Characterisation of intermetallics and mechanical behaviour in the reaction between SnAgCu and Sn-Pb solder alloys

Due to the imminent wide-spread introduction of Pb-free solders, materials properties, manufacturing and reliability of solder joints are currently being investigated in the electronics packaging community. The interactions of new Pb-free alloys with other materials are of particular concern in order to understand and thereby enable the optimisation and control of the processes for reliable products. In the transition period from Sn-Pb to Pb-free soldering, existing Sn-Pb products that remain in the market, or with customers, is an issue when they have to be repaired with Pb-free solders. The complete elimination of Pb from the products that are specified in the legislation has to be account for Pb contamination that is inevitable in rework, repair or technology upgrades. As such, a number of technical problems in connection with mixing Sn-Pb with Pb-free alloys exist, including: (1) Uncertainty of the resulting microstructure and its properties; (2) The effect of unknown compositions and structures on the reliability; (3) Critical or tolerable levels of some elements that can be permitted in applications; (4) The relative content of the elements and the formation and morphology of intermetallic phases. In this paper thermodynamic calculations are presented that have studied the multicomponent material behaviour and possible formation of intermetallic precipitates during reactions between Sn-Pb and Sn-Ag-Cu Pb-free alloys. Two Sn-Ag-Cu alloys that are relevant to current industrial interests, namely Sn-3.9Ag-0.6Cu (known as '405 alloy' in Europe and North America), and Sn-3.0Ag-0.5Cu (known as '305' alloy in Asia), were selected to react with different contamination levels of eutectic Sn-37Pb solder. The paper also presents experimental work that has characterized the intermetallics and the mechanical behaviour following reaction of the Pb-free alloys with eutectic SnPb solder. The microstructure and phase identification was studied by optical microscopy and scanning electron microscopy (SEM), with the latter featuring the electron back scattered diffraction (EBSD) technique that offers details of phase morphology and orientation at nanoscales. Nanoindentation, which is suitable for the ultra-fine and complex microstructures in small volumes, was also used to investigate the micromechanical properties, including hardness, elastic modulus and creep, at both ambient and elevated temperatures.