CALCE EPSC Graduate Student Theses and Dissertation Abstracts (2017)

Ning, Yan (Ph.D.)
Reliability of Copper-Filled Stacked Microvias in High Density Interconnect Circuit Boards

The electronics industry strives to produce affordable, lightweight, and reliable products with higher performance. At the electronic component level, this translates to components with increased I/Os and reduced footprints, and on the package substrate and printed circuit board (PCB) level, to the use of high density interconnects (HDIs). HDI technology makes use of microvias as interconnects between different conductor layers. According to IPC standards, microvias are blind or buried vias that are equal to or less than 150 μm in diameter. Advances in miniaturized electronic devices have led to the evolution of microvias from single-level to stacked structures that intersect multiple HDI layers. A stacked microvia is usually filled with electroplated copper to make electrical interconnections and provide structural support.

A challenge for HDI circuit board processing is to fabricate microvias without generating defects in the deposited copper structures. Firstly, the copper plating process can easily generate voids in microvias. When voids are present, localized stress concentrations within the electrodeposited copper structure can degrade the reliability of microvias. Secondly, poor quality of electroless copper (a process step following microvia hole drilling and prior to electrolytic copper plating, that makes the microvia hole conductive) results in inferior bonding between the base of the microvia and the target pad underneath the microvia. Microvia base and target pad interface separation is a common failure observed in HDI circuit boards. The objectives of this dissertation are to determine the effects of voids on the lifetime of copper-filled stacked microvias, and to develop an analytical model that the electronics industry can use to predict microvia fatigue life and assess risks associated with production and use of the latest generation of HDI circuit boards. The dissertation also aims to quantitatively address the factors that affect microvia interface separation.

A parametric study was conducted to investigate the effects of voids on the thermo-mechanical reliability of copper-filled stacked microvias using 3-D finite element analysis and strain-based fatigue life estimation. It was found that microvia fatigue life is affected by geometrical void characteristics, such as shape, size, and location; microvia aspect ratio; and material properties of dielectric layers. Large voids decrease the lifetime of microvias—for example, a 16% conical void results in a microvia fatigue life that is only 1.4% of that of a non-voided microvia. Moreover, microvia aspect ratio and z-axis coefficient of thermal expansion (CTE) of the HDI dielectric material are critical parameters for the lifetime. The fatigue life of a voided microvia of 0.25 aspect ratio is more than two orders of magnitude longer than the fatigue life of a voided microvia of 0.75 aspect ratio with the same void size. An increase of the z-axis CTE by 40% (from 50 ppm/°C to 70 ppm/°C) decreases the microvia fatigue life by 95%. As an outgrowth of this study, a microvia virtual qualification method was proposed. Using the combination of finite element analysis and fatigue life estimation, the required amount of HDI board reliability testing will be reduced, cutting overall development time and cost.

The factors that affect microvia fatigue life were examined, and a design of experiment (finite element simulation) was performed to quantify the effects of those factors on microvia lifetime in terms of cycles to failure. A second-order regression life prediction model was developed using response surface mothed (RSM) to predict cycles to failure of copper-filled stacked microvias under thermal loading. The life prediction model accounts for not only the microvia design parameters and material properties, but also voiding defects introduced during the manufacturing process. The model can predict cycles-to-failure of microvias without voids and with voids of different sizes. The electronics industry can use this model as a convenient and inexpensive tool for HDI design and process validation. This is the first known regression model for copper-filled stacked microvia life prediction. Finally, the factors that affect microvia interface separation were quantitatively addressed. Finite element modeling was used to simulate microvias with imperfect electroless copper layers. This study revealed how thermal loadings and structure flaws (in terms of initial crack length) affect the chance of microvia interface separation.

Kashani Pour, Amir Reza (Ph.D.)
Optimal Requirement Determination For Pricing Availability-Based Sustainment Contracts

Sustainment constitutes 70% or more of the total life-cycle cost of many safety-, mission- and infrastructure-critical systems. Prediction and control of the life-cycle cost is an essential part of all sustainment contracts. For many types of systems, availability is the most critical factor in determining the total life-cycle cost of the system. To address this, availability-based contracts have been introduced into the governmental and non-governmental acquisitions space (e.g., energy, defense, transportation, and healthcare).However, the development, implementation, and impact of availability requirements within contracts is not well understood. This dissertation develops a decision support model based on contract theory, formal modeling and stochastic optimization for availability-based contract design. By adoption and extension of the “availability payment” concept introduced for civil infrastructure Public-Private Partnerships (PPPs) and pricing for Performance-Based Logistics (PBL) contracts, this dissertation develops requirements that maximize the outcome of contracts for both parties. Under the civil infrastructure “availability payment” PPP, once the asset is available for use, the private sector begins receiving a periodical payment for the contracted number of years based on meeting performance requirements. This approach has been applied to highways, bridges, etc. The challenge is to determine the most effective requirements, metrics and payment model that protects the public interest, (i.e., does not overpay the private sector) but also minimizes that risk that the asset will become unsupported. This dissertation focuses on availability as the key required outcome for mission-critical systems and provides a methodology for finding the optimum requirements and optimum payment parameters, and introduces new metrics into availability-based contract structures. In a product-service oriented environment, formal modeling of contracts (for both the customer and the contractor) will be necessary for pricing, negotiations, and transparency. Conventional methods for simulating a system through its life cycle do not include the effect of the relationship between the contractor and customer. This dissertation integrates engineering models with the incentive structure using a game theoretic simulation, affine controller design and stochastic optimization. The model has been used to explore the optimum availability assessment window (i.e., the length of time over which availability must be assessed) for an availability-based contract.

