-joint strength. C. J. Thwaites22 reported that 4 percent gold in a Sn-Pb solder joint is the threshold value above which there is a marked embrittlement and reduction in fatigue resistance. There are three important Au-Sn intermetallics: AuSn, AuSn2, and AuSn4. Eutectic points are at about 85 wt percent Sn (215 C) and 20 percent Sn (280 C). Gold can also form a ternary compound when joined with Sn-Pb solder, exhibiting a eutectic at about 175 C.23 This can radically alter reflow characteristics and solder-joint performance. Au, like Pb, is poorly soluble in Sn, which means that it dissolves in hot Sn, Pb, or Sn-Pb solder and precipitates out of solution during the liquid-to-solid phase change of the solder. When it does, it forms brittle, platelike crystals that appear needle-like in cross-section.24
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