Low modulus porous copper interlayer/nanocopper paste bonding with high shear strength and thermal cycling reliability

A low Young's modulus porous copper interlayer was designed as a strengthening interlayer for nanocopper paste bonding in power device packaging, with the aim of maintaining joint strength while improving the resistance to creep and fatigue during service. The porous copper interlayers were fabricated through a pre-oxidation/rolling/etching strategy, and the effects of etchant composition and pre-oxidation conditions on the resulting pore structure were systematically investigated. Among the examined etchants, the NH₃·H₂O/NH₄Cl mixed solution completely removed the internal oxide layer, while preserving the integrity of the copper skeleton. SEM and EBSD observations supported the presence of an interconnected fine-grained polycrystalline copper skeleton. By optimizing the pre-oxidation process, the porous copper interlayer prepared at 400°C for 30 min exhibited a relatively large skeleton size and low porosity, providing a favorable balance between load-bearing capability and structural deformability. The resulting porous copper exhibited a low Young's modulus of 2290 MPa and still sustained a tensile stress of about 17 MPa. Using this interlayer, porous copper /nanocopper paste joints were fabricated at 280°C under 5 MPa for 30 min, achieving a maximum shear strength of 42 MPa. Fracture occurred predominantly within the porous copper rather than at the interface, indicating strong metallurgical bonding between the porous copper and the nanocopper paste. After 1000 thermal cycles between −55°C and 150°C, the joints retained a shear strength of 36 MPa, with a reduction of only 14.3%. In comparison, the nanocopper paste joint without the porous copper interlayer showed a much more pronounced strength degradation from 54 MPa to 16 MPa after the same thermal cycling condition. Cross-sectional observations revealed that the degradation of the control joint was associated with the growth of internal voids and the formation of deep interfacial cracks, whereas the porous copper interlayer/nanocopper paste joint maintained better structural integrity. These results indicate that the low modulus porous copper interlayer can improve the shear strength retention and suppress damage accumulation under thermal cycling, providing a feasible bonding strategy for mechanically reliable die attachment in high temperature power electronics.