Research On Process And Brazing Mechanism For Joining AlN And Cu With Ag-Cu-Ti Based Composite Filler

The AlN/Cu composite substrates are one of the key packaging materials for IGBT modules,which have a coefficient of thermal expansion(CTE) matching with chip materials,low dielectric constant,excellent current carrying capacity and high thermal conductivity.However,there is some high residual stress at the brazing joint due to the diversity in CTE between AlN ceramic and oxygen-free Cu,which reduced the reliability of brazing joints.From the perspective of CTE,this topic used AlN/AgCuTi composite fillers,TiN/AgCuTi composite fillers and TiN+Al/AgCuTi composite fillers to join AlN ceramics to oxygen-free Cu,and systematically research the effects of the content of reinforcing phase,brazing temperature and holding time on the microstructure and mechanical properties of brazed joints.On the basis of this research,the strengthening mechanism of the composite filler and the connection mechanism of the joint interface were further analyzed.The results show that:(1)The interfacial microstructure of AlN/AgCuTi+AlN/Cu joint was Cu/Ag(s,s)+Cu(s,s)+AlN+TiN+Ti₂Cu/TiN/AlN,with the increase of AlN content,the microstructure of brazing seam was gradually refined,the thickness of the reaction layer gradually decreased,and the shear strength of the joint increases first and then decreases.The maximum shear strength was 98.2 MPa,when the joint was brazed with AgCuTi+4 wt.%AlN composite filler,which was 89.1% higher than that of the joint brazed using AgCuTi filler.(2)The interface microstructure of AlN/AgCuTi+TiN/Cu joint was Cu/Ag(s,s)+Cu(s,s)+TiN+Ti₂Cu/TiN/AlN.The content of TiN,brazing temperature and holding time mainly affect the performance of brazed joints by affecting the CTE,the thickness of the reaction layer and the content of the Ti-Cu phase.The shear strength of the joint increased first and then decreased with the increase of TiN content,brazing temperature and holding time.The maximum shear strength was 131.0 MPa when the joint was brazed at 900°C for 15 min with AgCuTi+4 wt.%TiN composite filler,which was 150.0%higher than that of the joint brazed using AgCuTi filler.(3)The interface microstructure of AlN/AgCuTi+TiN+Al/Cu joint was Cu/Ag(s,s)+Cu(s,s)+TiN+Al Cu₂Ti/TiN+Ti₄AlN₃/AlN.The addition of the trace Al element changed the ceramic/filler interface from the TiN layer to the TiN+Ti₄AlN₃ gradient layer.The shear strength of the joint increased first and then decreased with the increase of Al content,brazing temperature and holding time.The maximum shear strength was 161.6 MPa when the joint was brazed at 880°C for 15 min with AgCuTi+4TiN+0.50Al(wt.%)filler,which was 204.9%higher than that of the joint brazed using AgCuTi filler.(4)The strengthening effect of the AlN/AgCuTi composite filler reflected in reducing the thickness of the reaction layer and reducing the overall CTE value of the filler by means of AlN and Ti₂Cu phases.In addition to reducing the thickness of the reaction layer and reducing the CTE value by the TiN and Ti₂Cu phases,the TiN/AgCuTi composite filler can further improve the joint performance by the Orowan strengthening mechanisms of the submicron Ti₂Cu phase.In addition to reducing the thickness of the reaction layer,reducing the CTE value and Orowan strengthening by the TiN and Al Cu₂Ti phases,TiN+Al/AgCuTi composite filler can further relieve the residual stress in the brazed joint through the TiN+Ti₄AlN₃ gradient interface layer.(5)This topic combines thermodynamic theory to study the connection mechanism of TiN/AgCuTi composite filler.At the ceramic/filler interface,Ti diffused to the AlN ceramic and form a TiN layer,inside the filler layer,a trace of Ti react in the area around the TiN particles to form fine Cu-Ti phases.Cu-Ti phase mainly appeared in Cu(s,s)and the formation trend of Ti₂Cu phases was the largest.