Search

Polyurethane (PU)—based conformal coatings which protect electronic components and boards from harsh environmental exposures were further developed to mitigate the tin (Sn) whisker growth in lead (Pb)—free electronics. As a means of achieving this goal, the adhesion mechanics of PU-based conformal coating on tin surfaces was investigated. We attempted to improve the interfacial adhesion between PU-based conformal coatings and tin (Sn) as it slows down the formation of tin whisker growth. A multi-layer sandwiched specimen made between two Si beams was employed to create an interface between PU coating and Sn for four-point bending testing (4PBT). The result shows that PU has strong adhesion to Sn while polyurethane acrylate (PUA) has much weaker adhesion to Sn. The PUA adhesion, however, increased by introducing a silane agent ((3-Aminopropyl)triethoxysilane) on the tin surface, which is covalently bonded to PUA. The adhesion improvement of a modified tin surface was also confirmed by a simple cross-cut tape peeling test. Post fracture analysis suggested that blocking the reaction between bare Sn surface and the acrylate component contribute to improved adhesion.

Bismuth (Bi)-based systems are of great interest as it is considered to potentially replace high lead (Pb)-content solders used in high temperature electronics. In particular, molten Bi strongly reacts with Ni to form higher melting point intermetallic compounds (IMCs) via transient liquid phase (TLP), which can offer a joining method alternative to the traditional solder approach. A fundamental understanding of nucleation and growth of intermetallic phases is crucial to create a reliable joint. Two intermetallic phases form between Bi and Ni (Bi3Ni, BiNi). In this study, growth kinetics for Bi3Ni and BiNi was investigated, both of which show a parabolic growth behavior. Bi3Ni exhibits rapid growth and apparent activation energy of 65.5 kJ/mol at lower temperatures (from 160 to 240 °C) and of 132.9 kJ/mol at higher temperatures (> 240 °C), where the transition is likely due to a viscous-flow nature near melting temperature of Bi. On the other hand, BiNi grows at a later stage with a slower rate with the apparent activation energy of 125.6 kJ/mol (from 260 to 340 °C). In addition, based on the formation sequence and growth direction of these IMCs, interdiffusion coefficients for each of these IMCs were determined. Micro-hardness tests show that Bi3Ni is softer and more brittle than BiNi.

Copper micropillars plated onto a silicon die and soldered with Sn-Ag solder to a copper lead frame in a flip chip on lead package have been subjected to high-temperature storage at 150°C and 175°C for 500 h, 1000 h, and 1500 h. Cu6Sn5 and Cu3Sn intermetallic compounds were found on both sides of the solder, but the growth rates were not the same as evidenced by different values of the growth exponent n. Cu and Sn diffusion controlled the Cu3Sn growth in the Cu pillar interface (n ≈ 0.5), while interface reactions controlled the growth in the Cu lead frame interface (n ≈ 0.8). Increasing the aging temperature increased the growth of Cu3Sn as well as the presence of microvoids in the Cu lead frame side. Adding Ni as a barrier layer on the Cu pillar prevented the growth of Cu3Sn in the Cu pillar interface and reduced its growth rate on the lead frame side, even at higher aging temperatures.

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.

The fluxless solderability of pure indium on gold-coated surfaces is investigated in this study using measurements of wetting angle and joint strength. This study focuses on the effects of indium’s native oxides and those which form during heat treatment. The initial oxide thicknesses are obtained by heating indium samples at various temperatures, and then, during the reflow above the melting temperature, oxidation at the interface is either allowed in open air or prevented by reflowing in an inert environment. The testing results presented include characterization of indium oxide (In2O3), the effects of indium oxide on joint strength, and the effect of hot pressing during reflow process.

Ceramic/self-assembled monolayer (SAM) bilayer coatings can provide adequate protection for silicon devices, or act as a multipurpose coating for other electronic applications, due to synergistic effects by forming a hybrid coating structure. The organic SAM layer acts as a “template” for the growth of the ceramic layer, while the ceramic layer can provide protection from environmental and mechanical impact. Low-temperature solution-based deposition techniques, namely, an in-situ solution method (biomimetic) and a hydrothermal method, have been employed in this study. Specifically, phosphonate-based (diethyl phosphatoethyl triethoxy silane) SAMs were used as a template to generate a zirconia ceramic layer at low temperatures. Other organic templates such as -SiCl3-, -OH-, -HSO3-, or -CH3-terminated SAMs were also examined. The reactions to grow the ceramic film were found to be pH sensitive. The ceramic and SAM coatings were characterized by a variety of analytical techniques. A pathway for the formation of the ceramic coating is also discussed.

Oversized rare-earth dopant ions such as Y3+, Nd3+, and La3+ segregate to grain boundaries and reduce the tensile creep rate of α-Al2O3 by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.

The tensile creep behavior of yttrium- and lanthanum-doped alumina (at dopant levels below the solubility limit) was examined. Both compositions (100 ppm yttrium, 100 ppm lanthanum) exhibited a uniform microstructure consisting of fine, equiaxed grains. The creep resistance of both doped aluminas was enhanced, compared with undoped alumina, by about two orders of magnitude, which was almost the same degree of improvement as for materials with higher dopant levels (in excess of the solubility limit). In addition, measured creep rupture curves exhibited predominantly steady-state creep behavior. Our results, therefore, verified that the creep improvement in these rare-earth doped aluminas was primarily a solid-solution effect.

