A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item Solder Interconnect Life Prediction under Complex Temperature Cycling with Varying Mean and Amplitude(2013) Chai, Fei; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electronic devices are under concurrent loading of the power cycling of the devices and the temperature cycling from the surrounding environment. Temperature histories resultant from these concurrent loading would be a complex temperature cycling with varying cyclic temperature mean and amplitude, as well as spatial thermal gradient. This study developed modeling approaches and quantified accuracies for predicting solder interconnect life under complex temperature cycling. Three modeling approaches were presented in this study: 1) modeling the strain energy under the resultant complex temperature cycling and employing the energy based fatigue life models; 2) segmenting the resultant complex temperature cycle into multiple simple temperature cycles with a single temperature range for each first, then assessing the life expectancy of the solder interconnect under the segmented simple temperature cycles and at last applying Miner's rule to superpose the damage; 3) estimating solder damage under the resultant complex temperature cycling by a standard temperature cycling with a single temperature range. Two case studies were included in this thesis: 1) chamber controlled complex temperature cycling with mini cycles occurring at the upper excursion on ceramic leadless chip carriers assembled by Sn36Pb62Ag2 and SnAg3.0Cu0.5 solder (without spatial thermal gradient); 2) combined temperature and power cycling on plastic ball grid array assembled by Sn63Pb37 and SnAg3.0Cu0.5 solder (with spatial thermal gradient). Physical tests were also conducted to quantify the developed modeling approaches.Item EVOLUTION OF THE MICROSTRUCTURE AND VISCOPLASTIC BEHAVIOR OF MICROSCALE SAC305 SOLDER JOINTS AS A FUNCTION OF MECHANICAL FATIGUE DAMAGE(2009) Cuddalorepatta, Gayatri; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework. Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb. Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys. Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.Item PROBABILISTIC PHYSICS OF FAILURE ASSESSMENT OF THERMOMECHANICAL FATIGUE IN HIGH-I/O AREA-ARRAY INTERCONNECTS(2009) Roettele, Shaughn Michael; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Thermal cycling durability of Plastic ball grid array (PBGA) interconnects is known to decrease as I/O count increases. This is due, in part, to mechanistic effects; such as increasing thermal expansion mismatches between component and PWB, due to increasing package sizes. Failure prediction due to these mechanistic effects is a deterministic process and is based on the load level found in the critical joint (joint with the most severe loading). However, due to probabilistic effects, for example manufacturing variabilities, premature failure may result in one of several joints in the neighborhood of the critical one. Failure probability increases as the number of joints in this critical region increases. Thus, observed failure rates are due to a convolution of deterministic and probabilistic effects. In effect, for large BGAs, deterministic predictions may overestimate interconnect durability. This thesis uses thermal cycling experiments and detailed mechanistic modeling to present a methodology for adjusting deterministic predictions of solder joint failure with a suitable probabilistic correction factor.Item Effects of Solder-Dipping as a Termination Re-Finishing Technique(2006-08-14) Sengupta, Shirsho; Pecht, Michael G.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Solder-dipping may be used to replace tin-rich finishes with eutectic tin-lead for tin-whiskering risk mitigation purposes. However, re-finishing also subjects electronic parts to new risks, including damage from the thermal re-finishing profile, finish non-uniformity, incomplete replacement of the pre-existing finish and poor solderability from re-finishing. This study overviews solder-dipping as a re-finishing technique and identifies key process and part parameters that could result in risks. A physical analysis procedure was developed and implemented to assess these risks on electronic parts. A quantitative metric was established to assess propensity for thermo-mechanical damage for solder-dipping parts. Surface mount dipped parts were prone to exposure of base-metal or interfacial intermetallics at termination corners, knees and heels. Solder-dipped insertion-mount parts showed regions of low finish thickness and possible deviations from eutectic tin-lead composition at portions close to the part-body.