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Determination of the Creep Properties of Pb-Free Solders for Harsh Environments Using Meso-Scale Testing
Keywords: Pb-free solder, creep, harsh environments
Solder joints in electronic packages are prone to failure due to the evolution of thermal expansion mismatch strains during thermal cycling. The comparatively wide operating temperature range and long lifetimes of aerospace electronics requires high reliability solder joints. Since 2006, high reliability industries (aerospace and military amongst others) that are exempt from lead-free RoHS regulation on account of concerns over the reliability of Pb-free solders have found it increasingly difficult and expensive to continue using traditional Sn-Pb-based solders. Hence there is a pressing need to find a suitable alternative that can match the manufacturing and reliability performance of Sn-Pb. Design for reliability approaches and life-time predictions of electronic assemblies requires a full understanding of failure modes and accurate constitutive behaviour. However, there remains a dearth of data for the constitutive behaviour of lead-free solders under harsh environment scenarios. Unfortunately, conventional test approaches, particularly in the case of creep behaviour which is critical to solder lifetimes, are expensive and time-consuming. This paper describes a new meso-scale test approach that lies between nanoindentation and bulk creep testing. Real ball grid arrays using lead-free solders have been creep tested in the temperature and stress ranges of operating solder joints. High temperature creep data has been obtained quickly and the behaviour of Pb-Sn and lead-free variants has been compared and contrasted. Creep constitutive data has been complemented by comprehensive microstructural analysis, and the technique is shown to be sensitive to discrete failure events. The technique offers promising time and materials savings in obtaining important mechanical property data and its subsequent use in life-prediction models.
Sophie Godard Desmarest, Ph.D. Student
University of Oxford

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