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Received Dec 6, 2017; Accepted Mar 5, 2018
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1. Introduction
Solder, a fusible metal alloy used to join metal pieces, is a staple material in the electronics industry. It functions as an adhesive or joining material to provide electrical continuity between the active silicon die, the substrate, and the printed wiring boards [1]. Solder alloys made up of tin and lead (Sn-Pb) predominate the manufacturing and electronics industries for years. However, due to the bad effects of lead and its compounds to human health and the environment [2], its use has been widely limited if not banned.
Global lead-free regulations started with the US banning lead (Pb) in gasoline additives, plumbing, and construction [3]. Japan pioneered the use of lead-free solders in the electronics industry and the European Union (EU) passed the restriction of the use of certain hazardous substances (ROHS) in electrical and electronic equipment banning the use of lead, mercury, and cadmium, among others [4]. Thus, there was a critical necessity to look for alternatives to Pb solders for the electronics industry. For solders to be a good environmentally benign substitute to Pb, it should be cost-effective and should mimic the properties of Pb, that is, low melting temperature (around 183°C), good mechanical, thermal, and electrical properties. Almost every lead-free solder in the market is Sn-based solders such as Sn-Ag-Cu and Sn-Cu [1]. At present, there is no common standard for lead-free electronic products since each country or region has its own recommended lead-free solders. In the US, the National Electronic Manufacturing Initiative, Inc., (NEMI) recommends Sn-3.9Ag-0.6Cu as the lead-free solder. Japan, through the Japan Electronic and Information Technology Association (JEITA), recommends the use of Sn-3.0Ag-0.5Cu solder alloy, while the European Consortium recommends Sn-3.8Ag-0.5Cu as the lead-free solder. Overall, the most common type of lead-free solder (LFS) used worldwide is the alloy made up of Sn-Ag-Cu. Alternative Pb-free solder systems such as Sn-Ag, Sn-Bi, Sn-Zn, and Sn-Cu have been developed [1, 5, 6] but uncertainties in their integrity and reliability limited their use in consumer products [7]. New types of lead-free solders are...