Valentine, Nathan (M.S.)
Failure Modes and Mechanisms Analysis of Silicon Power Devices

Silicon power devices are a major reliability concern for power electronics converters. Failure modes, mechanisms, and effects (FMMEA) is a well-established method for identifying, analyzing, and improving the reliability of a system. Effects for an FMMEA are system dependent; therefore, this work establishes a Failure Modes and Mechanisms Analysis (FMMA) for silicon power devices which identifies the relevant failure modes and mechanisms of silicon power devices. Following the FMMA, a set of failure analysis case studies of silicon power devices which aid in the identification of failure causes and mechanisms for the FMMA. Finally, the criticality of the different mechanisms is discussed based on the severity of a failure within a given system, the occurrence of a failure mechanism for a given component, and the ability to detect a failure using techniques such as PHM, criticality can be identified for the mechanisms.

Formica, Tyler (M.S.)
The Effectiveness of Warranties in the Solar Photovoltaic and Automotive Industries

A warranty is an agreement outlined by a manufacturer to a customer that defines performance requirements for a product or service. Although long warranty periods are a useful marketing tool, in 2011 the warranty claims expense was 2.6% of total sales for computer original equipment manufacturers (OEMs) and is over 2% of total sales in many other industries today. Solar PV systems offer inverters with 5-15 year warranties and PV modules with 25-year performance warranties. This is problematic for the return on investment (ROI) of solar PV systems when the modules are still productive and covered under warranty but inverter failures occur due to degradation of electronic components after their warranty has expired. Out-of warranty inverter failures during the lifetime of solar panels decrease the ROI of solar PV systems significantly and can cause the annual ROI to actually be negative 15-25 years into the lifetime of the system. This thesis analyzes the factors that contribute to designing an optimal warranty period and the relationship between reliability and warranty periods using General Motors (GM) and the solar PV industry as case studies. A return on investment of a solar photovoltaic system is also conducted and the effect of reliability, changing tax credit structures, and failure areas of solar PV systems are analyzed.

Sridharan, Raman (M.S.)
Stress Response of Tall and Heavy Electronic Components Subjected to Multi-axial Vibration

Electronic assemblies often experience multiaxial vibration environments in use and tall, heavy components are more vulnerable when exposed to multiaxial vibration than are shorter, lighter assemblies. The added vulnerability comes from higher stresses that are a result of nonlinear dynamic amplification which large components are susceptible to under simultaneous multiaxial excitation, termed multi degree of freedom (MDoF) excitation. However, it is still common practice to conduct vibration durability testing on electronic assemblies one axis at a time – in what is termed sequential single degree of freedom (SSDoF) testing. SSDoF testing has been shown to produce lower fatigue damage accumulation rates than simultaneous MDoF testing, in the leads of tall and heavy electronic components. This leads to overestimating the expected lifespan of the assembly. This paper investigates the geometric nonlinearities and the resulting cross-axis interactions that tall and heavy electronic components experience when subjected to vibration excitation along two orthogonal axes – one direction is in the plane of the PWB and the other is along the normal to the PWB. The direction normal to the PWB aligns with the axial direction of the leads, while the in-plane direction aligns with the primary bending direction of the leads. Harmonic excitation was simultaneously applied to both axes to study the vibration response as a function of frequency ratio and phase “difference” along the two axes. The experimental observations were verified with a nonlinear dynamic Finite Element study. The effect of geometric nonlinearity on cyclic stresses seen in the vibrating component are analyzed.

Pandian, Guru (M.S.)
Effects of long-term aging on lead-free solders (SAC405) and surface finish (pure tin)

Since 2006, commercial electronics manufacturers have been banned from using lead-based materials and other toxic materials in their products due to the RoHS directive from the European Union. This led to industries transitioning to lead-free materials to be used in solder and surface finishes of their products. Although all of commercial electronics industry has transitioned to lead-free materials, some of the reliability and safety critical products used in industries such as defense, aerospace, automobile, and healthcare sectors are still exempted from the lead-free regulation. These industries are hesitant to transition to lead-free due to lack of data and hence the confidence on the long-term reliability of lead-free electronics. Known issues of tin whiskers and solder interconnect fatigue which can arise later in a products life have raised concerns related to the use of lead-free materials in electronic assemblies. To address these concerns, 10 year old lead-free systems were examined to determine the solder interconnect degradation level and tin whisker risk level.

Heid, Tim (M.S.)
Effect of Temperature on Vibration Fatigue of SAC105 Solder Material After Extended Room Temperature Aging

This study focuses on temperature dependent vibration durability of Sn1.0Ag0.5Cu (SAC105) solder material after extended aging at room temperature. The test specimens are printed wiring assemblies (PWA) with leadless chip resistors (LCR) soldered on a daisy-chained printed wiring board (PWB) with SAC105 solder that have been aged for 10 years at room temperature. The test vehicles are subjected to sinusoidal excitation at the fundamental resonant frequency at low temperature (-50 °C), room temperature (25 °C) and high temperature (120 °C) at a constant acceleration level. The stresses in the interconnects depend on the component location on the PWA as well as on two different pad designs. The resulting differences in the stress levels provide multiple data points on the fatigue curve from a single test. Nonlinear dynamic finite element analysis (FEA) is used to create strain transfer functions (STF) to determine the relation between the in-test measured strain on the PWB and the local strain in the solder pads. The vibration testing results are eventually combined with the solder strains to estimate temperature dependent S-N (Strain - Life) curves. The fatigue durability is found to decrease significantly with increasing temperature, in part due to the increase in solder strain, and in part due to degradation of the solder cyclic fatigue properties.

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