Nanoindentation of Lead Free Solders for Harsh Environments
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1079-N05-13
Nanoindentation of Lead Free Solders for Harsh Environments Vitor Farinha Marques, Patrick Grant, and Colin Johnston Department of Materials, University of Oxford, Oxford, OX5 1PF, United Kingdom ABSTRACT To better understand the factors governing the reliability of lead free solders during severe excursions in temperature, the hardness and elastic modulus of the micro-phases formed in a Sn-Ag-Cu/Cu solder joint were characterized using nanoindentation at temperatures from 25 to 175°C. The creep behaviour of the different micro-phases was also studied as function of temperature. The hardness and elastic modulus of Cu6Sn5, Cu3Sn had a weak dependence on temperature, while primary Sn, eutectic regions and electroplated Cu hardness and modulus were sensitive to temperature. Creep studies indicated that intermetallic were more creep resistant than softer phases that readily underwent creep, the type and rate of which was shown to be strongly temperature dependent.
INTRODUCTION
Current environmental legislation restricts the use of lead containing solders in microelectronic packages for domestic use, and a comprehensive range of “drop-in” alternatives have been developed successfully [1]. The aerospace industry is currently exempt from this legislation on safety grounds. However, it is inevitable and widely accepted that the aerospace sector will become eventually subject to similar restrictions. At the current time, domestic lead free alternatives are not qualified for the aerospace sector and industrial studies suggest that the performance and failure behaviour of lead free solders in the harsher aerospace environment is different to reported domestic behaviour [1-2]. During service, microelectronic assemblies are subjected to coefficient of thermal mismatch induced strains because of the multiplicity of materials used, such as alumina or reinforced fibers PCBs, as well as the interconnect solder themselves. Resulting stresses are dependent upon the elastic modulus and the ability of the materials – primarily the solders – to relieve the thermally induced stresses by non-elastic processes including yield and/or creep [3].
Further, the microstructure, phases and their fraction, and joint geometry also tend to change with time and temperature. Consequently, development of successful lifing methodologies for interconnects is extremely challenging, Room temperature nanoindentation has been explored for its potential to characterize the materials used in real microelectronic joints/interconnects, rather than studying the constituent material in bulk form [4-6]. However, there was been a relatively little work on the nanoindentation of evolving interconnect phases as function of temperature and time. The present study investigates the hardness, elastic modulus and creep behaviour of Cu6Sn5, Cu3Sn, Ag3Sn, Cu, primary Sn and eutectic regions in Sn-Ag-Cu/Cu joints as a function of temperature. Ultimately it is envisaged that this type of data will improve the understanding of lead free solder reliability under
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