Interface Development in Cu-Based Structures Transient Liquid Phase (TLP) Bonded with Thin Al Foil Intermediate Layers
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exchangers (MHEs) offer a device configuration through which the rate of solid– liquid convective heat transfer can be greatly increased.[1] Due to higher thermal conductivities and increased ductility of metals, metal-based MHEs possess better heat transfer performance and mechanical robustness as compared to Si-based counterparts. Microscale molding replication is an efficient method for fabrication of metallic high-aspect-ratio microscale structures, and can be used for high-throughput fabrication of open microchannel structures in a range of metals and alloys.[2–7] Open microchannel structures need to be bonded with mating structures to form enclosed microchannel devices. Proper bonding techniques are critical to fabrication of functional metal-based MHEs. Transient liquid phase (TLP) bonding, also known as diffusion brazing,[8] has applications from the aerospace industry to the microelectronic industry.[9–12] A previous study showed that Al-based microchannel devices can be formed by TLP bonding with near-eutectic Al-Ge nanocomposite thin film intermediate bonding layers.[13–15] General characteristics of the TLP bonding process have been described by MacDonald and Eager.[16] A KE CHEN, Graduate Research Assistant, and WEN JIN MENG, Professor, are with the Department of Mechanical Engineering, Louisiana State University, Baton Rouge, LA 70803. Contact e-mail: [email protected] J.A. EASTMAN, Materials Scientist, is with the Materials Science Division, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted August 1, 2013. Article published online April 20, 2014 3892—VOLUME 45A, AUGUST 2014
thin layer acting as a melting point depressant (MPD) is sandwiched between faying metal surfaces, and the entire assembly is heated up. Interaction between the MPD and the base metal leads to the formation of an interfacial liquid layer upon heating, before bulk melting of the base metal occurs. With continued heating, the interfacial liquid layer widens to a maximum thickness through dissolution of the base metal. Isothermal solidification follows, thereby forming the requisite bond. Such a TLP approach can lower the bonding temperature, lessen thermal stress damage, and dissolve residual contamination on faying surfaces. Tuah-Poku et al.[17] divided the TLP process into four stages, which was further divided by MacDonald and Eager[16] into five stages of solid-state interdiffusion during heat up, dissolution of the bonding interlayer, widening of the interfacial liquid layer, isothermal solidification, and homogenization of the bonding interface region. As shown in the Cu-Al phase diagram of Figure 1,[18] an eutectic invariant point exists at 821.2 K (548.2 C) and ~Al83Cu17, much below the Cu bulk melting temperature of 1357.6 K (1084.6 C). Al is therefore a candidate for MPD in TLP bonding of Cu-based structures. We have previously shown that Cu-based microchannel devices can be formed by TLP bonding with thin Al foil intermediate bonding layers.[19,20] Figure 2 shows a scanning electron microscopy (SEM) image of a port
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