Study On Bonding Performance Of Sintered Silver Joints On Bare Copper Substrates With Different Grain Structures

Silver paste have gained considerable attention as promising die-attach material in power electronics,which can be attributed to the low sintering temperature benefiting from reduced-size effect,and high melting temperature as well as excellent thermal and electrical conductivities of sintered bulk silver.As the most widely used die-attach substrate,bare copper substrate has lower production cost and better long-time reliability than Ag-and Au-plated substrate.Therefore,it is of great significance to investigate and optimize bonding performance between silver paste and bare copper substrate.In this paper,we investigated the effect of grain structure(i.e.grain size,crystal orientation)of copper on bonding performance of sintered Ag joint on bare copper substrate,and illuminated the interfacial bonding mechanisms of bare copper joints sintered in air and formic acid atmospheres,respectively.As for bare copper joint sintered in air condition,the oxide layer generated between sintered Ag layer and copper substrate was composed of Cu₂O and Cu.The formation of Ag-Cu₂O covalent bond and Ag-Cu metallic bond was clearly confirmed at the interface between sintered Ag and oxide layer.Due to the smaller mismatch of lattice constant with Ag,the Ag-Cu₂O covalent bonding could be even stronger than Ag-Cu metallic bonding.During sintering processing,the diffusion and further adhesion of Ag atoms on copper surface protected copper substrate from oxidation to some degree.The copper substrate with smaller grain size provided more grain boundaries to accelerate Ag atoms diffusion on the surface,hence increasing the adhesive area of Ag.As a consequence,the oxidation degree of copper substrate with smaller grain size would be slighter,and bonding strength of bare copper joint sintered in air decreased with increased grain size of copper substrate.As for formic acid atmosphere,the bare copper joint with optimum bonding performance was obtained with copper substrate possessing smallest grain size(1.88?m),strongest(001)texture and largest surface roughness(750.31 nm).It was thought copper substrate with smaller grain size provided more grain boundaries to facilitate the interfacial diffusion of Ag atoms.The improved diffusion rate of Ag atoms on the surface of copper substrate and into the substrate led to the greater contact area(i.e.Ag-Cu bonding quantity)and longer inter-diffusion band(i.e.Ag-Cu bonding quality),hence reinforcing the Ag-Cu interfacial bonding strength.Meanwhile,faster interfacial diffusion did not interfere with the densification of the sintered Ag layer.Therefore,the stronger interfacial bonding and well-densified Ag layer strengthened the sintered Ag joint.Besides,we also believed that(001)orientation could possibly be the most contributing to bonding strength among the orientations of copper substrate.It was also found rougher surface of copper substrate with smaller grain size after polishing could be another factor to improve bonding strength due to the larger contact area and mechanical interlocking.