David Shaddock

Development of a Compliant Nanothermal Interface Material

The power density requirements continue to increase and the ability of thermal interface materials has not kept pace. Increasing effective thermal conductivity and reducing bondline thickness reduce thermal resistance. High thermal conductivity materials, such as solders, have been used as thermal interface materials. However, there is a limit to minimum bondline thickness in reducing resistance due to increased fatigue stress. A compliant thermal interface material is proposed that allows for thin solder bondlines using a compliant structure within the bondline to achieve thermal resistance <0.01 cm2 C/W. The structure uses an array of nanosprings sandwiched between two plates of materials to match thermal expansion of their respective interface materials (ex. silicon and copper). Thin solder bondlines between these mating surfaces and high thermal conductivity of the nanospring layer results in thermal resistance of 0.01 cm2 C/W. The compliance of the nanospring layer is two orders of magnitude more compliant than the solder layers so thermal stresses are carried by the nanosprings rather than the solder layers. The fabrication process and performance testing performed on the material is presented.Copyright

Mechanical strengthening, stiffening, and oxidation behavior of pentatwinned Cu nanowires at near ambient temperatures

The complex effects of near ambient temperature exposure, i.e. 20?150 ?C, on the oxidation and the mechanical properties of thermal solution grown faceted Cu nanowires were investigated. The mechanical behavior was quantified with experiments on individual Cu nanowires using a MEMS-based method for nanoscale mechanical property studies. The elastic modulus of pristine Cu nanowires with diameters 300?550 nm was 117???1.2 GPa which agreed very well with polycrystalline bulk Cu, while the ultimate tensile strength was more than three times higher than bulk Cu, averaging 683???55 MPa. Annealing at just 50 ?C resulted in marked strengthening by almost 100% while the elastic modulus remained unchanged. Heat treatment in ambient air distinguished three different regimes of oxidation, namely the (a) formation of a thin passivation oxide at temperatures up to 50 ?C, (b) formation of thermal oxide obeying an Arrhenius type process for Cu+ migration at temperatures higher than 70 ?C, which was accelerated by grain boundary diffusion resulting in activation energies of 0.17?0.23 eV, and (c) complete oxidation following the Kirkendall effect at temperatures higher than 150 ?C and for prolonged exposure times, which did not obey an Arrhenius law. Notably, the formation of a weaker and more compliant thermal Cu2O did not compromise the effective strength and elastic modulus of oxidized Cu nanowires: experiments in Ar at temperatures higher than 70 ?C showed mechanical strengthening by ?50% and ultimate stiffening to ?190 GPa, which is near the upper limit for the elastic modulus of single crystal Cu in the direction.

Assembly Materials and Processes for High-Temperature Geothermal Electronic Modules

Geothermal well logging and instrumentation applications require high-temperature logging tools and sensors with long-term operation capability at 300°C. Advanced SiC technology has enabled high-temperature electronics. To build functional systems operating at high temperatures, an interconnection and packaging technology must be developed to interconnect SiC devices and passive components. Off-eutectic AuSn has been evaluated for SiC and passive component attachment to PtPdAu/Au thick film metallization on alumina substrates and on thick film dielectric. A functional SiC-based oscillator module was fabricated and tested to demonstrate the assembly technologies developed for 300°C applications.

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

Bismuth-Based Transient Liquid Phase (TLP) Bonding as High-Temperature Lead-Free Solder Alternatives

Predominant high melting point solders for high temperature electronics contain lead (Pb), which will soon be banned by environmental regulations as in most of consumer electronics. In an effort to replace the Pb-based solders with a new high-temperature capable material, we developed a transient liquid phase (TLP) bonding of bismuth (Bi) and nickel (Ni). A molten Bi (m.p. of 271°C) strongly reacts with Ni to form a Bi3Ni or BiNi intermetallic layer, both of which can withstand over 400°C. To study microstructural developments and their influences on reliability performance, the die attached coupons were assembled using Bi-Ni TLP bonds. It was shown that Bi3Ni is the first phase to form, after which the diffusion of Bi controls the growth kinetics of this intermetallic phase with the activation energy of 65.5 kJ/mol for the temperature range from 160 to 240°C. The solid-state transformation of Bi3Ni to BiNi follows, which is a slower process and only occurs at a long-term aging (activation energy of 17.8 kJ/mol, for the temperature range from 260 to 300°C). When tested at high temperatures (up to 350°C) or exposed under long-term storage at 200°C over 1000 hours, such bonds showed no degradation in die shear strength. It indicates the potential of the Bi-Ni TLB bonds as a Pb-free alternative to replace high-Pb solders for high temperature electronics that operate at 200°C or higher.

