ORCID Identifier(s)

0000-0003-3694-7493

Graduation Semester and Year

2016

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dereje Agonafer

Second Advisor

Ashfaq Adnan

Abstract

Microelectronic devices are an essential part of human life. The idea of Internet of Things (IoT) where all the everyday devices will be connected with each other transmitting information and operating on their own is becoming very popular. A silicon chip which has millions of transistors in it, has been the heart and brain of the everyday components ranging from personal entertainment, communication to Automotive ECU (electronic control Unit), and health care management systems to home security systems. Every aspect of modern human experience is dependent on this tiny silicon chip. Microelectronic devices can be divided into two broad categories based on their functionality: active and passive devices. Active devices are capable of changing the direction of electron flow. Some of the examples of actives devices are transistors, diodes, LEDs. Passive devices cannot change the direction of the electron flow. Passive devices include resistors, capacitors, and inductors. Essentially the chip making process is an intricate technology where bulk silicon is transformed into thin silicon wafers. After that series of surface micromachining, etching, material deposition processes are done to achieve a known good die. This tiny silicon is so delicate that it can’t be placed on the Printed circuit board on its own. Due to outside heat, moisture and corrosive environment, this chip has to protected by a layer of polymer also known as mold compound. Now inside a silicon chip there are thousands of nanometer pitch interconnects. So to scale up the interconnection level on the silicon chip, wire bonds or micro-bumps are created in a semiconductor package. Even for passive devices such as Multi-layered Ceramic Capacitor (MLCC), termination end has to be connected with the outside world. Conductive terminations are soldered with the land pads on the printed circuit boards. Active and passive devices have multiple layers of materials in them and also they are connected on the PCB through solder joint. The CTE mismatch among the component (Device, PCB and Solder) will create thermo-mechanical fatigue and eventually components fail due to breaking of interconnection between them. Drop/impact is a common reason for failure. Modern handheld gadgets are prone to repetitive drops on a rigid surface. This high strain rate loading will cause severe damage to interconnect and the device itself, causing failure of the system to meet operational expectations. This present work focuses on the failure mitigation of the active and passive devices and interconnects. 1st part of the work involves extensive investigation of the MLCC flex cracking problem due to PCB bending, thermo-mechanical fatigue and drop. For mitigating the MLCC flex cracking problem, innovative packaging and interconnection technique are introduced. The 2nd part of the work involves thermo-mechanical reliability estimation and improvement of the active devices (QFN and BGA) on custom printed circuit board. Due to different layup of the prepreg and conductive material in the PCB, the bulk mechanical properties of changes and impacts the board level solder joint reliability of the QFN and BGA Packages. A failure mitigation technique involving tailoring the mechanical properties of the outermost prepreg layers have been proposed to enhance thermo-mechanical reliability.

Keywords

Microelectronic devices, Flex cracking, Solder Joint Reliability (SJR)

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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