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Electronic Connections
Published in Milenko Braunovic, Valery V. Konchits, Nikolai K. Myshkin, Electrical Contacts, 2017
Milenko Braunovic, Valery V. Konchits, Nikolai K. Myshkin
Due to a limited solid solubility of Ag in Sn, this alloy is resistant to coarsening and thus has a more stable, uniform microstructure and is more reliable. Although the Sn–3.5Ag alloy itself exhibits good microstructural stability, when soldered to copper base metal, the combination of a higher Sn content (96.5Sn compared to 63Sn) and higher reflow temperature environments accelerates the diffusion rates for copper base metal in Sn. Upon reaching the composition corresponding to Cu6Sn5, the nucleation and growth of brittle intermetallic compound occurs. The use of alternative surface finishes, such as immersion gold (Au over Ni over Cu), slows the diffusion rate and thereby decreases the growth kinetics. Diffusion of Cu into the solder and formation of the brittle Cu6Sn5 intermetallic compound can be limited using Ni in the immersion gold coating as a diffusion barrier. Other surface finishes such as immersion silver (Ag over Cu) and immersion palladium (Pd over Cu) do not contain a Ni barrier layer.622
Reliability issues of lead-free solder joints in electronic devices
Published in Science and Technology of Advanced Materials, 2019
Nan Jiang, Liang Zhang, Zhi-Quan Liu, Lei Sun, Wei-Min Long, Peng He, Ming-Yue Xiong, Meng Zhao
Cu has good solderability and thermal conductivity and has been widely used as a substrate material in electronic packaging [19]. At present, Ni, Ag, and Au were plated on Cu substrate to inhibit the growth of interfacial IMC. Ni is used as a barrier between solder and Cu substrate. Since the reaction rate between solder and Ni is effectively lower than that between Cu and solder, thereby causing the thickness of interfacial IMC is lower [20]. On Ni/Ag and Ni/Au plated substrates, Au and Ag can inhibit the velocity that Ni and Sn reactions to form Ni3Sn4, so its IMC layer is relatively thinner than that of Ni plated substrates [21]. The electroless nickel/immersion gold layer has also been extensively used because of its superior wettability. The Au layer can prevent the surface of Cu substrate from being corroded and oxidized. The electroless Ni plating layer can significantly reduce the interdiffusion rate of Cu atoms and Sn atoms [22]. The addition of trace alloying elements to the Cu substrate can change the microstructure of the substrate, thus improving the performance of the substrate. Maeshima et al. [23] discussed the effect of adding Ni on Cu substrates on the IMC of solder and found that the addition of Ni can effectively inhibit the germination and growth of IMC. The main reason is that the addition of Ni element causes the IMC to form a layer (Cu1-xNix)6Sn5 near the side of the Cu substrate, thereby inhibiting the germination and growth of the IMC during the aging process and improving the reliability of the solder joint. Figure 2 shows the interfacial structure of Ni content after adding different contents of Ni to Sn-0.7Cu. It can be seen that the thickness of Cu3Sn layer decreases gradually with the increase of Ni content, and the thickness of Cu6Sn5 layer increases remarkably. The higher the activation energy, the faster the growth rate of IMC. Table 1 shows the activation energies for growth of different IMC layers. It is found that the addition of Ni reduces the activation energy of Cu6Sn5 and total IMC, and improves the activation energy of Cu3Sn. However, when the Ni content is too high, loose IMC are likely to form at the interface of solder joints, leading to the reduction of solder joint strength and influencing the reliability of solder joints. Annealing the substrate is to heat the substrate to a temperature for a sufficient period of time and then cool. Annealing the substrate can change its microstructure, thereby inhibiting the germination and growth of IMC and improving the reliability of solder joints. Kim et al. [24] studied the influence of annealing Cu substrate on interfacial IMC and found that annealing Cu substrate can effectively inhibit the growth of IMC. The grain of the Cu substrate was refined during the annealing process, which made the structure more uniform and stable and eliminate the internal stress and tissue defects of the Cu substrate. That can effectively reduce the diffusion rate of Cu atoms, thereby suppressing the germination and growth of the interface IMC.