Kyle Yazzie

High-temperature nanoindentation behavior of Al/SiC multilayers

Nanoscale Al/SiC composite laminates have unique properties, such as high strength, high toughness, and damage tolerance. In this article, the high-temperature nanoindentation response of Al/SiC nanolaminates is explored from room temperature up to 300°C. Selected nanoindentations were analyzed postmortem using focused ion beam and transmission electron microscopy to ascertain the microstructural changes and the deformation mechanisms operating at high temperature.

Mechanical Characterization of Lead-Free Sn-Ag-Cu Solder Joints by High-Temperature Nanoindentation

The reliability of Pb-free solder joints is controlled by their microstructural constituents. Therefore, knowledge of the solder microconstituents’ mechanical properties as a function of temperature is required. Sn-Ag-Cu lead-free solder alloy contains three phases: a Sn-rich phase, and the intermetallic compounds (IMCs) Cu6Sn5 and Ag3Sn. Typically, the Sn-rich phase is surrounded by a eutectic mixture of β-Sn, Cu6Sn5, and Ag3Sn. In this paper, we report on the Young’s modulus and hardness of the Cu6Sn5 and Cu3Sn IMCs, the β-Sn phase, and the eutectic compound, as measured by nanoindentation at elevated temperatures. For both the β-Sn phase and the eutectic compound, the hardness and Young’s modulus exhibited strong temperature dependence. In the case of the intermetallics, this temperature dependence is observed for Cu6Sn5, but the mechanical properties of Cu3Sn are more stable up to 200°C.

Modeling Fracture of Sn-Rich (Pb-Free) Solder Joints Under Mechanical Shock Conditions

With the increasing focus on developing environmentally benign electronic packages, lead-free solder alloys have received a great deal of attention. Mishandling of packages during manufacture, assembly, or by the user may cause solder joint failure. In this work, we conducted finite-element analysis to model solder joint fracture under dynamic loading conditions. The solder is modeled as a porous plastic material, and the intermetallic compound (IMC) material is characterized as an elastic material. The fracture of the solder is governed by void nucleation, and the IMC fracture is brittle in nature. The randomness of the void volume fraction in the solder and the defects in the IMC are considered and implemented in the finite-element package ABAQUS. The finite-element results show that the fracture mechanisms of the solder joints depend on the strain rate and IMC thickness. High strain rate and larger IMC thickness favor IMC-controlled fracture, which is brittle in nature. Low strain rate and smaller IMC thickness lead to solder-controlled fracture, which is governed by void growth and nucleation. Based on this finding, a mechanistic explanation for solder joint fracture is suggested.

Finite element simulation of swelling-induced crack healing in gels

Finite element simulations have been conducted to study the evolution of stress fields at the crack tip and the crack opening around the crack tip with time for a gel subject to mode I loading. It is found that the stress singularity at the crack tip is eliminated by the swelling of the gel when the solvent is applied at the crack tip. The swelling of the gel also heals the crack tip and the new crack tip generated by swelling does not have a stress singularity. This crack healing process is also verified by experiments and the strain field is measured by digital image correlation. This crack healing mechanism seems to provide a useful means to improve the mechanical integrity of gels and self-healing in general.

The Effect of Random Voids in the Modified Gurson Model

The porous plasticity model (usually referred to as the Gurson–Tvergaard–Needleman model or modified Gurson model) has been widely used in the study of microvoid-induced ductile fracture. In this paper, we studied the effects of random voids on the porous plasticity model. Finite-element simulations were conducted to study a copper/tin/copper joint bar under uniaxial tension using the commercial finite-element package ABAQUS. A randomly distributed initial void volume fraction with different types of distribution was introduced, and the effects of this randomness on the crack path and macroscopic stress–strain behavior were studied. It was found that consideration of the random voids is able to capture more detailed and localized deformation features, such as different crack paths and different ultimate tensile strengths, and meanwhile does not change the macroscopic stress–strain behavior. It seems that the random voids are able to qualitatively explain the scattered observations in experiments while keeping the macroscopic measurements consistent.

On the effective coefficient of thermal expansion (CTE) of bilayer/trilayer in semiconductor package substrates

The coefficient of thermal expansion (CTE) is a material parameter used to measure the degree of expansion divided by the change in temperature. For single layer films, there exists an explicit definition and well-established technique (Thermomechanical Analysis) to measure the CTE. However, there is no explicit definition of effective CTE for multilayer films/composites composed of multiple materials with different individual CTEs. Definition and calculation of effective CTE of multilayer films/composites are still unexplored fundamental problems. Bilayer and trilayer films are major components in the semiconductor package substrate. Three different effective CTEs located at the top surface, bottom surface and mid-plane of bottom film of bilayers were derived explicitly and compared to finite element modeling. Furthermore, the effective CTE of symmetrical trilayer films was derived explicitly and compared to finite element modeling. It shows that not only the individual CTEs of constituent materials but also the ratios of thicknesses and moduli contribute to the effective CTE behavior. The theoretical and numerical study of effective CTE of bilayer and trilayer fills our knowledge gap of effective CTE of multilayer films/composites and provides better understanding of the thermal-mechanical behavior of semiconductor package substrate. The principles also provide solutions and insights to solve critical issues (i.e. warpage) induced by thermal mismatch strains.

On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu

When solder interconnects are fabricated, a Sn-based alloy is melted between two substrates with metallization layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu6Sn5 intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The morphology of the IMC layer greatly influences the mechanical behavior of the solder joint. Here, we report on the characterization of a novel, asymmetric growth behavior of IMC layers in Sn-3.9Ag-0.7Cu solder joints, based on gravity-induced spalling of the IMC.

Symmetric and Asymmetric Double Cantilever Beam Methods for Interfacial Adhesion Strength Measurement in Electronic Packaging

This paper discusses Double Cantilever Beam (DCB) test methods that were developed for characterizing adhesion strength of several critical interfaces in advanced microelectronic packaging. Those interfaces include silicon-epoxy underfill and solder resist-epoxy underfill. A unique sample preparation technique was developed for DCB testing of each interface in order to avoid the testing challenges specific to that interface—for example, silicon cracking and voiding in silicon-underfill samples and cracking of solder resist films in solder resist-underfill samples. Additionally, asymmetric DCB samples (i.e., different cantilever beam thickness on top compared to the bottom) were found to be more effective in maintaining the crack at the interface of interest and in reducing the occurrence of cohesive cracking when compared to symmetric DCB samples. Several case studies using DCB for material selection and assembly process optimization are also discussed. Furthermore, fractography results from SEM examination of the fractured surfaces are also presented for better understanding of the failure mode.

Thermal-Mechanical Impact of Thermal Solutions in Handheld Devices

Use of Thermal Interface Materials (TIM) is a common thermal solution approach in handheld devices to reduce junction temperature and control device skin temperature. This work summarizes the thermal benefits of using a TIM for enhancing the user experience and increasing System on Chip (SoC) performance. On the other hand, TIM induces a load on the package which in turn can impose stress on the package solder joints. This paper explains the impact of a variety of parameters such as TIM material and thickness and system boundary conditions on thermal performance of the SoC/system and the load distribution on solder joints. The complexity of mechanical load distribution is discussed through extensive data collection and simulation in phone and tablet form factors. Design guidelines for selection of appropriate TIM are proposed to improve the thermal performance without compromising the reliability of the SoC package.Copyright