Optimizing the reliability of power electronics module isolation substrates

Optimal design of a power electronics module isolation substrate is assessed using a combination of finite element structural mechanics analysis and response surface optimisation technique. Primary failure modes in power electronics modules include the loss of structural integrity in the ceramic substrate materials due to stresses induced through thermal cycling. Analysis of the influence of ceramic substrate design parameters is undertaken using a design of experiments approach. Finite element analysis is used to determine the stress distribution for each design, and the results are used to construct a quadratic response surface function. A particle swarm optimisation algorithm is then used to determine the optimal substrate design. Analysis of response surface function gradients is used to perform sensitivity analysis and develop isolation substrate design rules. The influence of design uncertainties introduced through manufacturing tolerances is assessed using a Monte-Carlo algorithm, resulting in a stress distribution histogram. The probability of failure caused by the violation of design constraints has been analyzed. Six geometric design parameters are considered in this work and the most important design parameters have been identified. Overall analysis results can be used to enhance the design and reliability of the component

A Study on the Effect of Bond Stress and Process Temperature on Palladium Coated Silver Wire Bonds on Aluminum Metallization

In the past ten years, the increasing price of gold has motivated the wire bonding industry to look for alternative bonding wire materials in the field of microelectronics packaging. A new candidate wire to replace gold is palladium coated silver wire. In this thesis, the effect of the two specific process parameters “bond stress” and “process temperature” on the ball bonds made with the new candidate wire are investigated. Using 20 μm diameter wire and various level-combinations of these process parameter, ball bonds are produced according to a special accelerated optimization method to result in a target diameter of 46 ± 0.5 μm and target height of 16 ± 0.5 μm. Three different levels are used for each of the specific process parameters. After pre-selecting a few process parameters, the accelerated method determines the levels for the process parameters “impact force” and “electric flame-off current” with a 2×2 design of experiments. Then, the ultrasound parameter is maximized up to a level where a pre-selected ultrasonic deformation occurs to the bonds, maintaining the target bond diameter and height. The bond quality is measured by measuring the shear strength of the bonds. The results show that • the bond geometry is not affected by the bond stress, • the optimized specific process parameters vary by less than ~0.5 % when bond stress values are varied from 60 to 100 MPa, • the variations in optimized parameters are larger than ~3.0 % when the BT is changed from 100 to 200 ºC, • ball bonds achieve acceptable shear strength (> 120 MPa) when the values for both, bond stress and bond temperature, are high, • ultrasound level and shear stress interact, the higher shear stress the lower the ultrasound level required. An average shear strength of ~120 MPa is achieved with 11.4 % ultrasound, 100 MPa bond stress, and 200 ºC bond process temperature. In summary, a robust methodology is presented in this thesis to efficiently optimize the ball bonding process as demonstrated with the new candidate wire has a bondability similar to that of gold wire with only minor adjustment in the bonding process needed

Hyper-Sensitive MEMS Pressure Sensor Array for Microscale Bubble Pressure Measurement

As technology is advancing, more complex, efficient, and powerful devices are being made. These powerful devices generate a lot of heat which needs to be taken out to maximize their performance. Hence, efforts are being made to improve cooling techniques for these devices. Boiling is one such technique used in the cooling of devices. The heat transfer performance in the flow boiling systems is higher than that in pool boiling systems. With a simple add-on tapered manifold over a plain surface, we can convert pool boiling to flow boiling. This study will lead to improved performance and reliability of microelectronic devices, supercomputers, server chips, etc. The forces from bubbles growing can provide a pumpless, self-sustained unidirectional flow effectively transforming pool boiling into the extremely efficient flow boiling, resulting in energy savings. MEMS pressure sensor array will be mounted at the end of tapered manifold to map the pressure field around a nucleating bubble. The thesis describes the design, fabrication, packaging, and testing of a bulk micromachined sensor array that is capable of monitoring the pressure progression of a bubble. The sensor utilizes an extremely thin 225 nm square Si3N4 diaphragm which is produced by etching away the bulk silicon with XeF2 through holes present in the diaphragm. A unique process flow was developed to achieve the diaphragm thickness in nanometers. Four polysilicon piezoresistors, mounted on the surface of the diaphragm, where the stress is maximum, are used by the sensor. The thesis also discusses the results obtained from the response of the fabricated sensor. Various attempts were made to get a voltage output in response to applied pressure. These values were acquired over a number of experiments repeated at similar experimental conditions to demonstrate the repeatability of the calibration data. The value of sensitivity, derived from the slope of the linear calibration plot of Vout (V) vs. Pressure (Pa), is 5.26 μV/Pa, which is very close to the required target hyper-sensitivity of 5 μV/Pa

