Created: 10/19/99

CALCE EPSC Graduate Student Theses (1995)

Bauernschub, Richard A. (M.S. Mechanical Engineering)

A Physics-of-Failure Approach to Defect-Related Reliability

Remaining competitive in the electronic packaging industry requires that engineers shrink geometries, reduce costs, and decrease production time, while at the same time increase reliability. An impedance to all of these concerns is manufacturing defects. As device and package sizes diminish, the relative size of common defects becomes a reliability problem. In addition, manufacturing costs and production times consist largely of screening procedures performed at the end of every production stage. Clearly, reducing and optimizing screening procedures will reduce production costs and attaining an understanding of the impact that defect magnitudes will have on device reliability is critical to improving the product.

With the above as motivation, an effort was initiated to produce a resource from which the root-causes, effects, methods of prevention, and methods of detection could be found for common manufacturing defects at all packaging levels. The result is the Microelectronics Defects Database, a resource which is located on the CALCE InfoNet on the World Wide Web and is available to anyone with Internet access.

Chen, An-Tung (Ph.D. Mechanical Engineering)

Plated Through Hole Life Estimation in Rigid Flex Using Finite Element Analysis

In recent years flexible printed circuits (FPCs) have been used successfully in virtually all market products. They are architecturally similar, yet behave quite different structurally than conventional electronic rigid printed wiring boards. In the rigidized portion of FPCs, plated through holes (PTHs) are commonly used to interconnect copper trace layers so as to provide a continuous signal path. When FPCs are subjected to cyclic temperature changes, plated through holes can fail due to the thermomechanical stresses arising from the mismatch in coefficients of thermal expansion (CTE) of the various layer materials and the copper barrel.

This dissertation is to analyze the thermomechanical failure of plated-through-h structures in the rigid portion of FPCs and evaluate the design parameters which affect the critical stresses. Parametric studies with a finite element model are used to examine the qualitative/quantitative effects of several geometric parameters on the critical barrel stress in the PTH. Design of experiments (DoE) methods including linear regression analysis and quadratic analysis are conducted for the determination of an empirical equation for this critical stress. Actual nonlinear stresses and strains are approximated using Neuber's rule from the derived linear elastic model. The fatigue life of plated through holes is then estimated using the Coffin-Manson relation. These results are compared with nonlinear finite element analyses.

In order to accomplish the finite element analyses, thermomechanical properties of the constituent materials are crucial for accurate failure modeling. In this dissertation general methodologies are developed for characterizing thermomechanical properties of acrylic adhesive material in the rigid portion of FPCs. Thermomechanical effective properties of FPC assembly are also investigated under various temperature ranges so as to verify those properties. Various schemes are evaluated to predict the global response of the FPC assembly by comparing to experiment measurements and finite element simulations.

Chen, Qing Yan Jenny (M.S. Mechanical Engineering)

Failure Mechanisms and Reliability Assessment of Optoelectronic and Electronic Packages

An overview is given of failure mechanisms and failure modes on laser diode and LED, fiber reliability issues, and possible failure of laser package in different environments. A thermal and vibrational CALCE software simulation was performed on a high-speed Laser Transmitter module printed circuit board (PCB) assembly subjected to two environment profiles: a space environment profile, and a military avionic profile. Reliability assessment of components on the PCB was conducted by using CADMP-II software. The average times to failure of the assembly of twenty-three different failure mechanisms are predicted. Acceleration factors of lifetime of laser diodes in a space environment versus room temperature is also given. Coupling loss due to misalignment caused by vibration stress is discussed. Finally, derating function curves of the least time to failure mechanisms and optical misalignment are presented.

Evans, Jillian (Ph.D. Reliability Engineering)

Effects of Thermal and Humidity Cycling on 3-D Electronic Packaging Technology

hree dimensional electronic packaging technologies are emerging for many electronic system applications requiring high density. The reliability of these emerging technologies remains relatively unknown. This research focused on understanding the reliability of two of these important new technologies, through characterization of their dominant failure mechanisms. This research showed that interfacial de-bonding of the polyimide dielectrics used in layered stack interconnection is a dominant mechanism in humidity cycling. De-bonding occurs under progressive interfacial crack growth due to fluctuating loads during absorption and desorption of moisture. Eventually, a bus line on the chip stack will rupture in the wake of the crack front, causing electrical failure. Fatigue in bus interconnect metallization is a dominant mechanism in layered stacks under thermal cycling conditions, driven by a large out-of-plane coefficient of thermal expansion of the polyimide dielectrics. A procedure to model these mechanisms, using a stress-life approach, was also developed, which employed finite element modeling as a basis for stress assessment in key stack structures.

