Microstructure Formation in Reinforced Sn-Cu Lead-free Solder Alloys

Abstract

Electronics manufacturers are pushing the limits in reducing the physical size of circuitrywhile simultaneously increasing the number of transistors to satisfy Moore’s Law [1]. Thisincludes investing in new materials, and configuring new ways to manufacture complex 3D(three dimensional) electronic packaging [1]. One key requirement of new materials andtechniques is ensuring the high reliability of the resultant products in various challengingoperating environments including thermal and mechanical extremes [2-4]. A viable methodto enhance the properties and performance of a solder joint is by incorporation ofreinforcement particles to the solder matrix, either by intrinsic or extrinsic methods. In thisthesis a series of Sn-Cu Pb-free solder alloys with extrinsic or intrinsic phase reinforcementwere manufactured and the microstructure and soldering behavior were investigated indetail.Additions of extrinsic reinforcement in the form of nano-sized ceramic material were madeusing a microwave sintering powder metallurgy (PM) method, which is a viable method toimprove the mechanical and thermal properties of Pb-free solder materials. In addition, theadvanced processing routes ensures a homogenous distributions of reinforcement particlesis present. To investigate the performance of the reinforced bulk solders including thermaland mechanical properties and relate this to the microstructure, samples were investigatedusing techniques such as synchrotron micro-XRF, HRTEM, SEM, XPS, dilatometery, DSCand shear and microhardness testing. A hypothesis of how reinforcement improves solderproperties is developed and discussed. Synchrotron X-ray radiography imaging (SXRI) wasused to analyse the development of microstructure and the complex interactions occurringin the solders. Based on the properties of the fabricated solder, the microwave sintering PMroute was discussed as a promising method for the reinforcement of Pb-free solders.The initial formation of interfacial IMC products was studied in Sn-Cu based solder alloys byin situ experiment techniques such as SXRI and UHV-TEM. The results provide directexperimental evidence of real-time initial Cu6Sn5 layer development during soldering andalso the stress creation and release events that arise due to the polymorphic transformationsof the Cu6Sn5 phase and the associated volumetric change.In addition, the nucleation and growth behavior of primary intermetallics which can beconsidered an intrinsic reinforcing material in solder joints was studied. Here, the nucleationand growth behavior of primary Cu6Sn5 and β-Sn crystals in some of the most commonlyused solder alloys including Sn-0.7Cu and Sn-3.0Ag-0.5Cu is explained. This also includesthe effects of Ni additions for refining primary Cu6Sn5 in Sn-Cu solder joints. Using SXRI,observations were made during solder joint solidification, which is difficult using conventionalmethods. The initial nucleation and solidification kinetics of primary Cu6Sn5 crystals werediscussed.The growth of primary and interfacial Cu6Sn5 intermetallics after multiple reflow andannealing and the effect of this growth on the solder joint shear strength was studied tounderstand the effect of electronic component assembly processes on microstructuredevelopment. It was found that additions of TiO2 reinforcement were able to reduce thenumber and total length of primary Cu6Sn5 particles and suppress the interfacial layer duringmultiple reflows. It is possible that TiO2 particles in intimate contact with the interfacialCu6Sn5 hinder the Cu dissolution paths. By reducing the undercooling, additions of TiO2result in a lower average thickness in the interfacial IMC and a more stable growthmorphology.The collective results of this thesis demonstrate a detailed understanding of the manufactureof reinforced Sn-Cu Pb-free solder alloys and the mechanism of microstructure formation.The results are of significance scientifically and have industrial relevance and implicationsin controlling the microstructure and improving the performance and reliability of Pb-freesolder joints.