Die-attach for high-temperature electronics

Abstract

Applications in harsh environments push the boundaries for electronic systems. High temperatures put great stress on electronic components. The die-attach is an enabling component that makes the system work. It needs to reliably and predictably attach electronic components to circuit boards. It must function mechanically, thermally and electrically for the system to work properly. Two applications that offer great challenges are thermoelectric devices and power electronics. Thermoelectric generators require a temperature difference to able to convert heat energy into electrical energy. High-temperature heat sources offer abundant heat that may be harvested. Power electronics dissipate lots of heat during operation. These losses make them particularly hot and unforgiving when applied in high-temperature environments. This work set out to investigate two different die-attach technologies for high-temperature applications: Liquid solid diffusion (LSD) bonding and solid-liquid interdiffusion (SLID) bonding. LSD was at an idea stage when this project started. The hypothesis was that it could be possible to form off-eutectic joints that comprised a microstructure that could have structural load capacity in a partially liquid state. This thesis has shown that such joints may be formed using the binary Au–Ge system. A melting point depressant material, i.e., eutectic Au–Ge preforms, were sandwiched between Au substrates to form joints, forming a Au | Au72Ge28 | Au structure. The preforms were melted, and solid-liquid interdiffusion between the adjoined materials change the composition into a Au rich off-eutectic composition. Cooling solidifies the joint into a hypoeutectic (Au-rich) compound with an overall Au | Au–Ge | Au structure. Investigations on the microstructure reveled that a network of columnar like solid single-phase structures of Au forms a connection between joined components. These Au structures were surrounded by a hypereutectic (Ge-rich) compound. These Au structures have a significantly higher melting point (up to 1064 °C) than the eutectic preform that was used to fabricate them, which melts at 361 °C. The fabricated joints had a significant structural capacity ranging from approximately 140 MPa at room temperature to about 40 MPa in a partially liquid state at 410 °C. SLID bonding is done by melting a melting point depressant material between two substrates that are to be joined. Solid and liquid interdiffusion between the adjoined materials transform these into an intermetallic compound (IMC). The joint solidifies isothermally. The Ni–Sn system was used to fabricate SLID joints. Joints were successfully formed using a layered Ni | Sn | Ni structure that was transformed into a Ni | Ni–Sn IMC | Ni structure. The fabricated joints were flawed with voids caused by idiomorphic (needle-like) Ni3Sn4 structures growing at the Ni surface into the Sn melt during fabrication. Acting as spacers, they restrict volumetric contraction of the joint materials. The contraction is forced by phase transformation from Sn(l) and Ni(s) into Ni–Sn IMCs by as much as up to 17 vol.%. This cause the voids to form. Despite this, the shear strength was very high, up to 230 MPa was measured. These findings were confirmed by contemporary researchers.