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

Abstract

Electronics manufacturers are pushing the limits in reducing the physical size of circuitry while simultaneously increasing the number of transistors to satisfy Moore's Law [1]. This includes investing in new materials, and configuring new ways to manufacture complex 3D (three dimensional) electronic packaging [1]. One key requirement of new materials and techniques is ensuring the high reliability of the resultant products in various challenging operating environments including thermal and mechanical extremes [2][3][4]. A viable method to enhance the properties and performance of a solder joint is by incorporation of reinforcement particles to the solder matrix, either by intrinsic or extrinsic methods. In this thesis a series of Sn-Cu Pb-free solder alloys with extrinsic or intrinsic phase reinforcement were manufactured and the microstructure and soldering behavior were investigated in detail.Additions of extrinsic reinforcement in the form of nano-sized ceramic material were made using a microwave sintering powder metallurgy (PM) method, which is a viable method to improve the mechanical and thermal properties of Pb-free solder materials. In addition, the advanced processing routes ensures a homogenous distributions of reinforcement particles is present. To investigate the performance of the reinforced bulk solders including thermal and mechanical properties and relate this to the microstructure, samples were investigated using techniques such as synchrotron micro-XRF, HRTEM, SEM, XPS, dilatometery, DSC and shear and microhardness testing. A hypothesis of how reinforcement improves solder properties is developed and discussed. Synchrotron X-ray radiography imaging (SXRI) was used to analyse the development of microstructure and the complex interactions occurring in the solders. Based on the properties of the fabricated solder, the microwave sintering PM route 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 by in situ experiment techniques such as SXRI and UHV-TEM. The results provide direct experimental evidence of real-time initial Cu6Sn5 layer development during soldering and also the stress creation and release events that arise due to the polymorphic transformations of the Cu6Sn5 phase and the associated volumetric change.ii In addition, the nucleation and growth behavior of primary intermetallics which can be considered an intrinsic reinforcing material in solder joints was studied. Here, the nucleation and growth behavior of primary Cu6Sn5 and β-Sn crystals in some of the most commonly used solder alloys including Sn-0.7Cu and Sn-3.0Ag-0.5Cu is explained. This also includes the 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 conventional methods. The initial nucleation and solidification kinetics of primary Cu6Sn5 crystals were discussed.The growth of pri...

Spontaneous formation of sub-4 nm nanocrystalline alloy via polymorphic phase transformation

“…The surface of grains became more rugged than that of the intact large grains having a smooth surface. As confirmed by previous research [24,25], 186 o C is the phase transformation temperature (PTT) for Cu 6 Sn 5 (the PTT of NPs is normally lower than in the case of a bulk material [26]), and it is not an order-disorder transition, but a polymorphic transformation with surface reconstruction and volume change (2.15%) [24,27]. Therefore, it is highly possible that the polymorphic phase transformation of Cu 6 Sn 5 is responsible for the spontaneous grain differentiation, refinement, and surface reconstruction.…”