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Highly Reliable Double-Sided Bonding used in Double-sided Cooling for High Temperature Power Electronics
Keywords: Double-sided Cooling, Automotive Power Electronics, Transient Liquid Phase Bonding
This paper demonstrates the feasibility of double-sided die attachment, a key technology for double-sided cooling structures, using transient liquid phase (TLP) bonding. Recently, double-sided cooling has drawn particular interest by providing a notable improvement in thermal management and increasing allowable power density for automotive power electronics. The use of TLP bonding for double-sided attachment avoids a number of complications in the assembly process (a. multiple bonding processes/materials, b. misalignment, c. electrical connection/isolation limitation), enables multiple attachments, and provides a high bonding quality and reliability at high temperature operation due to its high re-melting temperature. Therefore, double-sided TLP bonding facilitates a simple fabrication of double-sided cooling structures. In this paper, we demonstrate double-sided copper-tin (Cu-Sn) TLP bonding via proof-of-concept fabrication (for double-sided cooling structures) and bond quality characterization. The high thermal and electrical conductivity offered by Cu-Sn TLP is especially advantageous compared to other TLP processes. A test die having copper layers (on top and bottom) and two substrates having copper layers were prepared. Both sides of the die were TLP bonded to each substrate by inserting thin (25 um) Sn foils between the die and substrate. The bonding interfaces on the top and bottom of the die were investigated using optical microscopy, SAM, and SEM/EDX analysis. Analysis confirms that Cu-Sn TLP bonding was completed through transformation of all Sn to Cu-Sn alloys (Cu3Sn and Cu6Sn5). The transformed Cu-Sn alloys had melting temperatures (C3Sn: >600 degC and Cu6Sn5: >400 degC) significantly higher than that of Sn (~230 degC), thus allowing high reliability at elevated operating temperatures. No bondline damage was observed after the sequential bonding process. These characterizations validate TLP bonding as being more suitable for double-sided structures when compared to alternative techniques such as Pb-based or Pb-free solder (in terms of high temperature reliability) or sintering techniques (in terms of process pressure).
Sang Won Yoon, Senior Scientist
Toyota Research Institute of North America
Ann Arbor, MI

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