Interconnect Fatigue Research Articles

Energy based modeling for temperature cycling induced tin silver copper solder interconnect fatigue life

Lead free (LF) solders such as tin silver copper (SAC) and its variations have been used in microelectronic products for the past decade due the Restriction of Hazardous Substances regulation. In use, temperature cycling is a major contributor to the wear-out failure mechanisms of solder joints during the life span of electronics. To predict the reliability of electronic solder joints, a fatigue life model is needed. Currently, strain energy density based models are widely used for solder joint fatigue life estimation. This paper reviews existing LF solder fatigue life models and provides strain energy based life model constants for a variety of LF solder materials.

Impact of underfill and other physical dimensions on Silicon Lateral IGBT package reliability using computer model with discrete and continuous design variables

An effort to design and build a prototype LED driver system which is energy efficient, highly compact and with few component count was initiated by a consortium UK universities. The prototype system will be based on Silicon Lateral IGBT (LIGBT) device combined with chip on board technology. Part of this effort, finite element modelling and analysis were undertaken in order to mitigate the underfill dielectric breakdown failure and solder interconnect fatigue failure of the LIGBT package structure. Electro-static analysis was undertaken to predict the extreme electric field distribution in the underfill. Based on electro-static analysis, five commercial underfill were selected for thermo-mechanical finite element analysis on solder joint fatigue failure prediction under cyclic loading. A design optimisation analysis was endeavoured to maximise the solder interconnect reliability by utilising a computer model with continuous variable (physical dimensions) and discrete variables (underfill type) and a stochastic optimiser such as multi-objective mixed discrete particle swarm optimisation. From the optimisation analysis best trade off solution are obtained.

Characterization of micro-contact resistance between a gold nanocrystalline line and a tungsten electrode probe in interconnect fatigue testing.

An electromechanically-coupled micro-contact resistance measurement system is built to mimic the contact process during fatigue testing of nanoscale-thickness interconnects using multiple probe methods. The design combines an optical microscope, high-resolution electronic balance, and micromanipulator-controlled electric probe, and is coupled with electrical measurements to investigate microscale contact physics. Experimental measurements are performed to characterize the contact resistance response of the gold nanocrystalline pad of a 35-nm-thick interconnect under mechanical force applied by a tungsten electrode probe. Location of a stable region for the contact resistance and the critical contact force provides better understanding of micro-contact behavior relative to the effects of the contact force and the nature of the contact surface. Increasing contact temperature leads to reduced contact resistance, softens the pad material, and modifies the contact surface. The stability of both contact resistance and interconnect resistance is studied under increasing contact force. Major fluctuations emerge when the contact force is less than the critical contact force, which shows that temporal contact resistance will affect interconnect resistance measurement accuracy, even when using the four-wire method. This performance is demonstrated experimentally by heating the Au line locally with a laser beam. Finally, the contact resistances are calculated using the LET (Li-Etsion-Talke) model together with combined Holm and Sharvin theory under various contact forces. Good agreement between the results is obtained. This research provides a way to measure change in interconnect line resistance directly under a stable contact resistance regime with a two-wire method that will greatly reduce the experimental costs.

Strain-Range-Based Solder Life Predictions Under Temperature Cycling With Varying Amplitude and Mean

To investigate modeling approaches for predicting solder interconnect fatigue life under field thermal conditions, a complex temperature cycle composed of three temperature dwell points was modeled and physically tested. Modeling the solder interconnect life expectancy under these complex conditions is accomplished by segmenting the complex temperature cycle into multiple simple temperature cycles, predicting the life expectancy of the solder interconnect under the simple temperature cycles using the Engelmaier model, and applying Miner's rule to superpose the damage of the segmented cycles and predict solder interconnect fatigue life under the complex temperature cycle. Several modeling strategies based on different segmenting schemes are presented, and the one with the best comparison with the physical test results is identified. A case study is presented for ceramic leadless chip carriers assembled with Sn62Pb36Ag2, Sn96.5Ag3Cu0.5, and SN100C solders. The Engelmaier model constants to model the simple temperature cycles are also provided as a basis for modeling the complex cycle.

Interconnect Reliability Characterization of a High-Density 3-D Chip-on-Chip Interconnect Technology

This paper investigates the solder interconnect reliability of a high-density 3-D chip-on-chip technology under an accelerated thermal cycling (ATC) test condition through finite element (FE) modeling and experimental validation. The fabrication of the 3-D chip-on-chip technology is accomplished with a two-step gap control bonding process to minimize the solder squeezing phenomenon. The alternative goal of this paper is placed on the influences of underfill on the interconnect failure mechanism and reliability. With the calculated plastic strain, the thermal fatigue life of the most critical solder interconnect can be estimated through an empirical Coffin-Manson fatigue life prediction model. The effectiveness of the proposed FE modeling is demonstrated through ATC tests. Finally, to identify the parameters most affecting the lead-free solder interconnect reliability, both parametric FE analysis and a simulation-based experimental design scheme based on a response surface methodology are carried out with the validated FE model. Both the numerical and experimental results that underfill can not only change the interconnect failure mechanism from an interfacial crack between the Al pad and the copper (Cu) layer of the under bump metallurgy to a cohesive solder failure, but also greatly improve the solder interconnect thermal fatigue life by as much as 2.5 times. Furthermore, the experimental design demonstrates that both the Young's modulus of intermetallic compound and thermal expansion coefficient of underfill are identified as the parameters most affecting the solder interconnect reliability of the 3-D chip-on-chip technology.

