Thermal Management In Stacked Dies

The present trend in integrated circuit (IC) packaging, geared towards reduction in size and higher functionality in IC packages, has called about the need for integrating dies vertically in a single package. Added functionality and capacity of stacked dies packages within the same footprint as a single die package practically means higher power densities packages, and this is where thermal issues arise. Through the present study, it is of utmost interest to determine how the temperature and stress distribution within the package varies with the different loads of power applied in the silicon dies. The findings from the present study are of in-depth meaning particularly in the thermal management aspect because the challenge arises in attempting to remove heat efficiently from stacked dies packages. The research study is facilitated with ANSYSTM 7.0, which is used as a finite element modelling and analysis tool. The temperature and stress distribution in stacked dies packages under different source power is studied. Thermal stresses are induced in the package as a result of mismatch in the coefficient of thermal expansion (CTE) properties of the various package materials. Warpage of the package is to be limited to avoid the loss of electrical and mechanical connections. For all cases, the junction temperature and stress increases linearly with total power generation rates in the package. The junction temperature in each package is found to be independent of the power splitting ratios among the dies. The increment in junction temperature with respect to total power decreases from the single die package to the four stacked dies package. The printed circuit board provides a conduction path for effective heat removal from the bottom of the package to ambient as proven from the simulation results

Methods and Models for Assessing Solder Interconnect Reliability of Control Boards in Power Electronic Systems

Over the past 20 years, power electronic systems have been increasingly required to operate in harsh environments including automotive, deep-well drilling and aerospace applications. In parallel, the higher power densities and miniaturization of the power switching module result in elevated stress levels on the control circuitry. The objective of this study was to develop methods and models for assessing the interconnect reliability of components used in the control circuitry for power electronic systems. Physics-of-Failure modeling and a series of thermal and reliability simulations were conducted on a 2.2 kW variable-frequency drive to evaluate the susceptibility of system level and component level failure mechanisms. Assessment methods consisted of developing CalcePWA simulation models of the primary sub-assemblies and constructing a power cycling apparatus to perform accelerated testing of the drive

Damage Initiation and Evolution in Voided and Unvoided Lead Free Solder Joints Under Cyclic Thermo-Mechanical Loading

The effect of process-induced voids on the durability of Sn-Pb and Pb-free solder interconnects in electronic products is not clearly understood and researchers have reported conflicting findings. Studies have shown that depending on the size and location, voids are not always detrimental to reliability, and in fact, may sometimes even increase the durability of joints. This debate is more intensified in Pb-free solders; since voids are more common in Pb-free joints. Results of experimental studies are presented in this study to empirically explore the influence of voids on the durability of Ball Grid Array (BGA) Pb-free solder joints. In order to quantify the detailed influence of size, location, and volume fraction of voids, extensive modeling is conducted, using a continuum damage model (Energy Partitioning model), rather than the existing approaches, such as fracture mechanics, reported in the literature. The E-P approach is modified in this study by use of a successive initiation method, since depending on their location and size; voids may influence either the time to initiate cyclic fatigue damage or time to propagate fatigue damage, or both. Modeling results show competing interactions between void size and location, that results in a non-monotonic relationship between void size and durability. It also suggests that voids in general are not detrimental to reliability except when a large portion of the damage propagation path is covered with either a large void or with many small voids. In addition, this dissertation also addresses several fundamental issues in solder fatigue damage modeling. One objective is to use experimental data to identify the correct fatigue constants to be used when explicitly modeling fatigue damage propagation in Pb-free solders. Explicit modeling of damage propagation improves modeling accuracy across solder joints of vastly different architectures, since the joint geometry may have a strong influence on the ratio of initiation-life to propagation-life. Most solder fatigue models in the literature do not provide this capability since they predict failure based only on the damage accumulation rates during the first few cycles in the undamaged joint. Another objective is to incorporate into cyclic damage propagation models, the effect of material softening caused by cyclic micro-structural damage accumulation in Pb-free solder materials. In other words the model constants of the solder viscoplastic constitutive model are continuously updated with the help of experimental data, to include this cyclic softening effect as damage accumulates during the damage-propagation phase. The ability to model this damage evolution process increases the accuracy of durability predictions, and is not available in most current solder fatigue models reported in the literature. This mechanism-based microstructural damage evolution model, called the Energy Partitioning Damage Evolution (EPDE) model is developed and implemented in Finite Element Analysis of solder joints with the successive initiation technique and the results are provided here. Experimental results are used as guidance to calibrate the Energy Partitioning fatigue model constants, for use in successive initiation modeling with and without damage evolution. FEA results show 15% difference between the life predicted by averaging technique and successive initiation. This difference could significantly increase in the case of long joints such as thermal pads or die-attach, hence validating the use of successive initiation in these cases. The difference between using successive initiation with and without damage evolution is about 10%. Considering the small amount of effort that has to be made to update the constitutive properties for progressive degradation, it is recommended that softening be included whenever damage propagation needs to be explicitly modeled. However the damage evolution exponents and the corresponding E-P model constants obtained in this study, using successive initiation with damage evolution, are partially dependent on the specimen geometry. Hence, these constants may have to be re-calibrated for other geometries

Fatigue testing method for fine bond wires in an LQFP package

A mechanical testing setup was developed to study the fatigue response of fine thermo-sonic wire bond connection in low profile quad flat packages (LQFP). The testing set-up was designed to induce pre-defined multi-axial stresses in the wire bond loops of non-encapsulated packages in order to mimic their deformation behavior during the thermo-mechanical loading. Lifetime curves were obtained up to 1.0E7 loading cycles with fatigue failure occurring in the heat affected zone of the ball bond. The experimental fatigue data in combination with extended FEA provided the basis for a Coffin Manson lifetime model. The proposed fatigue testing procedure can be applied as a highly efficient method for evaluation of various wire bonded packages by using a limited number of test samples and simultaneous testing of several wire bonds.Energie-Umweltmanagemen