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