Norhashimah M. Shaffiar

Extended cohesive zone model for simulation of solder/IMC interface cyclic damage process in Pb-free solder interconnects

The current formulation of stress- and energy-based cohesive zone model (CZM) is extended to account for load reversals. Cyclic degradation of solder/IMC interface properties, namely penalty stiffness, strengths and critical energy release rates follows power-law functions of fatigue cycles. Performance of the extended CZM is examined using finite element (FE) simulation of a single Sn-4Ag-0.5Cu (SAC405) solder interconnect specimen. Strain rate-dependent response of the solder is represented by unified inelastic strain equations (Anands model) with optimized model parameters for SAC405 solders. The 3D FE model of the specimen is subjected to cyclic relative displacement (Δδ = 0.003 mm, R = 0) so as to induce shear-dominant fatigue loading. Results show that interface crack initiated at the leading edge of the solder/IMC interface on the tool side of the assembly after 22 cycles have elapsed. Bending stress component induced by the solder stand-off height dominates the interface damage process. A straight interface crack front is predicted indicating the relatively brittle nature of the SAC405/Cu6Sn5 interface. The extended formulation of the CZM to account for load reversals has demonstrated the ability to describe the progressive solder/IMC interface damage process consistent with the mechanics of relatively brittle interface fracture.

Application I: Solder Joint Reflow Process

The solder joint reflow process exposes the test assembly with BGA solders and/or printed solder paste to prescribed temperature profiles resulting in complete melting, wetting and flowing of the molted solder. During reflow cooling, the solder solidifies, making up the solder joint. Higher reflow temperature also activates a chemical reaction at the solder/pad interface, resulting in the formation of a thin intermetallic layer that improves the strength of the interface. The large temperature difference between solder reflow and ambient conditions induces thermal strains and stresses in the solder joint due to mismatches in the coefficient of thermal expansion (CTE) among the different materials making up the assembly. Thus, a thorough understanding of the evolution of internal states in the solder joint is essential in mitigating the residual stresses in the solder joints and ensuring high reliability of the assembly. In this respect, finite element (FE) simulation with an appropriate model for the geometry of the assembly and suitable material models can be employed to describe the mechanics of the materials during the solder joint reflow cooling process.

Flexing test of HDPE/EPR filled CNT radiated nanocomposites for sport shoe soles

Abstract This study was conducted to examine the effect of electron beam (EB) irradiation on the flexing of high density polyethylene (HDPE)/ethylene propylene rubber (EPR) blends and HDPE/EPR filled carbon nanotube (CNT) nanocomposites. The blends are compression molded into the soles of shoes before exposed to EB irradiation. Radiated and non-radiated matrixes as well as nanocomposites were subjected to single force and flexed at specified angles of 90º according to ISO 17707 for flexing test. The flexes were more for samples that were exposed to EB irradiation. Moreover, samples filled with CNT showed a lower flex number.

Overview of the Simulation Methodology

This chapter presents an overview of the simulation methodology that comprises both the art and science involved in simulating physical phenomena. It adequately summarizes the various aspects of simulation, including identification of the physical problem of interest, determination of material properties and behavior through mechanical testing, and formulations of governing equations for the finite element (FE) method.

Application II: Solder Joints Under Temperature and Mechanical Load Cycles

The reliability of microelectronic packages and assemblies is established through reliability testing of the device. In a reliability test, assemblies with BGA solder joints are subjected to temperature cycles, as prescribed by reliability test standards (e.g. JEDEC 2000).

Damage Mechanics-Based Models

The mechanics of materials is a branch of mechanics that investigates the response of engineering materials and structures to loading and environment. It relates the externally applied load to internal states of the material, namely displacement, strain and stress, and their dependency on temperature and strain rate. The deformation and failure process of a material is a complex process involving nonlinear behavior and different mechanisms.

Mechanics of Solder Materials

The mechanics of a material describes the response of a material to load. Such response is usually quantified in terms of displacement, strain and stress acting at every point in the material. The mechanical behavior of the material is represented using a stress–strain diagram. The diagram is obtained from tension test data on a sample of the material. Procedures for conducting a tension test on metallic materials are well documented in test standards such as ASTM-E8 [6] and ISO 6892 [9].

Essentials for Finite Element Simulation

Finite Element (FE) simulation solves the mathematical model of the physical problem. The following sections discuss the essential elements for the FE simulation process with respect to an illustrative problem in the assessment of solder joint reliability. The problem considers a surface mount microelectronic test assembly with a flip-chip package mounted on a Printed Circuit Board (PCB) using an array of solder joints.

Application III: Board-Level Drop Test

Consumer electronic products, particularly mobile devices such as mobile phones, thermal scanners, tablets and laptop computers, contain BGA packages and assemblies. These devices are susceptible to impact loading from unintentional dropping of the device during operation and handling. Such loading transfers the induced mechanical shock to solder joints in the electronic assembly that could initiate damage, leading to premature failure of the solder joints. Consequently, mechanical shock loading is considered in reliability assessment of solder joints. Impact loading is introduced through a board-level drop test of a PCB with a surface mounted electronic assembly containing BGA solder joints.

Application IV: Fatigue Fracture Process of Solder Joints

An assessment of solder joint reliability for BGA packages and assemblies under cyclic mechanical loading was addressed in Chap. 6. Finite Element (FE) simulations of the test assembly with BGA solder joints subjected to cyclic flexural loading and torsional loading was described in Sects. 6.3 and 6.4, respectively. The simulations established stress-strain hysteresis behavior of the critical solder joint in the array. Phenomenological fatigue life models, as described in Sect. 4.6, were developed based on least-squared fit correlations between the calculated characteristic fatigue-life variables (such as the accumulated inelastic strain, e in,acc and inelastic work density per load cycle, W in,acc ) and measured fatigue lives of identical assemblies. However, this classical life prediction method does not account for the observed multiple failure types occurring during the fatigue fracture process. In this respect, damage mechanics-based models, described in Chap. 7, are employed to simulate the dominant failure mechanisms observed during solder joint fatigue, namely ductile bulk solder failure and the relatively brittle solder/IMC interface fracture.