Study On Damage Constitutive Model And Failure Mechanism Of Lead-Free Solder In Microelectronic Packaging

Due to increasing environmental and health concerns about the toxicity of lead, aswell as the legislation limiting the usage of lead-beating solders, the applying oflead-free solder alloys in microelectronic applications has been an inevitable packagingtrend. As major mechanical, thermal, and electrical interconnects between thecomponent and the PCB(Printed circuit board), solder joints are crucial for thereliability of the most electronic packages. It is urgent to realize mechanicalcharacterization and constitutive properties of lead-free solders. In the most of efforts, itassumed materials perfectly and disregarded effects of micro-structure evolution andmicro-damage on material constitutive behaviors.Based on the theories and methods of micromechanics and damage mechanics, thisdissertation studied mechanic properties and microstructure damage mechanism oflead-free solder, developed a viscoplastic-damage constitutive model incorporatinggrain size effect and void damage. Parameters of the model were identified by neuralnetwork and optimization routine implemented SQP (Successive quadraticprogramming algorithm). Utilizing user-defined material subroutine (UMAT) of finiteelement program ABAQUS, the numerical simulation system was set up to analyzemechanical behaviors and damage failure of lead-free solder. The model was verifiedthrough comparing data between the predictions and experiments. Then, damageprocess and reliability of solder joints in BGA packaging was investigated. In thisdissertation, the main investigation work focuses on the following several aspects:①Based on DSCM(Digital Speckle Correlation Method), a novel video controlexperimental technique was developed for assessing mechanical behaviors of Sn4.0Ag0.5Cu and SnSb8.5 solder alloys. A series of experimental tests in tension onthem have been conducted under various constant strain rates ranging from 10E-5/s to10E-3/s and at room temperature to 150℃. By scanning electron microscope (SEM)obsevervation, microstructure evolution and damage mechanism was also examined.②Based on Gurson-Tvergaard plastic potential equation and orthogonal rules, amodified viscoplastic-damage constitutive model for lead-free solder was proposed tosimulate the effect of voids on reliability and the macroscopic mechanical response. Theeffect of microstructure is taken into account by incorporating the grain size in matrixviscoplastic flow rule. Temperature effect is incorporated into the material properties.The void volume fraction as the internal damage variable is introduced into the materialconstitutive relation and provides direct reflection of fatigue life.③The model was implemented into finite element program ABAQUS through itsuser defined material subroutine to analyze mechanical response and damage evolutionof lead-free solder alloys. The parameters are identified by combined experimental andnumerical techniques: instead of FEM simulations, a multi-layer feed forward neuralnetworks based on resilient back propagation algorithm has been used; a non-linearoptimization routine implemented SQP(successive quadratic programming) algorithmhas been developed to optimize the model's parameters in order to obtain the bestagreement between the experimental data and the predicted curve.④The model was verified against various tensile test data for a wide range oftemperatures and strain rates. The predictions have shown the ability of the modifiedviscoplastic-damage model to correctly describe the experimental observations:nonlinearity, strain rate sensitivity and damage evolution. Then, this model andnumerical simulation system was effectively applied to the thermal fatigue and damageprocess analysis of BGA packages. The numerical analysis provides the theory evidencefor design and reliability of BGA packaging.