Strength of joints produced by transient liquid phase bonding in the Cu–Sn system Original Research Article

The paper focuses on the strength and toughness of joints produced by transient liquid phase (TLP) bonding in the Cu–Sn system. It is motivated by potential applications of TLP bonding in attachment of high-temperature wide bandgap devices to ceramic substrates. Model test specimens suitable for mechanical testing are developed, utilizing substrates of oxide dispersion strengthened copper. Three microstructural conditions are probed: a uniform layer of the δ intermetallic phase (Cu41Sn11), a two-phase microstructure comprising the δ-phase and a dispersion of ductile (Cu) particles, and a uniform Cu solid solution. Notched and unnotched bend tests are used to ascertain strength and toughness. The δ-phase exhibits a reasonably high strength (300 MPa), but low toughness (≈5 MPa √m). Addition of (Cu) particles increases both the strength and the toughness by about 30% (400 MPa and 7 MPa √m, respectively). These property enhancements are rationalized on the basis of existing models of ductile phase toughening. The conversion of the intermetallic to Cu solid solution leads to a decrease in strength (to 200 MPa), but an increase in toughness (to 13 MPa √m). The latter trends appear to be a consequence of the reduction in the flow stress of the joint material. Additionally, the conversion to (Cu) is accompanied by the formation of voids, predominantly near the prior boundary between the δ-phase and the adjoining Cu. The voids likely diminish the joint properties, relative to the intrinsic values associated with the defect-free copper solid solution.