Solder joint fatigue models: review and applicability to chip scale packages

Damage mechanics describes the degradation process that takes place in materials and structures. Traditionally, Coffin–Manson type empirical curves are used to determine the fatigue life. Damage mechanics allows us to determine the fatigue life without the need for empirical curves. The main problem in damage mechanics has always been a lack of universally agreed upon definition of a damage metric. In this paper a damage metric based on the second law of thermodynamics and statistical mechanics is presented. The proposed thermodynamic framework treats a solid body as a thermodynamic system and requires that the entropy production be nonnegative. Verification of the damage model has been performed by extensive comparisons with laboratory test data of low cycle fatigue of Pb40/Sn60 solder alloy.

A Thermodynamic Framework for Damage Mechanics of Solder Joints

This study investigates the effects of employing different two-dimensional (2-D) and three-dimensional (3-D) finite element analysis (FEA) models for analyzing the solder joint reliability performance of a flip chip on board assembly. The FEA models investigated were the 2-D-plane strain, 2-D-plane stress, 3-D-1/8th symmetry and 3-D-strip models. The different stress and strain responses generated by the four different FEA models were applied to various solder joint low cycle fatigue life prediction relationships. The investigation shows that the 2-D-plane strain and 2-D-plane stress models gave the highest and lowest solder joint strains, respectively. The 3-D-strip and 3-D-1/8th symmetry model results fall in between the 2-D-plane strain and 2-D-plane stress model results. The 3-D-1/8th symmetry model agrees better with the 2-D-plane strain model, while the 3-D-strip model agrees better with the 2-D-plane stress model results. The results for the fatigue life prediction analyses also show similar trends.

Flip chip on board solder joint reliability analysis using 2-D and 3-D FEA models

Viscoplastic finite-element simulation methodologies were utilized to predict solder joint reliability for a same die size, stacked, chip scale, ball grid array package under accelerated temperature cycling conditions (-40C to +125C, 15 min ramps/15 min dwells). The effects of multiple die attach material configurations were investigated along with the thickness of the mold cap and spacer die. The solder structures accommodate the bulk of the plastic strain that is generated during accelerated temperature cycling due to the thermal expansion mismatch between the various materials that encompass the stacked die package. Since plastic strain is a dominant parameter that influences low-cycle fatigue, it was used as a basis for evaluation of solder joint structural integrity. The paper discusses the analysis methodologies as implemented in the ANSYS finite element simulation software tool and the corresponding results for the solder joint fatigue life.

Finite element based solder joint fatigue life predictions for a same die size-stacked-chip scale-ball grid array package

A thermomechanical fatigue life prediction model based on the theory of damage mechanics is presented. The damage evolution, corresponding to the material degradation under cyclic thermomechanical loading, is quantified thermodynamic framework. The damage, as an internal state variable, is coupled with unified viscoplastic constitutive model to characterize the response of solder alloys. The damage-coupled viscoplastic model with kinematic and isotropic hardening is implemented in ABAQUS finite element package to simulate the cyclic softening behavior of solder joints. Several computational simulations of uniaxial monotonic tensile and cyclic shear tests are conducted to validate the model with experimental results. The behavior of an actual ball grid array (BGA) package under thermal fatigue loading is also simulated and compared with experimental results. @DOI: 10.1115/1.1536171# With the increasing use of surface mount bonding technology in microelectronics industry, the reliability concerns for solder joints are increasing exponentially. Eutectic solder alloys are most commonly used bonding materials in electronic packaging, which provide electrical and thermal interconnection, as well as mechanical support. The temperature fluctuations due to device internal heat dissipation and ambient temperature changes, along with the coefficient of thermal expansion ~CTE! mismatch between the soldered layers, result in thermo-mechanical fatigue of the solder joints. Progressive damage in solder balls eventually leads to device failure. Fatigue life prediction of solder joints is critical to the reliability assessment of electronic packaging. Standard state of practice in the electronic industry for the number of cycles to failure prediction is based on using empirical relations, such as Coffin-Manson approach. Typically, using the CTE differential between the bonded components, the maximum elastic and plastic strain in the solder joint is calculated. Most of the time, using the plastic strain value, Coffin-Manson curves are used to predict the fatigue life of solder joints. Usually this approach yields very conservative results for BGA packages, Zhao et al. @1#. Recently, numerous physics-of failure based models have been developed for the evaluation of reliability of solder alloys under thermo-mechanical fatigue loading, such as Busso

A Damage Mechanics-Based Fatigue Life Prediction Model for Solder Joints

This study explores the possibility of using a unified theory of creep-fatigue, similar to the Halford-Manson strain-range partitioning method, for examining the effect of cyclic temperature range on fatigue life, over a wide range of temperatures. Other investigators have attempted similar techniques before for solder fatigue analysis. The present study is different since it proposes an energy-partitioning technique rather than strain-partitioning to examine the dependence of solder fatigue behavior on temperature dependent changes in the relative amounts of plastic and creep strains. The solder microstructure also dictates creep behavior but is assumed to be a given invariant parameter in this study. In other words, this study is targeted at as-cast microstructures and does not address post-recrystallization behavior. A sample solder joint of axisymmetric configuration, commonly found in leaded through-hole mounting technology, is analyzed with the help of nonlinear finite element methods. The strain history is determined for constant-amplitude temperature cycling with linear loading and unloading, and with constant dwells at upper and lower ends of the cycle. Large-deformation continuum formulations are utilized in conjunction with a viscoplastic constitutive model for the solder creep-plasticity behavior. Relevant material properties are obtained from experimental data in the literature. The results show significant amounts of rachetting and shakedown in the solder joint. Detailed stress-strain histories are presented, illustrating the strain amplitude, mean strain and residual stresses and strains. For illustrative purposes, the hysteresis cycles are partitioned into elastic, plastic and creep components. Such partitioned histories are essential in order to implement either the Halford-Manson strain-range partitioning technique or the energy-based approach suggested here, for analyzing the creep-fatigue damage accumulation in solder material. This study also illustrates the role and utility of the finite element method in generating the detailed stress-strain histories necessary for implementing the energy partitioning approach for creep-fatigue damage evaluation. Solder life prediction is presented as a function of cyclic temperature range at a given mean temperature.

C. Oyan

Papers

Solder Creep-Fatigue Analysis by an Energy-Partitioning Approach