A NON-LINEAR DAMAGE MODEL WITH LOAD DEPENDENT EXPONENTS FOR SOLDERS UNDER SEQUENTIAL CYCLIC SHEAR LOADS

The damage state of a material subject to cyclic loads is often characterized by the cycle ratio of applied cycles to the number of survivable cycles. The damage in a material under sequential cyclic loading is widely estimated using Miner’s rule. Miner’s rule assumes that damage in a material accumulates linearly under cyclic loading and the damage path is independent of the applied load level. Due to these inherent assumptions, Miner’s rule inaccurately estimates life under sequential loading conditions for solders. To improve the accuracy of damage estimation, a non-linear damage accumulation model based on damage curve approach that takes into account the effect of loading sequence under sequential loading conditions is proposed for solders in this dissertation. In the proposed non-linear damage model, damage is related to the cycle ratio using a power law relationship where the power law (damage) exponent is defined as a function of the applied load level (cycles to failure). An experimental approach is proposed to determine the load dependent exponents of the non-linear model under three load levels. The test matrix consisted of a series of single level cyclic and sequential cyclic shear tests in a thermo-mechanical micro analyzer. Load dependent exponents were developed for SAC305 (96.5%Sn+3.0%Ag+0.5Cu) solder material and the applicability of these exponents were validated by tests under a new loading condition and reverse loading sequence. Experimental results revealed that the value of damage exponent decreased with the severity of the applied load level. Additionally, taking damage analogous to crack growth, an analytical relationship between the damage exponent and the applied load level was developed from the Paris’ law for crack propagation. This enables determination of non-linear damage curves at different load levels without conducting extensive experimentation. The damage due to crack initiation was assumed to be 10% of the total damage and sensitivity analysis was carried out to determine the effect of this assumption. The load dependence of the Paris’ law exponent (m) was also derived for SAC305 solder material. Analysis of the failed specimens revealed fatigue crack in the solder joints along the tin grain boundaries

Characterisation of the cyclic softening properties of solder

Master'sMASTER OF ENGINEERIN

Reliability evaluation of stacked die BGA assemblies under mechanical bending loads

This thesis presents a reliability evaluation of stacked die ball grid array (BGA) assemblies under mechanical bending loads. The test specimens used in this investigation were four die stacked BGAs assembled on printed circuit boards (PCBs) with eutectic tin-lead solder and gold over nickel finishes, both as-reflowed and after aging. The failure envelopes of both types of specimen were quantified in terms of PCB flexural strain and strain rate. The experimental data from cyclic bending tests at three strain amplitudes with a constant strain rate have been used to determine the effect of strain amplitudes on cycles to failure. The experimental data from cyclic bending tests were combined with the data from impact tests to determine the effect of strain rate to cycles to failure. The failure sites associated with each test condition were identified, and failure site transition phenomena are reported and discussed

Evaluating the Susceptibility of Electronic Components Assembled with Leaded Solder to Flexural Failures, with High Rate Considerations

Multilayer ceramic capacitors (MLCCs) are subjected to high strain rate flexural loading via drop tower and air gun tests, with PWB strain rates ranging from 1/s to 10/s. Three MLCC part sizes, three different manufacturers, and standard and flexible termination parts are included in the study. Standard termination capacitors failed via the well documented flex crack failure mechanism. However in all cases this crack followed a vertical path not typical of this failure mechanism. Flexible termination capacitors failed via a newly discovered failure mechanism involving delamination in the end cap metallization between the silver filled epoxy and the nickel-tin plating. For size 1206 parts, this delamination was seen in both end caps, and the part detached from the test board. For size 0603 parts, this delamination occurred in one end cap, while the opposite end cap fractured though the ceramic in a manner similar to a flex crack. Size 0603 parts also failed via the vertical flex cracks documented in standard termination parts. All of the documented failures of MLCC devices at PWB strain rates of ≥ 1/s occurred at maximum PWB strain values greater than an order of magnitude lower than those seen in lower strain rate testing. This rate dependency of MLCC part failures has vast implications for products intended for high rate environments. Additionally, when the PWB strain rate was increased along with PWB maximum strain, flexible termination capacitors performed worse than their standard termination equivalents. This brings to issue the role of these next generation parts in portable consumer electronic devices as well as other designs with high rate implications. Ball grid array (BGA) devices are subjected to four point bend tests via a servo-hydraulic testing machine at PWB strain rates ≤ 0.1/s. The resulting BGA data is found to adhere reasonably well to the Coffin-Manson low cycle fatigue relationship. Independently generated BGA data that differs with respect to many testing variables is plotted alongside the experimental data. The high correlation of the data set indicates the possibility of creating a BGA mechanical failure model that is independent of failure site, package type, and test specifications

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

Multi-Scale Dynamic Study of Secondary Impact During Drop Testing of Surface Mount Packages

This dissertation focuses on design challenges caused by secondary impacts to printed wiring assemblies (PWAs) within hand-held electronics due to accidental drop or impact loading. The continuing increase of functionality, miniaturization and affordability has resulted in a decrease in the size and weight of handheld electronic products. As a result, PWAs have become thinner and the clearances between surrounding structures have decreased. The resulting increase in flexibility of the PWAs in combination with the reduced clearances requires new design rules to minimize and survive possible internal collisions impacts between PWAs and surrounding structures. Such collisions are being termed ‘secondary impact’ in this study. The effect of secondary impact on board-level drop reliability of printed wiring boards (PWBs) assembled with MEMS microphone components, is investigated using a combination of testing, response and stress analysis, and damage modeling. The response analysis is conducted using a combination of numerical finite element modeling and simplified analytic models for additional parametric sensitivity studies

Investigation into Solder Joint Failure in Portable Electronics Subjected to Drop Impact

Ph.DDOCTOR OF PHILOSOPH

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH