![]() The resulting interconnect will be a mixed BiSn-SAC solder joint. If BiSn paste is utilized during assembly, the package (likely balled with a SAC variant) would have to be placed on a BiSn paste deposit and then reflowed at a lower temperature appropriate for the low melting point paste (i.e. The given reality in the packaging industry is that most electronic components will not be easily available with BiSn preforms. Results showed a clear transition from ductile solder failure to a brittle separation failure at the higher strain rates.Īn additional complication is the unfortunate near-term reality of integrating this alloy into the industry on a large scale. Additionally, the evolution of the microstructure was characterized. Investigations into the fracture mechanisms were conducted by examining the shear fracture surface with optical and scanning electron microscopy. Using this information, the strain rate sensitivity of the interconnects was mapped and correlated with the observed failure modes. Using a joint level micromechanical tester, ball shear tests were conducted at a range of strain rates for samples in the as-reflowed and aged state. A selection of the solder joints were then isothermally aged at 90☌ for 200 hours. A 22 mil Cu-OSP pad on a 1.0 mm thick FR4 substrate was used for all samples. SAC 305 control samples were also made using a conventional Pb-free reflow profile with a peak temperature of 247☌. These samples were then reflowed at either 175☌ or 200☌. Mixed SAC-BiSn solder joints were formed by placing SAC 305 spheres on BiSn paste deposits for a paste to ball volume ratio of. The degree of mixing of these two regions has been shown to be highly dependent upon reflow temperature and the paste to ball volume ratio. Recent work by several researchers has shown that this hybrid microstructure is unstable and quite active with respect to the movement and localized concentration of the Bismuth. It is of specific industrial interest then to not just investigate the BiSn solder system but also within the context of a realistic mixed interconnect. This non-equilibrium microstructure will be composed of two regions, one Bi-rich region which is well past saturation and a second region which is Bi-deficient. This means that the resulting solder interconnect, reflowed below conventional SAC reflow temperatures, will form a type of mixed hybrid microstructure. Therefore, it will be necessary to use components already balled with SAC 305 solder. Bismuth is widely acknowledged as a brittle element and its presence in such quantities raises concerns of not just Cu 6Sn 5 embrittlement but also solder fragility in high strain rate types of environments.Ī challenge with regards to near term implementation is that most packages are not available with BiSn solder bumps. While a body of knowledge currently exists regarding this system, and the near eutectic variant BiSnAg, there are still concerns regarding its ductility, especially as a function of thermal exposure and strain rate. The BiSn system itself is not particularly novel as it was posited as a SAC alternative during the initial shift from Pb based solders. One of the most popular low melt solder alloys currently being investigated by the industry is the Bi-Sn eutectic system, which has a melting point of 139☌. The ability to minimize the thermal exposure that an assembly is subjected to affords significant benefits with respect to both the reliability and the materials that can be used. As the electronics industry continues to evolve a concerted effort has developed to implement lower melting point solders. ![]()
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