MicroElectroMechanical System (MEMS) devices often require low-temperature, fluxless soldering techniques due to their high temperature sensitivity and performance requirements of the components. While seeking the development of a soldering technology using pure indium, the major focus of this study is to assess the thermodynamics and kinetics of indium oxidation at various solder reflow environments that will ultimately provide a processing window for solder reflow and surface oxide cleaning. With a glove box employed to generate reducing environments, oxygen, moisture, and hydrogen contents were varied to examine their effects on oxidation and reduction behavior of indium. We also explored oxidation mechanisms at different regimes of temperature and time. In particular, electron transport from indium to indium oxide is shown to be the rate controlling mechanism under specific oxidizing conditions. For accurate thickness measurements, a spectroscopic ellipsometer was employed. In addition, the effect of indium oxidation on solder joint reliability was observed via wetting angle and interfacial shear strength measurements.

A bio-inspired approach is employed to deposit the oxide films on the substrates coated with self-assembled organic layers. Particularly, titania and zirconia films are grown in aqueous precursor solutions at near room temperatures. This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. Multiscale structural evolution resulting from initial bulk nucleation, nanoparticle aggregation, and ultimate film formation are systematically studied by adjusting the precursor solution conditions. Dynamic light scattering (DLS) is utilized to characterize initial nanoparticles and their associated aggregates/clusters formed in situ in solution. Corresponding nano- and microstructure developments of the oxide films are investigated through high-resolution transmission electron microscope (TEM) and scanning electron microscope (SEM). In addition, mechanical performance is evaluated with the aid of a dynamic nanoindentation testing to establish the structure-property relationships of the bio-inspired oxide films. The goal of this study is to have a capability to tailor microstructures and mechanical behaviors by identifying the controlling mechanisms responsible for nucleation and growth of such oxide films.

A biomimetic approach is employed to deposit ceramic films on organic self-assembled monolayers (SAMs) coated substrates. Specifically, zirconia (ZrO2) films are grown in a zirconium sulfate precursor solution at near room temperatures (∼70°C). This process, directed by the nanoscale organic template, mimics the controlled nucleation and growth of the biominerals such as bones and teeth. The resultant zirconia films consist of nanosized particles (5-10 nm) that are precipitated out in a supersaturated precursor solution. Cross-sectional TEM and STEM works were performed to quantitatively analyze the film structure and chemistry, as well as interfacial region of the ceramic-SAM films. A stepwise deposition process was developed to avoid excessive formation of aggregation. Further, the dynamic nanoindentation testing was employed to assess the thickness and film-only intrinsic mechanical properties for direct comparison among the films processed with different processing parameters and microstructures. The films with finer particulate structure displayed higher intrinsic modulus than did those with coarser structure.

We explore the effect of the substrate on mechanical behavior of thin films using a depth-sensing indentation. For this purpose, nanoindentation has been performed on thermally grown silicon oxide films on the silicon substrate. One primary goal of this study is to extract ‘film-only’ mechanical properties from the nanoindentation data by subtracting the substrate effect. It is also shown that elastic modulus of the film is more influenced by the substrate than hardness due to a larger elastic extension beyond a plastically deformed region, as well as a larger elastic mismatch between the SiO2 film and the Si substrate. Further, an inverse analysis of indentation data is proposed to estimate a thickness of thin films. Consequently, this study provides a fundamental understanding in mechanical phenomena of thin films occurring at nanoscales.

Flip-chip technology is becoming one of the most promising packaging techniques for high performance packages. Solder balls are used as the connection technique in the flip-chip method and the connections are reinforced by filling in the spacing between the chip and substrate with underfill. The function of the underfill is to reduce the stresses in the solder joints caused by a coefficient of thermal expansion (CTE) mismatch. The presence of polymeric underfill material will, however, make the flip-chip packaging system susceptible to interfacial failure. Thus, the purpose of this study is to examine the interfacial delamination between the dissimilar materials in order to increase the reliability of the flip-chip interconnection method, and to understand the effect of underfill curing conditions on the interface adhesion. In particular, we use a linear elastic fracture mechanics (LEFM) approach to assess interfacial toughness. For this purpose, four-point bending testing is performed to determine a critical strain energy release rate, Gc. In addition, nano-indentation testing equipped with atomic force microscope (AFM) is employed to determine structure and properties of the underfill layer.

A better understanding of the origin and evolution of the stresses is a crucial step in improving reliability of packaging systems for microelectromechanical systems (MEMS). Given its importance, we examine the stresses developed in hermetically packaged MEMS inertial sensors. For this purpose, an optical surface profilometer is employed to assess the stresses by measuring the curvature of dummy silicon dies (3.5×3.5 mm2) assembled in different types of packages and die attach adhesives. We also explore a temporal evolution of stresses during thermal exposure of the test packages in an effort to emulate actual packaging processes and device operation conditions. The result shows different levels of stresses generated from various adhesives and package types, and also a stress evolution during packaging processes. The mechanical stress data also show a good agreement with MEMS performance data obtained from actual accelerometers. Therefore, the stress data will not only be useful in better understanding performance of MEMS packages, but the testing protocol can also provide a diagnostic tool for very small packaging systems.