Harry Schoeller

Effects of Microstructure Evolution on High-Temperature Mechanical Deformation of 95Sn-5Sb

Lead-free solders have garnered much attention in recent years due to legislation banning the use of lead in electronics. As use of lead solders is phased out, there is a need for lead-free alternatives for niche applications such as high temperature environments where traditionally high lead solders are used. Electronics and sensors exposed to high-temperature environments such as those associated with deep well drilling require solder interconnects that can withstand high thermal-mechanical stresses. In an effort to characterize solder alloys for such applications, this study focuses on deformation behavior of the Sn95-Sb5 solder under high-temperature exposures (from 298°K to 473°K). As compared to conventional high-temperature Pb-based solder 90Pb–10Sn, Sn95–Sb5 exhibited very high tensile strength and modulus, as well as superior creep properties despite its lower melting temperature. Importantly, high-temperature deformation was shown to be influenced by the presence of the second phase (SnSb) distributed within the Sn-rich matrix. These second phase precipitates appeared to be dissolved into the Sn-rich phase above 453°K, which converted the solder into a single-phase alloy and resulted in a change in its deformation mechanism. Furthermore, as the service temperature is of such high homologous temperature (T > 0.5Tm), creep deformation will contribute significantly toward the life of the solder joint during thermal cycling. In order to characterize the creep behavior and to identify controlling mechanism(s), creep tests were carried out, from which the stress exponent and activation energy were determined. In this study, detailed microstructures under high-temperature are presented in conjunction with the corresponding mechanical behavior to further understand the controlling deformation mechanisms.Copyright

Thermodynamics and Kinetics of Oxidation of Pure Indium Solders

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.

Constitutive Relations of High Temperature Solders

This study focuses on microelectronic package design for the oil and natural gas drilling of wells with depths in excess of 20,000 ft, where package temperatures can exceed 204°C. At these high temperatures, solder interconnect sites are subject to fatigue and creep failures due to the stress generated by the thermal expansion mismatch between various components in the package. Typically this phenomenon is modeled by finite element analysis (FEA) to predict the number of cycles to failure. To ensure meaningful model results, however, accurate time and temperature-dependent mechanical properties are needed. This study examines five solders suitable for high temperature: 90Pb-10Sn, 95Sn-5Sb, 92.5Pb-5Sn-2.5Ag, 95Pb-5In, and 93Pb-3Sn-2Ag-2In. Uniaxial tension tests of the solder wires are carried out on a MTS servohydraulic machine using wedge grips. To evaluate the time-dependence on deformation, a strain rate study was carried out at 0.5%/sec, 1%/sec, and 5%/sec. Nanoindentation of solder wire is performed and compared to the corresponding solder wires tested through uniaxial tension tests. Dynamic nanoindentation through continuous stiffness measurement is performed on the wires to obtain the indentation data less sensitive to creep of the material, as well as to assess the effect of indentation depth on elastic modulus for each solder. One purpose of nanoindentation testing is to determine its suitability for the mechanical testing of soft solders. Mechanical properties obtained from these tests will be used in future modeling studies to estimate the cyclic fatigue life of these solders under thermal loading.Copyright

Harsh-Environment Packaging for Downhole Gas and Oil Exploration

This research into new packaging materials and methods for elevated temperatures and harsh environment electronics focused on gaining a basic understanding of current state-of-the-art in electronics packaging used in industry today, formulating the thermal-mechanical models of the material interactions and developing test structures to confirm these models. Discussions were initiated with the major General Electric (GE) businesses that currently sell into markets requiring high temperature electronics and packaging. They related the major modes of failure they encounter routinely and the hurdles needed to be overcome in order to improve the temperature specifications of these products. We consulted with our GE business partners about the reliability specifications and investigated specifications and guidelines that from IPC and the SAE body that is currently developing guidelines for electronics package reliability. Following this, a risk analysis was conducted for the program to identify the critical risks which need to be mitigated in order to demonstrate a flex-based packaging approach under these conditions. This process identified metal/polyimide adhesion, via reliability for flex substrates and high temperature interconnect as important technical areas for reliability improvement.

Adhesive Tiecoat/Polyimide Interactions in High Temperature Flex Packaging

The high temperature reliability of flex-based Cu/tiecoat/polyimide structures was evaluated through finite element simulation and experimental approach. This study is part of an effort to characterize and optimize polyimide flex as a substrate material for electronics packages rated to greater than 204°C. The peel strength of several common adhesion metals (Ti, Cr, Ni, Cu) on Kapton E was quantified at room temperature and after high temperature storage in inert and highly oxidizing environments. These results were used in tandem with thermal-mechanical simulations to characterize the behavior of several tiecoat materials. Experimental results showed diminished peel strengths of both the Ti and Cr after a 100-hour 250°C heat treatment in air. However when annealed in an inert N2 environment at 250°C for 100 hours, Cr, Ni, and Ti retained their as-sputtered peel strength. Ni and Cu exhibited lower mechanical stresses in the simulation; however, their relatively low reactivity limits their adhesion strength at the interface in oxidizing environments. To further understand the origin of the thermal-mechanical stress, the effect of mismatched CTE was compared to mismatched elastic modulus. Both properties were found to contribute to stress generation; however elastic modulus mismatches had a much greater influence on the overall magnitude of the stress. Through experimentation and FEA analysis this study aims to develop a flexed-based high temperature packaging solution and to shed light onto high temperature tiecoat/polyimide interactions.Copyright

Solderability of Indium With Various Oxide Thickness

The purpose of this paper is to investigate the indium joint strength with known indium oxide thickness. The indium joint strength was assessed by measuring the maximum load of indium solder with the comparison of the wetting angle. The oxide thickness was already grown in different ambient conditions, and provided for the joint strength measurement. The indium joint strength with different oxide thicknesses was tested at a fixed strain rate by tensile loading. This investigation will be very useful to characterize the indium solderability in different environment in terms of the quantitative joint strength.Copyright