Lead-free solutions for high and low temperature solder interconnects
Abstract
Three distinct approaches were investigated in the development of new high and low temperature lead-free solders. These solutions include using the thermodynamic size effect to reduce melting temperature of current lead-free solders, creating a solderless interconnect through core-shell copper-silver particles, and designing of a partially molten Bi-Ag alloy for high temperature applications. Suppressed melting temperatures were observed at temperatures up to 100°C below the material's bulk melting temperature through DSC analysis for various tin and tin alloy nanosolders. Secondary Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) analysis confirmed the phases present as well as the particle sizes. Melting of nanoparticle solder pastes on copper coupons revealed some adhesion to the copper substrate, although the presence of flux residue prevented solid joints from forming. Alloys of varying composition within the Bi-Ag binary system were investigated for high temperature applications to replace Pb5Sn. Microstructure analysis revealed that the binary alloy consists of a silver-rich dendritic phase within a bismuth-rich eutectic mixture. Increasing silver concentrations leads to improved mechanical properties such as hardness and strength. Electrical resistivity decreased with the addition of silver reducing from 1.72e-6 Ω*m to 4.23e-7 Ω*m. Micro tensile test microstructure revealed the presence of silver dendrites impeding crack propagation through the bismuth-rich eutectic by silver dendrite bridging. Wetting to a copper substrate and grain boundary grooving were observed with alloys that were heated below their liquidus temperature. Copper-silver core-shell particles were used to create a solderless joint for electronics. DSC analysis identified temperatures (200-250°C) which sintering of the silver shell was activated. Pressed pellets which were heated in argon were analyzed with SEM and TEM. Neck formation both before and after annealing were assessed by SEM. TEM analysis confirmed the initial silver shell thickness of 5nm in addition to observing the silver diffusion to the neck regions at the particle contacts using EFTEM analysis before and after annealing. Compression tests on pressed pellets both as pressed and annealed revealed fractured necks in the sintered compacts. These samples showed increased neck formations with increasing annealing temperatures leading to increased modulus and yield strengths.
Degree
M.S.M.S.E.
Advisors
Handwerker, Purdue University.
Subject Area
Electrical engineering|Materials science|Materials science
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