Abstract
In aerospace applications, some devices, instruments and equipment suffer the harsh environment, as extremely low temperature and wide temperature ranges for instance. The solder joints which are fragile and sensitive to the thermal and mechanical environment are part of the electronic control systems that are important to the devices. Sn-based alloy has been the mostly used solder material of electronic circuit in recent years. Sn-based solder alloy has better performance such as low melting point and good plasticity. However, it shows brittle behavior at low temperatures and brittle failures may eventually lead to catastrophic failures. Study on mechanical behaviors of Sn-based solder alloy at cryogenic temperatures becomes more and more important. Charpy test was performed to measure the dissipated energy in the impact process. There were 6 kinds of solder alloys, as follows: Sn, 99.3Sn0.7Cu, 96.5Sn3Ag0.5Cu, 62Sn36Pb2Ag, 63Sn37Pb, 10Sn90Pb. There is obvious ductile-to-brittle transition for all the Pb-free solders at lower temperature. For Sn solder, this transition begins between -40 °C and -60 °C. The ductile-to-brittle transition temperature (DBTT) of 96.5Sn3Ag0.5Cu solder is between -30°C and -40°C which is higher than Sn solder. When adding Pb to solder, the toughness will be so perfect that the ductile-to-brittle transition tend to be gradual and not obvious. The results shows that for Sn solder the fracture modes have both intergranular fracture and transgranular fracture. A rod-like structure has been found in 99.3Sn0.7Cu solder and 96.5Sn3Ag0.5Cu solder that it facilitates transgranular fracture. The composition of the specific structure is likely to be Cu6Sn5 for 99.3Sn0.7Cu solder and Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn for 96.5Sn3Ag0.5Cu solder measured by EDS (Energy Disperse Spectroscopy). Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn particles are observed in the fracture morphology of 62Sn36Pb2Ag solder and the fracture mode is quasi-cleavage fracture.
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