Correlation of microstructural and mechanical properties in particle reinforced lead-free solders

Solders are widely used as interconnect material for the electronic industry. The solder interconnect provides both electrical contact and mechanical connection between the integrated circuit devices and their substrate. Thus solders with good reliability are required to facilitate successful functioning of electronic devices and components that we encounter in daily life. However traditionally used Sn-Pb solders have to be replaced by Sn rich lead-free solders because of the toxicity concerns surrounding the use of lead. Many Sn-rich lead-free solders do exists, however they are not drop-in substitutes for Sn-Pb solders as the knowledge concerning their mechanical behavior and reliability are not well understood. A particularly critical aspect is the lead-free solders' reliability at high service temperatures required in several microelectronics applications. Such applications require solders possessing good microstructural stability, strength and creep resistance. In this regard, literature reports the alloying and the composite approach to improve the properties of the lead-free solders. Among these two methods, the composite approach wherein suitable particles are added in to the solder matrix seems to be promising. However, the scientific understanding between property improvement and characteristic of the particle reinforcement is too limited. Also the processing of the composite solder is limited by stringent process parameters that require significant improvement to increase its industrial applicability. Another important issue arises from the fact that solders as such exhibit complex elasto-plastic properties. Furthermore the solder's mechanical response is greatly influenced by specimen geometry and process parameters. Therefore novel localized strain measurement techniques that could assess and understand the mechanical response of the solder without much effect from substrate are required. The goals of the current PhD thesis are identified within the framework of the issues previously mentioned. The composite approach is demonstrated to improve the microstructural and the mechanical properties of existing lead-free solders. As reinforcing particles either Cu or Ni is used with volume fraction from 0.8-4.2 vol. %, while Sn-4.0Ag-0.5Cu (SAC405) alloy is chosen as matrix. Throughout the study the results of the composite solder is compared against the un-reinforced SAC405 reference solder. The solder material in bulk form (reflowed in ceramic crucible) is used for microstructural analysis and also as joining material between Cu substrate to investigate the mechanical response in tension and shear. The current research demonstrates a new processing method developed for producing composite lead-free solders utilizing micro and nanometer scale metallic particles. Generally the preparation of nano-composite solders requires several process parameters that restrict their industrial use. In literature even the exact process route is still not