Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Thermo-mechanical reliability studies of lead-free solder interconnects

N/ASolder interconnections, also known as solder joints, are the weakest link in electronics packaging. Reliability of these miniature joints is of utmost interest - especially in safety-critical applications in the automotive, medical, aerospace, power grid and oil and drilling sectors. Studies have shown that these joints' critical thermal and mechanical loading culminate in accelerated creep, fatigue, and a combination of these joints' induced failures. The ball grid array (BGA) components being an integral part of many electronic modules functioning in mission-critical systems. This study investigates the response of solder joints in BGA to crucial reliability influencing parameters derived from creep, visco-plastic and fatigue damage of the joints. These are the plastic strain, shear strain, plastic shear strain, creep energy density, strain energy density, deformation, equivalent (Von-Mises) stress etc. The parameters' obtained magnitudes are inputted into established life prediction models – Coffin-Manson, Engelmaier, Solomon (Low cycle fatigue) and Syed (Accumulated creep energy density) – to determine several BGA assemblies' fatigue lives. The joints are subjected to thermal, mechanical and random vibration loadings. The finite element analysis (FEA) is employed in a commercial software package to model and simulate the responses of the solder joints of the representative assemblies' finite element models. As the magnitude and rate of degradation of solder joints in the BGA significantly depend on the composition of the solder alloys used to assembly the BGA on the printed circuit board, this research studies the response of various mainstream lead-free Sn-Ag-Cu (SAC) solders (SAC305, SAC387, SAC396 and SAC405) and benchmarked those with lead-based eutectic solder (Sn63Pb37). In the creep response study, the effects of thermal ageing and temperature cycling on these solder alloys' behaviours are explored. The results show superior creep properties for SAC405 and SAC396 lead-free solder alloys. The lead-free SAC405 solder joint is the most effective solder under thermal cycling condition, and the SAC396 solder joint is the most effective solder under isothermal ageing operation. The finding shows that SAC405 and SAC396 solders accumulated the minimum magnitudes of stress, strain rate, deformation rate and strain energy density than any other solder considered in this study. The hysteresis loops show that lead-free SAC405 has the lowest dissipated energy per cycle. Thus the highest fatigue life, followed by eutectic lead-based Sn63Pb37 solder. The solder with the highest dissipated energy per cycle was lead-free SAC305, SAC387 and SAC396 solder alloys. In the thermal fatigue life prediction research, four different lead-free (SAC305, SAC387, SAC396 and SAC405) and one eutectic lead-based (Sn63Pb37) solder alloys are defined against their thermal fatigue lives (TFLs) to predict their mean-time-to-failure for preventive maintenance advice. Five finite elements (FE) models of the assemblies of the BGAs with the different solder alloy compositions and properties are created with SolidWorks. The models are subjected to standard IEC 60749-25 temperature cycling in ANSYS 19.0 mechanical package environment. SAC405 joints have the highest predicted TFL of circa 13.2 years, while SAC387 joints have the least life of circa 1.4 years. The predicted lives are inversely proportional to the magnitude of the areas of stress-strain hysteresis loops of the solder joints. The prediction models are significantly consistent in predicted magnitudes across the solder joints irrespective of the damage parameters used. Several failure modes drive solder joints and damage mechanics from the research and understand an essential variation in the models' predicted values. This investigation presents a method of managing preventive maintenance time of BGA electronic components in mission-critical systems. It recommends developing a novel life prediction model based on a combination of the damage parameters for enhanced prediction. The FEA random vibration simulation test results showed that different solder alloys have a comparable performance during random vibration testing. The fatigue life result shows that SAC405 and SAC396 have the highest fatigue lives before being prone to failure. As a result of the FEA simulation outcomes with the application of Coffin-Manson's empirical formula, the author can predict the fatigue life of solder joint alloys to a higher degree of accuracy of average ~93% in an actual service environment such as the one experienced under-the-hood of an automobile and aerospace. Therefore, it is concluded that the combination of FEA simulation and empirical formulas employed in this study could be used in the computation and prediction of the fatigue life of solder joint alloys when subjected to random vibration. Based on the thermal and mechanical responses of lead-free SAC405 and SAC396 solder alloys, they are recommended as a suitable replacement of lead-based eutectic Sn63Pb37 solder alloy for improved device thermo-mechanical operations when subjected to random vibration (non-deterministic vibration). The FEA simulation studies' outcomes are validated using experimental and analytical-based reviews in published and peer-reviewed literature.N/