Ganguly, Gautam (M.S. Mechanical Engineering)

Damage Due To Lead Insertion in Through-Hole Compliant Pin Connectors

The increasing use of insertion mount connectors with compliant pins in modern electronic packaging raises serious concerns about local damage and permanent deformation of the PTH (plated through hole) wall caused by the insertion forces. Current techniques to ensure acceptable levels of deformation involve cross-sectioning of samples and visual qualification for every new configuration, which are time-consuming and non-value-added procedures. In this study a faster and more cost-effective predictive methodology is proposed, to replace the empirical qualification procedure with a simulation model based on the physics-of-failure (PoF) approach. A pseudo-closed-form model is developed in this study to predict nonlinear deformations and stresses caused during insertion of compliant pins in a PTH. the model is based on a Fourier series technique and separation of variable method. A verification and correlation technique is recommended through finite element modeling, for assessing and refining the accuracy of the closed-form analytical model.

Gannamani, Ranjit (M.S. Mechanical Engineering)

Assembly of PEMs Without Popcorning

Moisture-induced popcorning in plastic encapsulated microelectronics (PEMs) during the reflow process is a significant issue in microchip packaging. Though popcorning may not result in immediate failure, it jeopardizes the long-term reliability of the device. Current techniques to avoid popcorning involve baking and dry-bagging, which are time-consuming and non-value-added procedures. This study proposes an alternate technique to deter popcorning - the use of optimal reflow temperature ramp rates. The results indicate that, for a specific device subjected to known preconditioning, an optimum temperature ramp rate exists, at which popcorning does not occur.

Three reflow cycles are specified as part of current guidelines for the preconditioning of PEMs prior to reliability testing; effect of the second and third cycles is investigated, with the intention of optimizing current standards. The results show that they may be unnecessary.

Experimental setup: The setup, designed and built at the CALCE EPRC, is an extremely effective tool for the real time investigation of popcorning in PEMs. It consists of an infrared heating system incorporated within an X-ray radiography unit. The PEM can be precisely heated through any desired temperature profile, while simultaneously monitoring its X-ray image for popcorning. The equipment can be used to identify the specific stage of the reflow profile at which doming of the encapsulant and package cracking occurs. The effect of variations in parameters such as the moisture content of the PEM and reflow temperature ramp rates on popcorning in PEMs can be investigated.

Munamarty, Ramesh (M.S. Mechanical Engineering)

Failure Mechanisms of Plastic Encapsulated Microcircuits

The motivation of the study was to identify the failure sites and mechanisms of plastic ball grid array packages when the devices are subjected to moisture and reflow conditions. Two types of plastic ball grid array packages, a 225-lead full matrix array and a 256-lead perimeter array, were subjected to moisture preconditioning followed by simulated infra-red reflow. The packages were subsequently examined for delamination and cracking using scanning acoustic microscopy and environmental scanning electron microscopy. Delamination and cracking was observed in both package types, originating in the die attach and propagating along the weakest interfaces. This was the interface between the copper and solder mask in areas of the substrate covered with copper, and the interface between the fiber and the resin of the substrate in areas where copper was not present. This study also shows that there is a critical ramp rate below which popcorn cracking does not occur. These results agree with a similar finding that there is a critical ramp rate for popcorning in plastic quad flat packs, and can be used to define manufacturing conditions which allow devices to be processed popcorn-free without resorting to baking or dry-bagging.

Panchwagh, Tanmay (M.S. Mechanical Engineering)

Reliability of Surface Mount Capacitor Subjected to Wave Soldering

Stress cracking from thermal stresses generated by the wave soldering operation is a major factor contributing to yield loss for multilayer ceramic capacitors (MLCC). A f inite-element model (FEM) was generated to determine the sites of stress concentration in MLCC s during wave soldering. The location of the maximum principal stress was found to be at the surface of the capacitor dielectric under the edge of the end-termination. This location remained the same for all the conditions studied. The site of maximum stress correlates well with the location of crack initiation sites observed in field failures. In addition, the maximum stress was found to depend on the end-termination material, the end-termination thickne ss and the dielectric material, but to be independent of the overall capacitor dimensions wi thin the range studied.

