Figure - available from: Journal of Materials Science: Materials in Electronics
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a Cross-sectional SEM (left) and EBSD (right) images of a solder joint after 950 h at 5.0 kA/cm² and 150 °C. Enlarged SEM images of the b cathode side and c anode side. In the EBSD image, the orientation of large grains is marked by arrows in the unit cells indicating the c-axes of the Sn crystals
Source publication
Electromigration (EM) in solder joints has recently been recognized as a serious reliability issue in the field of car electronics. EM in power modules is also of concern for next-generation environmentally-friendly vehicles. The current density of 10 kA/cm² is well-known as the threshold for EM failure. Few researches have studied the EM behavior...
Citations
... In recent years, electromigration in the solder joints of power modules has also been studied. For example, Kadoguchi et al. [1,10] experimentally investigated electromigration in the solder joints that connect the upper and lower arms of power modules. They found that the electromigration breakdown mode occurred at a current density of 5.0 kA/cm 2 , which is smaller than the 10 kA/cm 2 that generally causes serious electromigration [1]. ...
... They found that the electromigration breakdown mode occurred at a current density of 5.0 kA/cm 2 , which is smaller than the 10 kA/cm 2 that generally causes serious electromigration [1]. It was also found that even at a current density of 2.5 kA/cm 2 , electromigration damage can be induced depending on Sn crystallographic orientation [10]. However, they did not focus on dieattach solder joints between the power device and substrate which is uniformly bonded to the lower surface of a power device [11] because the average current density of the solder joint in Si-based power modules without considering current crowding varies from about 0.1 kA/cm 2 to 0.4 kA/cm 2 , which is much smaller than 10 kA/cm 2 [12]. ...
The larger current densities accompanying increased output of power modules are expected to degrade solder joints by electromigration. Although previous research has experimentally studied electromigration in solder, die-attach solder joints in Si-based power modules have not been studied because the average current density of the die-attach solder is much smaller than the threshold of electromigration degradation. However, even in die-attach solder, the electromigration degradation may appear where current crowding occurs. This report describes electromigration analysis of die-attach solder joints for Si-based power modules using an electrical-thermal-stress-atomic coupled model. First, we validate our numerical implementation and show that it can reproduce the distributions of vacancy concentrations and hydrostatic stress almost the same as the analytical solutions even at current densities assuming current crowding. We then simulate the die-attach solder joint with a Si-based power device and a substrate. Due to current crowding, the current density at the edge of the solder exceeds the electromigration threshold. Unlike general electromigration phenomena, the vacancy concentration increases at the center and decreases at the edges of the solder joint, regardless of whether it is on the cathode side or anode side, due to the longitudinal driving force in the solder joint generated by the current crowding. Creep strain increased remarkably at the anode edge and the cathode center. The absolute vacancy concentration clearly increased with increasing current density and size ratio. Creep strain significantly increased with increasing current density, size ratio, and temperature.
... The intermetallic compound (IMC) of (Ni,Cu) 3 Sn 4 forms between Sn-0.7Cu and Ni plating. When the Ni plating is exposed to high temperature or high current density in a power module, Ni easily diffuses into the solder layer [12,13], thinning the Ni plating and thickening the (Ni,Cu) 3 Sn 4 . ...
Improving the reliability of solder joints for die attachment in power modules is one of the most important issues in creating environmentally friendly vehicles such as hybrid electric vehicles. Power modules must have highly reliable solder joints that must be thermostable at temperatures over 175 °C in the future. In die attachment, soldering surfaces are often finished with Ni plating, so for Cu/Ni plating/Sn–Cu solder joints it is necessary to suppress both Ni diffusion into the solder as well as growth of the (Ni,Cu)3Sn4 intermetallic compound (IMC). Ni diffusion in Ni plating can be suppressed by the presence of a continuous (Cu,Ni)6Sn5 IMC layer at the Ni plating/solder interface. To form this IMC, we investigated the interfacial reactions and growth behavior of IMC layers in the presence of composite Sn–0.7Cu solder with added Cu balls. Adding 2.5 mass% of Cu balls prompted the formation of a continuous (Cu,Ni)6Sn5 IMC layer at both the electroless Ni–P and the electrolytic Ni plating, and this IMC layer worked well as a Ni diffusion barrier during a high-temperature storage test at 200 °C for 1000 h.
To clarify potential issues that may arise in the future when high current densities are applied to joints in electronic power modules, high current density (25 kA/cm2) energization tests were conducted on sintered Ag joints sandwiched between Au and Ag layers. The degradation behavior differed depending on the current direction. The joint that placed the Au layer on the anode side showed a faster increase in voltage and temperature than the one placed on the Au layer at the cathode side, which also fractured at a much shorter energizing time (5 h). Generation and segregation of high-concentration coarse voids, in the same plane and growth of the Ag-Au solid solution layer, were observed in the sintered Ag layer of the joint, which placed the Au layer at the anode side before the fracture. Segregated voids existed near the cathode-side interface between the Ag layer and the surface of the base metal (Ti), and around the anode-side interface between the Ag layer and the Ag-Au solid solution layer. These results imply that the electromigration and diffusion coefficient imbalance between Ag and Au affect the behavior of the void generation and growth of the solid solution layer, which leads to a difference in the lifetime of the joints in the current direction under high current density energization. This indicates that the Au layer placed at the anode of the sintered Ag layer loses the reliability of the sintered Ag joints under the high current density driving electronic power modules.
Electric vehicles are becoming increasingly popular as environmentally friendly alternatives to conventional fossil fuel-based vehicles. The need to improve their performance demands design of materials that can withstand high current density without adversely affecting their durability. In this study, the influence of high-density current on the structure of a Ni-Sn solid–liquid interdiffusion joint with an Al interlayer was examined. The temperature at the joint region increased to 240°C without external heating under energization of 20 kA/cm2. After 100 h of energization, several structural evolutions from the initial state were observed in the joint region, including precipitation of Al3Ni at the cathode-side Al/Ni interface and enrichment of Ni concentration on both sides of the Ni-Sn intermetallic compound (IMC) layer. Electromigration influenced the precipitation position of the Al3Ni grains. However, there was only a trace of electromigration in the Ni enrichment behavior of the Ni-Sn IMC layer, because thermal and stress migration were very active there. The degree of Ni enrichment in the energized Ni-Sn IMC layer was much higher than that in the Ni-Sn IMC layer annealed at 300°C for 100 h. This result implies that a high-density current activates the thermal migration and stress migration. The influence on the reliability of the joint is a concern because the high degree of Ni enrichment in the energized Ni-Sn IMC layer generates many Kirkendall voids.