Technology of Welding and Joining

In this book, you will find information on new materials and new welding technologies. Problems related to the welding of difficult-to-weld materials are considered and solved. The latest welding technologies and processes are presented. This book provides an opportunity to learn about the latest trends and developments in the welding industry. Enjoy reading

Thermal and thermo-mechanical performance of voided lead-free solder thermal interface materials for chip-scale packaged power device

The need to maximise thermal performance of electronic devices coupled with the continuing trends on miniaturization of electronic packages require innovative package designs for power devices and modules such as Electronic Control Unit (ECU). Chip scale packaging (CSP) technology offer promising solution for packaging power electronics. This is as a result of the technology’s relatively improved thermal performance and inherent size advantage. In CSP technology, heat removal from the device could be enhanced through the backside of the chip. Heat dissipating units such as heat spreader and/or heat sink can be attached to the backside (reverse side) of the heat generating silicon die (via TIM) in an effort to improve the surface area available for heat dissipation. TIMs are used to mechanically couple the heat generating chip to a heat sinking device and more crucially to enhance thermal transfer across the interface. Extensive review shows that solder thermal interface materials (STIMs) apparently offer better thermal performance than comparable state-of-the-art commercial polymer-based TIMs and thus a preferable choice in packaging power devices. Nonetheless, voiding remains a major reliability concern of STIMs. This is coupled with the fact that solder joints are generally prone to fatigue failures under thermal cyclic loading. Unfortunately, the occurrence of solder voids is almost unavoidable during manufacturing process and is even predominant in lead (Pb)-free solder joints. The impacts of these voids on the thermal and mechanical performance of solder joints are not clearly understood and scarcely available in literature especially with regards to STIMs (large area solder joints). Hence, this work aims to investigate STIM and the influence of voids on the thermo-mechanical and thermal performance of STIM. As previous results suggest that factors such as the location, configuration (spatial arrangement) and size of voids play vital roles on the exact effect of voids, extensive three dimensional (3D) finite element modelling is employed to elucidate the precise effect of these void features on a Pb-free STIM selected after thermo-mechanical fatigue test of standard Pb-free solder alloys. Finite element analysis (FEA) results show that solder voids configuration, size and location are all vital parameters in evaluating the mechanical and thermal impacts of voids. Depending on the location, configuration and size of voids; solder voids can either influence the initiation or propagation of damage in the STIM layer or the location of hot spot on the heat generating chip. Experimental techniques are further employed to compare and correlate levels of voiding and shear strength for representative Pb-free solders. Experimental results also suggest that void size, location and configuration may have an influence on the mechanical durability of solder joints. The findings of this research work would be of interest to electronic packaging engineers especially in the automotive sector and have been disseminated through publications in peer reviewed journals and presentations in international conferences

A THERMOMECHANICAL FATIGUE LIFE PREDICTION METHODOLOGY FOR BALL GRID ARRAY COMPONENTS WITH REWORKABLE UNDERFILL

Underfill materials were originally developed to improve the thermo-mechanical reliability of flip-chip devices due to the large coefficient of thermal expansion (CTE) mismatch between the silicon die and substrate. More recently, underfill materials, specifically reworkable underfills, have been used to improve reliability of second level interconnects in ball grid array (BGA) packages in harsh end-use environments such as automotive, military and aerospace. In these environments, electronic components are exposed to mechanical shock, vibration, and large fluctuations in temperatures. Although reworkable underfills improve the reliability of BGA components under mechanical shock and vibration, some reworkable underfills have been shown to reduce reliability during thermal cycling environments. Consequently, this research employs experimental and numerical approaches to investigate the impact of reworkable underfill materials on thermomechanical fatigue life of solder joints in BGA packages. In the first section of the analysis, material characterization of a reworkable underfill is performed to determine appropriate material models for reworkable underfills. In the second analysis section, a variety of underfill materials with different properties are exposed to harsh and benign thermal cycles to determine the stress state responsible for reducing fatigue life of solder joints in BGA packages. In the final analysis section, simulations are performed on the BGAs with reworkable underfill to develop a fatigue life predication methodology that implements a modified mode separation scheme. The model developed in this work provides a working fatigue life approach for BGA packages with reworkable underfills exposed to thermal loading. The results of this study can be utilized by the automotive, military, and aerospace industries to optimize underfill material selection process and provide reliability assessment of BGA components in real world environments

Dynamic Mechanical and Failure Properties of Solder Joints

Ph.DDOCTOR OF PHILOSOPH