Development of Micro-Computed Tomography Data Based Simulation Technique For Deformation and Strain Measurement of Densely Packed Electronics

Abstract

Rising pace of innovation and continuously increasing investments in research and developments has resulted in higher value added to each electronics equipment around us. This has also resulted in massive rise in demand in each segment of this industry. As of 2018, the semiconductor industry is the largest sector in the world, worth $248 billion. Growth of the consumer electronics industry can be largely accounted to miniaturization of the electrical components following the Moore’s Law. This miniaturization and reduction in form factor has occurred in the electronics used in the defense sector as well and at a high cost. The proposed budget for the US Defense has gone from $574.5 billion in 2018 to $597.1 billion in 2019. The Air Force's procurement budget has increased by 5.9 percent from $47.73 billion in 2018 to $50.54 billion in 2019. A large part of this budget is annually allocated behind the research, development and procurement of missile or missile related technologies.

Constant efforts are made to maximize the life of the electronics used in the missiles and study the Remaining Useful Life of these electronics to improve the cost efficiency of the missile program. Improvements in maximizing the life of electronics or the remaining useful life, requires understanding of the current in-situ condition of the electronics used in these systems. This requires researchers be able to quantify and analyze the amount of deformations the small electrical sub-assemblies inside the systems may observe over its service life. This data can then be used to predict the failure or analyze the extent of damage to predict and quantify its performance over its remaining life.

The existing popular methods used by the US Missile Command involves destructive testing of statistically representative selected samples of missile electronics which results in significant expensive. As per the Stockpile Recovery Program launched in 2015, performing simulations in of (mathematical models) has been cited as one of the ways to reduce the cost per unit of each missile and thus gain much required cost efficiency. This dissertation is thus on the development of non- destructive, non-invasive simulation and experimental techniques to quantify the deformation and strain occurring on the inside of the fuze electronics used in missile systems.

In this study the Author has used micro-computed tomography (micro-CT) data to make Finite Element models of a comprehensive fuze assembly and used the same micro-CT data to make experimental deformation measurements using a technique called Digital Volume Correlation.

Over the life cycle of a missile, the missile electronics are subjected to two categories of loading scenarios. One at the time of manufacturing and storage and other, during its service life. During manufacturing and storage, the electrical assemblies are subjected residual stresses left by the curing of potting thermoset resins or underfills and long hours of thermal aging during storage. During the service life, high-g and low-g mechanical shocks, vibration and sudden temperature changes are the most common forms of loading experienced by missile electronics. In order to protect the electrical assemblies and components from these mechanical and thermal loads, they are often potted within thermoset adhesives. This design further makes the strain and deformation quantification more challenging as the electrical components and assemblies are hidden from the line of sight.

While current methods involve experimentation and testing on a sample set at intermittent stages during the life cycle of a missile, these methods are often destructive in nature. This study is based on use of micro-CT scan data to measure deformations and strains occurring on these electrical components, hidden from the line of sight due to protective adhesives. This is done using Finite element models (simulation technique) and use of micro-CT scan data based Digital Volume Correlation (experimental technique).

Conventional FE modeling approach is found to be prohibitively time consuming for modeling densely packed electrical assemblies. CAD modeling, assembly and meshing of numerous electrical components, big and small, with varied different material models is found to be the bottle neck and thus little to no literature exists on doing FE modeling of comprehensive fuze assemblies. This work involves development of a novel FE modeling strategy using micro-CT data to overcome the problem. An application of this technique would be to perform FE simulations of any field extracted electrical assemblies at any stage of its service life.

Digital Volume Correlation is a technique analogous to Digital Image Correlation for computing deformations and strains in a non-contact manner utilizing the voxel/pixel intensity data. This study reports on use of this technique to experimentally monitor the physical integrity of the electrical components by comparing two micro-CT scans and computing deformations and strains the components would have experienced over a particular time frame. A further application of this technique is explored by quantifying damage progression in the electronics as a function of time over the entire 3D domain on the assembly.

Chapter 1 gives a detailed introduction and literature review on the relevant topics. Chapter 2 denotes a brief introduction to X-ray micro-CT systems, micro-CT data and its usability. A detailed account of the simulation and experimental technique is given in Chapter 3. Chapter 3 also enlists case studies done to explore the capabilities of the technique developed. Chapter 4 and 5 are based on applications of the technique developed to investigate effect of voids in solder joints found in a popular electronic packaging in the present-day consumer electronics followed by Conclusions and Discussion in Chapter 6 and 7.