Roy, Loren Louis (M.S. Mechanical Engineering)

Experimental Determination of the Shear Modulus of Compliant Adhesives Using Single Lap-Shear Tests

In some high performance applications of electronics, compliant adhesive serve to attach ceramic circuit cards to aluminum backing plates. The aluminum transfers large amount of heat created by the electronics on the ceramic circuit card to the environment. Since the coefficients of thermal expansion (CTE's) of the aluminum and the ceramic differ greatly, the strains caused by a temperature change can be quite large. The compliant adhesive reduces the mechanical strains and corresponding stresses in the ceramic. In order to estimate the stress level in a ceramic circuit card prior to constructing a prototype, a designer must obtain the mechanical properties of the adhesive. The present study focuses on the experimental determination of the shear modulus of compliant adhesive. The author creates lap-shear specimens by matching two aluminum plates bonded with a thin layer of Ablestik ECF563, a compliant adhesive. A hydraulic tensile testing machine with modifications loads the specimen while eddy current non-contact transducers mounted on the aluminum measure the shear displacements; the relative motion of the two halves of the lap-shear specimen. A constant strain rate res ults due to the linear response of the adhesive and the constant displacement rate control of the tensile tester. The shear modulus results from the slope of the stress-strain curve in the linear response region modified using a Finite Element Analysis S imulation. The adhesive tested exhibits homogeneous isotropic behavior by assumption.

Sidharth, S. (Ph.D. Mechanical Engineering)

Fatigue Life Estimation of Leads in Local Electronic Assemblies Subject to Vibration

The rapid advancement of integrated circuits and associated electronic technologies have placed increasing demands on electronic packaging and its material structures in terms of reliability requirements. In addition to thermally induced stresses, electronic packages often experience dynamic external loads during shipping, handling and/or operation. This is especially important in automotive and military environments. These dynamic loads give rise to large dynamic stresses in the leads causing fatigue failures. The motivation for this dissertation is the fact that no comprehensive tools or guidelines are available for a designer to quickly estimate the lead stresses, and hence the fatigue life of these assemblies.

This dissertation lays out a global-local approach for the determination of fatigue life of leads in electronic packages mounted on the printed wiring board. In the local global-local approach, if the displacements at the ends of the leads can be estimated, they can be input as a boundary condition for a simple finite element model for a detailed stress analysis, which in turn can be used to estimate the fatigue life.

For the purpose of determining the out-of-plane displacement of leads, the electronic components are divided into three major categories : (a) Those with leads on two sides eg. DIP (b) Those with leads on all four sides eg. quad flat pack (c) Those with leads in an area array format eg. PGA. For the first category, an analytical model is developed to calculate the displacements of the leads using the concept of beams on elastic foundations. The leads are modeled as axial and rotational springs. For the analysis of components in the second category, finite element analysis simulations along with design of experiments have been used to determine the displacements for the corner and center leads, which are the two most critical ones among all. The models have been developed on five factors which affect the behavior of these assemblies. Guidelines on when corner lead and when center lead becomes the most severely stressed are provided. A rigorous approach for conducting experimental design has also been outlined. The third category has not been a focus in this dissertation. The models developed will help the design engineers to quickly estimate the fatigue life of these electronic assemblies.

Yalamanchili, Prasad (Ph.D. Mechanical Engineering)

Evaluation of Electronic Packaging Reliability Using Acoustic Microscopy

Reliability of integrated circuit packages depends in many respects on their mechanical integrity. The effect of structural weaknesses caused by poor bonding, voids, microcracks or delaminations may not be evident immediately in the electrical performance characteristics, but may cause premature failure. The C-mode scanning acoustic microscopy (C-SAM) is an excellent tool for non-destructive failure analysis of IC packages. To gain insights into the nature of images obtained from acoustic microscopy the acoustic wave parameters must be understood and used to interpret the data (images and waveforms) obtained via acoustic microscopy.

Basic material parameters pertinent to acoustic microscopy have been calculated in this thesis and they will assist researchers in the understanding and interpretation of acoustic images. The spot size, depth of focus, lateral and axial resolutions for different materials in electronic packaging and the pressure and intensity coefficients of reflection and transmission for typical material systems in electronic packaging are presented and analytical methods are developed to calculate these parameters. The basic resolution limitations of acoustic microscopy are teid to the limitations to electronic packaging material systems.

Transmission and reflection scanning acoustic microscopy techniques for reliability evaluation of electronic packaging are presented and the limitations of each of these techniques are discussed.

This thesis also demonstrates the use of acoustic microscopy to systematically identify the package defects and determine the mechanisms for package failures. The application of the acoustic wave parameters and principles of C-SAM to the solution of three key problems in the physics of failure of electronic packages are presented. The examples selected ranged from plastic to ceramic packages, thus showing the application of C-SAM to packaging materials from low to high acoustic impedance.