| The rapid development of the high-density solder interconnection in the electronic packaging industry brings about the continiuous decrease in the size and stand-off height (SOH) of solder joints. Therefore, the present paper focuses on the interfacial reactions and reliability of solder joints influenced by the SOH lower than100μm. The interfacial reactions in the solder joints with same or different base materials at the two sides become more complicated with the decreasing SOH, which significantly influences the microstructure and mechanical property. And it is a big challenge for the reliability of electronic products.In the present paper, three different kinds of solder alloys, Sn, Sn37Pb and Sn9Zn, were used to join Cu bulk and Ni bulk for obtaining the sandwich-structure solder joints with SOH of 100μm,50μm,20μm and 10μm. These researches revealed the influence of SOH on the microstructure and the mechanical property of solder joints. The research results are listed as follows:The study on the as-reflowed Cu/Sn/Cu revealed the influence of SOH on the solder bulk microstructure, IMC thickness, IMC proportion and the mechanical property of solder joints. With the reducing SOH, the IMC thickness decreases, while the volume ratio of IMC increases. And also, the consumed Cu thickness decreases, while, the Cu concentration increases in the solder bulk with the reducing SOH. Tensile tests with a constant crosshead speed of 0.1mm/min were carried out to assess the mechanical property. It was found that the strain ratio of solder bulk increases with the reducing SOH, which improves the ultimate tensile stress (UTS) of solder bulk. When the SOH is reduced to be 10μm, the solder bulk consists of one Sn grain in height. The tensile fracture occurs in the IMC layer with the delamination of Cu6Sn5/Cu interface and the cleavage of Cu6Sn5 grain. And this is different from the fracture modes of solder joints with the other three SOHs, which all fractures in the solder bulk in a plastic mode.The study on the Cu/Sn/Cu solder joints aged at 150℃revealed the changes of microstructure and mechanical property. In the Cu/Cu3Sn/Cu6Sn5/Sn diffusion couple at the one side of solder joint, Cu and Sn diffuse in opposite direction, while, the their diffusion fluxes change with the growth of Cu3Sn and Cu6Sn5 during the thermal aging process. Based on the ratio of Cu to Sn diffusion fluxes at the Cu3Sn/Cu6Sn5 interface, the growth model of Cu6Sn5 layer and Cu3Sn layer has been established. According to the growth model, the Cu3Sn grows fast at the cost of Cu6Sn5 layer at the early stage, while, the thickness of the total IMC layers increases with the aging process. Moreover, the thinner original Cu6Sn5 layer, the faster the IMC growth. With the aging process, the growth rate of Cu3Sn slows down, as well as the consumption rate of the Cu6Sn5 layer, at this time, the IMC growth depends on the reaction between diffusion fluxes of Cu and Sn at the Cu3Sn/Cu6Sn5 interface. Finally, the IMC thickness would grow similarly. Because of the coarsened Sn grains in the solder joints with multiple grains in height during the aging process, the UTS decreases, and the fractures occur in the solder bulk; the fracture of the aged solder joint with 10μm SOH occurs with the delamination of Cu3Sn/Cu6Sn5 interface, which is different from the fracture mode before aging.The study of the as-reflowed and aged Cu/Sn/Ni solder joints reveals the change rules of the micrstructure and mechanical property of the solder joints with different base materials at the two sides. Due to the coupling effect during the reflow process, the incompact duplex IMC layers of the thin Ni-Cu-Sn layer and the deposited (Cu,Ni)6Sn5 layer at Ni side are formed, therefore, fractures occur in the IMC layer at Ni side for all Cu/Sn/Ni solder joints in the tensile test with a constant crosshead speed of 0.1mm/min. In the aging process, the thin Ni-Cu-Sn layer would transform into (Ni,Cu)3Sn4, which also have a weak bonding with the deposited (Cu,Ni)6Sn5 layer, as a result, the fracture of the aged solder joints mostly occurs at the (Ni,Cu)3Sn4/(Cu,Ni)6Sn5 interface.The study compared the microstructure and mechanical property of the as-reflowed sandwich solder joints with the same or different materials at the two sides. All of the solder joints were prepared with a SOH value of 100μm. Among the Cu/Sn/Cu, Ni/Sn/Ni and Cu/Sn/Ni solder joints, it is found that the Ni/Sn/Ni has the highest UTS, then the Cu/Sn/Ni, finally the Cu/Sn/Cu; The fractures occur in the bulk of the solder joints with the same materials at the two sides, while in the IMC layer at Ni side of the Cu/Sn/Ni solder joints, and this difference results from the compact IMC layer in the Cu/Sn/Cu and Ni/Sn/Ni solder joints and the incompact duplex IMC layers in the Cu/Sn/Ni solder joints.The study on the as-reflowed and aged Cu/Sn-37Pb/Cu revealed the influence of SOH on the solder bulk microstructure, IMC thickness, IMC proportion and the mechanical property of solder joints. With the reducing SOH of the as-reflowed solder joints, the IMC thickness decreases, while the Pb concentration increases in the solder bulk. During the subsequent thermal aging process, the IMC thickness and the Pb concentration in the bulk increase faster in the solder joints with lower SOH. The UTS increases with the higher strain rate, while it decreases with the higher Pb concentration in the solder bulk. The tensile tests with a constant crosshead speed of 0.1mm/min were used to assess the mechanical property of solder joints, and it is found that the UTS increases with the SOH reduced from 100μm to 20μm, when the strain rate is the major influencing factor. The solder joint with 10μm SOH has the lowest UTS value, because the Pb concentration in the bulk could range from 50wt.% to 90wt.% before and after the thermal aging at 150℃for 500h. All of the fractures occur in the solder bulk in a plastic mode.The study of as-reflowed and aged Cu/Sn-9Zn/Cu reveals the influential mechanism of SOH on the solder bulk composition, IMC growth and mechanical property. For the as-reflowed solder joints, the Cu concentration increases, while the Zn concentration decreases in the solder bulk with the reducing SOH, therefore, the density of dissocative Cu5Zn8 particles increases in the solder bulk. The overmany dissocative Cu5Zn8 particles destroy the integrity of solder bulk, decreasing the UTS. All of the fractures occur in the solder bulk in the tensile tests with a constant crosshead speed of 0.1mm/min. During the aging process, the Cu6(Sn,Zn)5 forms earlier in the solder bulk with the reducing SOH.
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