National Educators' Workshop: Update 1991. Standard Experiments in Engineering Materials Science and Technology

Given here is a collection of experiments presented and demonstrated at the National Educators' Workshop: Update 91, held at the Oak Ridge National Laboratory on November 12-14, 1991. The experiments related to the nature and properties of engineering materials and provided information to assist in teaching about materials in the education community

National Educators' Workshop. Update 92: Standard Experiments in Engineering Materials Science and Technology

This document contains a collection of experiments presented and demonstrated at the workshop. The experiments related to the nature and properties of engineering materials and provided information to assist in teaching about materials in the education community

MICROELECTRONICS PACKAGING TECHNOLOGY ROADMAPS, ASSEMBLY RELIABILITY, AND PROGNOSTICS

This paper reviews the industry roadmaps on commercial-off-the shelf (COTS) microelectronics packaging technologies covering the current trends toward further reducing size and increasing functionality. Due tothe breadth of work being performed in this field, this paper presents only a number of key packaging technologies. The topics for each category were down-selected by reviewing reports of industry roadmaps including the International Technology Roadmap for Semiconductor (ITRS) and by surveying publications of the International Electronics Manufacturing Initiative (iNEMI) and the roadmap of association connecting electronics industry (IPC). The paper also summarizes the findings of numerous articles and websites that allotted to the emerging and trends in microelectronics packaging technologies. A brief discussion was presented on packaging hierarchy from die to package and to system levels. Key elements of reliability for packaging assemblies were presented followed by reliabilty definition from a probablistic failure perspective. An example was present for showing conventional reliability approach using Monte Carlo simulation results for a number of plastic ball grid array (PBGA). The simulation results were compared to experimental thermal cycle test data. Prognostic health monitoring (PHM) methods, a growing field for microelectronics packaging technologies, were briefly discussed. The artificial neural network (ANN), a data-driven PHM, was discussed in details. Finally, it presented inter- and extra-polations using ANN simulation for thermal cycle test data of PBGA and ceramic BGA (CBGA) assemblies

Low cost, SPF aluminum cryogenic tank structure for ALS

Past production work has shown that cryogenic tank structure for the Shuttle Booster Rockets and the Titan system have very high life cycle costs for the fuel tank structure. The tanks are machined stiffener-skin combination that are subsequently formed into the required contour after machining. The material scrap rate for these configurations are usually high, and the loss of a tank panel due to forming or heat treatment problems is very costly. The idea of reducing the amount of scrap material and scrapped structural members has prompted the introduction of built-up structure for cryogenic tanks to be explored on the ALS program. A build-up structure approach that has shown improvements in life cycle cost over the conventional built-up approach is the use of superplastically formed (SPF) stiffened panels (reducing the overall part count and weight for the tank) resistance spot welded (RSW) to outer tank skin material. The stiffeners provide for general stability of the tank, while the skin material provides hoop direction continuity for the loads

Low-Cost Fabrication Techniques for RF Microelectromechanical systems (MEMS) Switches and Varactors

