Reliability Assessment of Electronics Under Drop-Impact Using Cohesive Zone and XFEM Models

Abstract

The evolution of complexity in the handheld portable electronics accompanied with the miniaturization due to advancement in technology has contributed to their vulnerability under shock and drop conditions. Drop reliability of electronics has been addressed using various experimental and analytical techniques. Material characterization of Pb-free alloys at high strain rates typical of drop and shock is performed using these techniques. These transient dynamic phenomenons are addressed using advanced finite element methods such as extended finite element method, cohesive zone modeling and line spring element method to model crack/flaw initiation and propagation. The transient dynamic behavior of leadfree and leaded solder-interconnects have been studied in ball-grid array, copper-reinforced solder column and high lead column package architectures. Four interconnect types have been modeled using XFEM including Sn3Ag0.5Cu, 90Pb10Sn, Cu-Reinforced column, and 63Sn37Pb interconnects on ceramic ball-grid arrays. In addition, Sn3Ag0.5Cu on plastic ball-grid arrays has also been modeled. Extended finite element models have been correlated with cohesive-zone models along with experimental results. The board assemblies have been tested at 1,500g and 12,500g. The failed assemblies have been cross-sectioned and the failure modes correlated with model predictions. The predicted failure modes for all four interconnect types correlate well with the observed locations for failure. In an initial part of this thesis, damage and life prediction of transient dynamics in electronics interconnects is presented using XFEM in conjunction with digital image correlation and explicit submodeling. The second part of this thesis deals with measurement of fracture properties of Pb-free alloys at high strain rate conditions using FE modeling techniques. For this purpose, bimaterial and single material copper-solder specimens are tested using uniaxial tensile testing machine. Models for crack/flaw initiation and propagation are developed using Line spring method and extended finite element method (XFEM). Critical stress intensity factor for leadfree alloys such as Sn3Ag0.5Cu and Sn1Ag0.5Cu are extracted from line spring models. Stress intensity factor at Copper pad and bulk solder interface is also evaluated in ball grid array packages. Specimens are tested at various strain rates and events are monitored using high speed data acquisition system as well as high speed cameras operating more than 50,000 fps. In this work, fracture properties such as SIF and J integral are measured using simulation techniques and correlated with experimental results. The solder deformation and damage thresholds at the copper-solder interface have been measured at strain-rates representative of shock and vibration. These high strain rate properties of solder alloys are used to define the material models in all finite element simulations which run based on a technique of node based sub-modeling.