Microstructure evolution and thermostability of bondline based on Cu@Sn core-shell structured microparticles under high-temperature conditions

Cu@Sn core-shell structured particles with different coating layer thicknesses were fabricated and used as bonding materials. The microstructure evolution and thermostability of the bondline were studied under high-temperature conditions; the minimum thickness of the Sn coating layer required to consume all the inner Cu cores when forming ε-Cu₃Sn was determined; and the accompanying volume shrinkage was verified. The phase transformation pathway for particles with a thin Sn coating layer was Cu₆Sn₅ → ε-Cu₃Sn → δ-Cu₄₁Sn₁₁ → γ-phase → β-phase upon heating to 600 °C. Thermal instability occurs at 520 °C and 586 °C due to the formation of the Cu-rich phases. High-temperature shearing tests were conducted at 500 °C and 550 °C to verify the existence of a critical point for brittle rupture and tough rupture at approximately 520 °C. For particles with a thick Sn coating layer, the outer Sn shell completely consumes the inner Cu core when forming a ε-Cu₃Sn intermetallic phase, and no other Cu-rich phases formed when heating to 600 °C.