A novel low-cost microfabrication technique for manufacturing RF MEMS switches and varactors is proposed. The fabrication process entails laser microstructuring and non-clean room micro-lithography standard wet bench techniques. An optimized laser microstructuring technique was employed to fabricate the MEMS component members and masks with readily available materials that include, Aluminum foils, sheets, and copper clad PCB boards. The non-clean room micro-lithography process was optimized to make for the patterning of the MEMS dielectric and bridge support layers, which were derived from deposits of negative-tone photosensitive epoxy-based polymers, SU-8 resins (glycidyl-ether-bisphenol-A novolac) and photoacid activated ADEX™ dry films. The novel microfabrication technique offers comparatively reasonably yields without intensive cleanroom manufacturing techniques and their associated equipment and processing costs. It is an optimized hybrid rapid prototyping manufacturing process that makes for a reduction in build cycles while ensuring good turnarounds. The techniques are characterized by analysing each contributing technology and dependent parameters: laser structuring, lithography and spin coating and thin film emboss. They are developed for planar substrates and can be modified to suit specific work material for optimized outcomes. The optimized laser structuring process offers ablation for pitches as small as 75 µm (track width of 50 µm and gap 25 µm), with a deviation of 3.5 % in the structured vector’s dimensions relative to design. The lithography process also developed for planar and microchannel applications makes for the realization of highly resolved patterned deposits of the SU-8 resin and the laminated ADEX™ polymer from 1 µm to 6 µm and with an accuracy ±0.2 µm. The complete micro-fabrication technique fabrication techniques are demonstrated by realizing test structures consisting of RF MEMS switches and varactors on FR4 substrates. Both MEMS structures and FR4 substrate were integrated by employing the micro-patterned polymers, developed from dry-film ADEX™ and SU-8 deposits, to make for a functional composite assembly. Average fabrication yield up to 60 % was achieved, calculated from ten fabrication attempts. The RF measurement results show that the RF MEMS devices fabricated by using the novel micro-fabrication process have good figure-of-merits, at much lower overall fabrication costs, as compared to the devices fabricated by conventional cleanroom process, enabling it to be used as a very good micro-fabrication process for cost-effective rapid prototyping of MEMS

Design and reliability of polymeric packages for high voltage power semiconductors

This thesis focuses on the development of a novel polymer based housing for power thyristor devices typically used in long distance high voltage direct current (HVDC) transmission. Power thyristor devices used in HVDC power conversion stations are typically packaged in a hermetically sealed ceramic housing and have demonstrated an excellent history of reliability and performance. However, to avoid increasing the number of thyristors in future higher powered HVDC schemes thyristors having higher power ratings at 8.5 kV and sizes at 125 mm and 150 mm diameters are sought for implementation to achieve higher transmission ratings of, for example, 4000 A at +/- 800 kV. The main disadvantages of such large ceramic-based packages are higher processing cost and weight whilst robustness is also a concern. To overcome these issues, replacing the current ceramic housing with a polymeric material has been investigated in this project. The advantages it is anticipated such packages will provide include lower cost, less weight, robustness, recyclability, etc. However, some challenges it will also offer are: non-hermeticity i.e. polymers are moisture and gas permeable, potentially more complex manufacturing routes, and different electrical, mechanical and thermal properties compared to ceramic materials. The work presented in this thesis was part of a larger project where these challenges have been addressed by developing and testing a prototype polymeric thyristor housing. The prototype is aimed at demonstrating that polymer packages can deliver performance and reliability comparable to, if not better than, current ceramic packages. In this thesis, it is the package development and reliability related studies that are discussed. Because the housings will experience severe electrical stresses and various thermal excursions during their service life, the electrical and thermo-mechanical behaviour of the polymer housing was studied using finite element analysis to gain an understanding of the effects of various design variables and materials properties on performance and the tradeoffs between performance and manufacturability. From these modelling studies, design guidelines have been established for the future development of polymer housings. On the other hand, to identify the physics-of-failure of the prototype that was manufactured as part of the project, accelerated life tests were performed to study its reliability. The knowledge gained from the polymer prototype development was then applied to the design of a larger 125 mm diameter housing using the Taguchi method of experimental design