Alternatives for joining Si wafers to strain-accommodating Cu for high-power electronics |
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Authors: | Nicholas Faust Robert W Messler Subhash Khatri |
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Affiliation: | (1) Present address: Howmet, Whitehall, MI;(2) Was cognizant engineer at Silicon Power Corporation, Malvern, PA |
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Abstract: | Differences in the coefficients of thermal expansion (CTE) between silicon wafers and underlying copper electrodes have led
to the use of purely mechanical dry pressure contacts for primary electrical and thermal connections in high-power solid-state
electronic devices. These contacts are limited by their ability to dissipate I2R heat from within the device and by their thermal fatigue life. To increase heat dissipation and effectively deal with the
CTE mismatch, metallurgical bonding of the silicon to a specially-structured, strain-accommodating copper electrode has been
proposed. This study was intended to seek alternative methods for and demonstrate the feasibility of bonding Si to structured
Cu in high-power solid-state devices. Three different but fundamentally related fluxless approaches identified and preliminarily
assessed were: (1) conventional Sn−Ag eutectic solder; (2) a new, commercially-available active solder based on the Sn−Ag
eutectic; and (3) solid-liquid interdiffusion bonding using the Au−In system. Metallurgical joints were made with varying
quality levels (according to nonde-structive ultrasonic C-scan mapping, SEM, and electron microprobe) using each approach.
Mechanical shear testing resulted in cohesive failure within the Si or the filler alloys. The best approach, in which eutectic
Sn−Ag solder in pre-alloyed foil form was employed on Si and Cu substrates metallized (from the substrate outward) with Ti,
Ni and Au, exhibited joint thermal conduction 74% better than dry pressure contacts.
Formally an M.S. candidate at Rensselaer Polytechnic Institute, Troy, NY.
Professor of Materials Science and Engineering/Director of Materials Joining, advised Mr. Faust
sponsor of the development effort |
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Keywords: | High-power solid-state electronic devices electrical/thermal interconnect Si-to-Cu joining conventional [Pb-free] solder solid-liquid [Au− In] interdiffusion bonding |
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