Effect of Isothermal Aging on Anisotropic Creep Properties of SAC305 Single Crystals ANIKET BHARAMGONDA 1* , JOHNATHAN MARTIN 1 , YONGRAE JANG 1 , DR. ABHIJIT DASGUPTA 1 DR. TORSTEN HAUCK 2 , DR. YAXIONG CHEN 2 1 CALCE, UNIVERSITY OF MARYLAND, COLLEGE PARK, USA. 2 NXP Semiconductors *abharamg@umd.edu Abstract— Predicting life of SAC305 solder joints in service is difficult as many Sn-based solder materials undergoes microstructural changes as per the loading conditions. Aging of solder joint at even room temperature changes microstructure of the solder material, affecting its properties and thus life of solder joints. SAC solder joints are well known to contain few grains (oligocrystalline). Due to this, accurate estimation of thermo-mechanical properties of single SAC crystal and grain boundaries is required, to model grain-scale solder joint response under thermal cycling. Multitier modelling of oligocrystalline SAC solder joint has been demonstrated by this research group. During aging of solder joint intermetallic particle size, spacing between them, fraction of tin dendrites, grain and grain boundaries are continuously evolving. It is important to determine the contribution of each microstructural future so that accurate modelling of anisotropic behavior of oligocrystalline solder joints can be done and thus it will further reduce time in predicting life of solder joint. In this paper, single crystal high aspect ratio (HAR) solder joints are fabricated with low cooling rate, and then subjected to isothermal aging, to capture effects of such microstructural aging on creep resistance of single SAC crystals. Within SAC crystals, changes in the creep property is due to changes in IMC particle size, spacing and dendrite volume fraction. Monocrystalline morphology and grain orientation are determined from microscopy and electron backscatter diffraction (EBSD). Samples are aged at 125ºC for 0, 100 and 300 hrs., for studying the effect of isothermal aging. Scanning electron microscope and image processing are used to study microstructural evolution. Room temperature creep tests are conducted on single crystal solder joints and creep modelling is conducted using CV model, to determine the effect of the microstructural aging on Hill-Garofalo model constants. These calibrated continuum model constants are used to conduct thermomechanical simulations of QFN package to represent the effect of isothermal aging on durability of solder interconnects. Keywords- SAC305, oligocrystalline joint, single crystal, high temperature creep, hierarchical anisotropic modelling isothermal creep, Digital Image Correlation, Cross-Polarized Microscopy, image processing I. INTRODUCTION Lead free solder alloys, especially Tin-Silver-Copper (SAC) are used in most electronic products. It was observed that these solder joints have heterogeneous microstructure and due to body central tetragonal structure, oligocrystalline SAC solder joint exhibit anisotropic behavior. Multiple studies have been conducted to characterize and model creep and plasticity properties of solder joints [1-3]. Such anisotropic behavior of SAC solder contributes to variability observed during thermal and mechanical fatigue of solder joints [4,5] and can be captured through simulations by considering different grain orientation [6]. Anisotropic behavior of solder joint was conducted using crystal viscoelasticity model. Multi-tiered CV model consists of 4 tiers where tier-0 represents