BGA Solder Life Prediction under Combined Power and Temperature Cycling Condition

Solder interconnect failure is a known life limiting failure mechanism that is induced by cyclic temperature excursions. Thermal fatigue reliability of solder interconnects is conventionally assessed by simple temperature cycling test, which applies a constant temperature range, fixed dwell times and ramp rates during the test. However, due to the user controlled power cycles, non-constant workloads, and changes in the surrounding environment, electronics in the field often experience a complex combination of temperature and power cycling. In this study, the effect of power cycling superposed on a simple temperature cycling is experimentally examined. Furthermore, a scheme for modeling the solder interconnect fatigue life of Plastic Ball Grid Array (PBGA) parts under the concurrent power and temperature cycling. Damage, defined as the number of applied cycles over the number of survivable cycles, from the simple temperature cycle and the power cycle are linearly added using Miner's rule, and compared with the concurrent temperature and power cycling test. Cycles to failure of each condition is derived by life testing conducted on Plastic Ball Grid Array (PBGA) assembled with eutectic and SAC305 solder.

Critical Review of the Engelmaier Model for Solder Joint Creep Fatigue Reliability

A solder interconnect fatigue life model was developed by Werner Engelmaier in the early 1980s as an improvement upon the inelastic strain range-based Coffin-Manson model. As developed, the model provides a first-order estimate of cycles to failure for SnPb solder interconnects under power and thermal cycles. While the model has been widely adopted for SnPb solder joint reliability prediction, many issues that arise from simplifications in formulating input model parameters as well as from the complex physics of solder degradation challenge the model's ability to accurately estimate cycles to failure. Deficiencies with the model have been reported by a number of researchers. This paper reviews and summarizes the major issues with the Engelmaier model in its applicability to predict solder joint thermal fatigue life.

Fatigue of Chip Scale Package Interconnects Due to Cyclic Bending

This study investigates the effect of quasi-static bending loads (strain rate=0.05/s) on the durability of 0.5 mm pitch Chip Scale Package (CSP) interconnects when assembled on FR4 substrates. The substrates have rows of CSPs and are subjected to three-point bending loads. Overstress curvature limits are experimentally determined and used to identify limits for zero-to-max cyclic bending loads. The test configuration is simulated using finite element modeling (FEM) and the total strain accumulated in the solder joints is estimated. Using the FEM model, a calibration curve is constructed to relate the cyclic curvature range in the substrate to the cyclic strain range in the critical solder joint. Bending moments along the substrate are estimated from the forces applied at the center of the board during the fatigue test. Strains measured on the substrate surface and the bending displacements measured at the center are used to estimate curvatures at different locations along the substrate. Using the calibration curve, the total strains in the solder joint are obtained for the applied loading. A strain-range fatigue damage model proposed by Coffin and Manson, is used to predict the cycles to failure for the applied loading. Predicted durability is compared to experimental measurements. Concave substrate curvature is found to be more damaging than convex curvature, for interconnect fatigue. Finite element simulations are repeated for life-cycle loading to predict acceleration factors. Using the acceleration factors, the product durability is estimated for life-cycle environments.

Solar-Cell Interconnect Design for Terrestrial Photovoltaic Modules

An investigation of interconnect fatigue in photovoltaic systems has led to the development of useful reliability-design and life-prediction algorithms presented here. Experimental data gathered in this study indicate that the classical strain-cycle (fatigue) curve for the interconnect material fails to account for the broad statistical scatter, which is critical to reliability prediction. To fill this shortcoming, a functional form is fitted to experimental cumulative interconnect failure-rate data to yield statistical fatigue curves (with failure probability as a parameter) that enable (a) the prediction of cumulative interconnect failures during the design life of an array field, and (b) the unambiguous—i.e., quantitative—interpretation of data from field-service qualification (accelerated thermal-cycling) tests. Optimal interconnect cost-reliability design algorithms are derived, intended to minimize the cost of energy over the design life of the array field. This procedure yields not only the minimum break-even cost of delivered energy, but also the required degree of interconnect redundancy and an estimate of array power degradation during the design life of the array field. The usefulness of the design algorithms is demonstrated with realistic examples of design optimization, prediction, and service qualification testing.

Photovoltaic Array Reliability Optimization

Several statistical reliability studies have been conducted in areas of photovoltaic component design covering cell failure, interconnect fatigue, glass breakage and electrical insulation breakdown. This paper integrates the results from these studies and draws general conclusions relative to optimal reliability features for modules. The analysis is based on designing for specified low levels of component failures and then controlling the degrading effects of the failures through the use of fault tolerant circuitry and module replacement. Means of selecting the cost-optimal level of component failures, circuit redundancy, and module replacement are described.