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FEBRUARY 2019 I SMT007 MAGAZINE 15 Metallurgical Science vs. Properties [2] Metallurgically, Ag forms a second phase— primarily e (Ag 3 Sn) in the Sn matrix. The mor- phology of Ag 3 Sn particles, intermetallic in nature, can range from nodular to long-needle shapes; Cu interacts with Sn to form interme- tallic compound h (Cu 6 Sn 5 ) in essentially nod- ular shapes. The element Bi works differently in the Sn matrix. The solid solubility of Bi sol- ute in Sn solvent can reach about 21 wt% at the eutectic point (138°C). However, the solid solubility of Bi solute in Sn solvent can be dra- matically reduced with temperature, which is about 1.0 wt% at room or ambient tempera- ture. Additionally, the three elements (Ag, Cu, Bi) are expected to interact mutually in a ther- modynamically competitive manner. In mechanical behavior, the yield strength of the SnAgCuBi quaternary system generally follows an approximation of the linear rule of mixture in the volume fraction of the second phases (Ag 3 Sn), the volume fraction of inter- metallic compound (Cu 6 Sn 5 ) and the volume fraction of Bi precipitates in the Sn matrix. In accordance with Mott and Nabarro's strain field theory, the strengthening effect of Ag 3 Sn particles is interpreted as the result of the long-range internal stress built by the elastic modulus and volume differences between the second phase and the Sn matrix. The mobile dislocations in the soft Sn matrix can largely pass by the Cu 6 Sn 5 particles in a largely free manner due to the relatively larger interpar- ticle spacing. The strengthening mechanism of the elastic Cu 6 Sn 5 particles is attributed to the building of an elastic internal stress field in the Sn matrix, giving a backstress for dislocation movement. In fatigue life, like other alloy systems, the underlying operating mechanism is more com- plex, engaging in multiple events and varying with strain amplitudes. At the relatively large strain amplitudes, fatigue crack propagation is a dominating event throughout the fatigue lifetime. At the small strain amplitudes, fatigue crack initiation is a dominating event through- out the fatigue lifetime. In terms of the mechanisms identified, at the large strain amplitudes, Ag 3 Sn particles are a much more effective block for the fatigue crack propagation than the Pb-rich second phase in 63Sn37Pb. The Cu 6 Sn 5 particles in the Sn matrix are expected not to be fatigue-fractured at the fatigue conditions. Like the Ag 3 Sn particles, the Cu 6 Sn 5 particles serve as effective barricades for fatigue crack propagation. The formation of Cu 6 Sn 5 particles also can partition Sn grains, resulting in finer grains, which contributes to the extension of fatigue lifetime by enhancing grain boundary gliding mechanisms. At the small strain amplitudes, fatigue crack initiation is a dominating event throughout the fatigue lifetime. Since the cyclic deformation in the process of fatigue crack initiation almost entirely takes place in the Sn matrix, the Ag 3 Sn particles and Cu 6 Sn 5 particles are expected to play little role in retarding the cyclic deforma- tion damage or fatigue crack initiation. With the presence of Ag and Cu in the Sn matrix, the overall fatigue life is controlled by the fatigue fracture capacity of either the Sn matrix or in- terphase bonding. In this regard, Bi, on the other hand, offers additive-enhancing events in the Sn matrix, which compensates the defi- ciencies of Ag and Cu. However, its dosage is an overriding factor. As mentioned earlier, alloy's melting tem- perature near-linearly decreased with an in- creasing dosage of Bi. Does this imply that to achieve a lower melting temperature and thus the process temperature, we can build a high dosage of Bi into the SnAgCuBi? The answer is a resounding no. The attainment of a delicate and tricky bal- ance among the dosages of all three elements is the key to delivering the desired perfor- mance; simply put, this means the balanced strength and fatigue resistance as well as the desirable manufacturability. In electronics, to serve as sound solder joints that connect semi- conductor packages to the outside world, it is safe to say that the mechanical strength can readily be obtained (within the scope of the electronic circuit board). The ability of resis - tance to thermal fatigue is the top priority in the performance of solder material to produce reliable products (barring other extraneous failure modes).

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