Dynamic fracture process of solder/intermetallic interface in lead-free solder interconnects using cohesive zone model

Solder joint reliability (SJR) is an important requirement in electronics packaging. Most of the failures in a package are found in solder joints and interconnections. Brittle solder/intermetallic (IMC) interface fracture is the dominant failure mode in cases of impact loading and fast mechanical fatigue loading. In this study, the response of a single solder specimen subjected to cyclic shear deformation and a typical ball grid array (BGA) package undergoing board-level drop test is investigated. The finite element (FE) analysis of the single reflowed solder specimen and the BGA package is employed to understand the mechanics of the solder joints and the brittle solder/ IMC fracture process. Inelastic behavior of the solder joints is described using unified inelastic strain model (Anand model) with optimized model parameters. The brittle solder/IMC interface fracture is demonstrated using cohesive zone model (CZM). The accuracy of interface fracture description depends on the CZM model prescribed in the analysis. The CZM model is modified further to ensure better predictive capability especially in cyclic loading. FE results for single solder specimen under shear fatigue test simulation shows that the CZM parameters degraded as the number of cycles is increased. Rapid damage progression occurs at the beginning of cycle and propagated slowly for subsequent cycles. For a boardlevel drop test simulation, the critical solder joint is located the farthest away from the center of the board. The highest stress and inelastic strain are confined to a small edge region at solder/IMC interfaces. Damage initiated from the outer peripheral solder and propagated into the inner peripheral solder joint

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH

Effect of Dynamic Flexural Loading on the Durability and Failure Site of Solder Interconnects in Printed Wiring Assemblies

This dissertation investigates the durability of solder interconnects of area array packages mounted on Printed Wiring Assemblies (PWAs) subjected to dynamic flexural loads, using a combination of testing, empirical curve fitting and mechanistic modeling. Dynamic 4-point bend tests are conducted on a drop tower and with an impact pendulum. Failure data is collected and an empirical rate-dependent durability model, based on mechanistic considerations, is developed to estimate the fatigue failure envelopes of the solder, as a function of solder strain and strain-rate. The solder plastic strain histories are obtained from the PWA flexural strain and strain rate, using transfer functions developed from 3D transient Finite Element Analysis (FEA) with rate-dependent solder material properties. The test data also shows the existence of multiple competing failure sites: solder, copper trace, PWB under solder pads, and layers of intermetallic compound (IMC) between the solder and solder pads. The failures in the IMC layers are found to be either in the bulk of the IMC layers or at the interface between different species of IMC layers. The dominant failure site is found to be strongly dependent on the loading conditions. The empirical model is demonstrated for solder failures as well as Cu trace failures, and the transition between their competing failure envelopes is identified. This dissertation then focuses in detail on two of these competing failure sites: (i) the solder and (ii) the interface between two IMC layers. A strain-range fatigue damage model, based on strain-rate hardening and exhaustion of ductility, is used to quantify the durability and estimate the fatigue constants of the solder for high strain rates of loading. Interfacial fracture mechanics is used to estimate the damage accumulation rates at the IMC interface. The IMC failure model and the solder failure model provide a mechanistic perspective on the failure site transitions. Durability metrics, based on the mechanics of these two failure mechanisms, are used to quantify the competing damage accumulation rates at the two failure sites for a given loading condition. The results not only identify which failure site dominates but also provide estimate of the durability of the solder interconnect. The test data shows good correlation with the model predictions. The test vehicles used in this study consist of PWAs with Sn37Pb solder interconnects. But the proposed test methodologies and mechanistic models are generic enough to be easily extended to other emerging lead free solder materials. Wherever possible, suggestions are provided for the development of test techniques or phenomenological models which can be used for engineering applications. A methodology is proposed in the appendix to implement the findings of this thesis in real-world applications. Damage in the solder interconnect is quantified in terms of generic empirical metrics, PWA flexural strain and strain rate. It is shown that the proposed metrics (PWA strain and strain rate) can quantify the durability of the solder interconnect, irrespective of the loading orientation or the PWA boundary conditions