High temperature electronics packaging: An overview of substrates for high temperature

High temperature electronics (>200 °C) is an emerging technical area in electronics that requires new materials and processes or existing materials and processes in new ways. Results of reliability testing of packaging materials for >200 °C are presented so that the selection of appropriate packaging may be made to target an applications temperature and lifetime requirements. Polymer and ceramic substrates are compared using experimental testing. Lifetime comparisons at 200C for polymer substrates and limitations due to composition are made. Thick film alumina, Low Temperature Co-fired Ceramic, and thin film AlN substrate qualification test results and limitations in material selection are presented.

Mechanical Constitutive Properties of Two High-Temperature Lead-Rich Solders

This study focuses on the mechanical properties of two kinds of solders for high temperature interconnections (melting temperature above 280 °C after reflow). They are both lead-rich solders in wire form: 93.5Pb/5Sn/1.5Ag and 92.5Pb/5Sn/2.5Ag. The mechanical constitutive tests conducted in this study include: (i) monotonic elastic-plastic stress-strain response at constant strain rates and constant temperatures; and (ii) isothermal, viscoplastic creep strain response at several constant stress levels and different temperatures. The sensitivities of the material viscoplastic constitutive response to temperature, loading rate and stress level were systematically studied, so that the mechanical performance of these two solders could be compared with those of other solders, at similar test conditions. Both the Pb-rich solders studied had statistically similar behavior at the tested stress levels and temperatures in creep tests. The Ramberg-Osgood model and the Garofalo model were used to represent the elastic-plastic and creep behavior, respectively, of each solder and the corresponding model constants are presented in the paper.Copyright

DIP Test Socket Characterization for 300°C

Demonstrating functional reliability testing of high temperature electronic devices for long lifetime at 300°C requires electrical test fixtures with even better reliability. Advances in complexity of SiC devices and the need for increased accelerated tests motivate the need for a reliable test fixture at high temperature. The design, fabrication and testing of a prototype test board using commercially available materials shows stability beyond 2000 hours. The approach uses an alumina circuit board with thick film conductors interconnecting an array of BeNi contacts to surface pads. The pads are connected to high temperature wires using spring loaded contacts so that the circuit board may be removed.

Investigation of Thick Film Technology for High Temperature Applications

For electronics operating at 300°C, thick film technology has been proposed as a suitable interconnection technology to create modules. This work examines the leakage current with constant bias (100V) at 300°C. The leakage current increased significantly within the first few hours of aging. The effect of 300°C aging with dc bias on the adhesion of multilayer thick film test structures was also studied. The aged adhesion was a function of bias polarity. Fracture surface analysis results are presented. Bi in the PtPdAu conductor appears to play a role in both the leakage current and adhesion phenomena observed.

Recent Progress in Thin Film Multichip Packaging for High Temperature Digital Electronics

A thin film material and process technology is being developed and evaluated for high temperature (300°C) digital multichip modules for use in geothermal well instrumentation. The substrate technology selected is AlN to minimize the difference in the coefficient of thermal expansion between the substrate and the SiC digital die. A thin film/plated Ti/Ti:W/Au metallization is used with a plasma enhanced chemical vapor deposited Si3N4 to create multilayer interconnections. Active components are assembled to the interconnect substrate using Au stud bump thermocompression bonding. The Au stud bump maintains a monometallic interface between the substrate Au pad surface and the Au pads on the SiC die. A digital circuit has been built and successfully tested as an initial demonstration.