Microstructure Evolution Mechanism And The Performance Characterization Of Cu/Cu₃Sn-Cu/Cu Composite Micro-joints

The third generation semiconductor power devices,which draw great attention,as typified by SiC and GaN,have many outstanding advantages,such as big bandgap,high breakdown voltage,high power density,and servicing at high temperature.New interconnection methods,such as transient liquid phase(TLP)technology,have been widely used.Full intermetallic compounds(IMC)joints using TLP have become a hot topic because of their cost effective and"low temperature bonding and high temperature service".However,the traditional fabrication of full IMCs joints requires long time or high temperature,which would seriously affect the reliability of the packaging system.And the mechanical properties,electrical and thermal conductivity of the full IMCs joints can not fully meet the needs of the third generation semiconductor power devices.Therefore,how to efficiently manufacture the reliable IMCs solder joints with sufficient reliability and excellent property has become a research hotspot.In this paper,microporous copper was used as skeleton and pure tin as filler material.Based on the principle of transient liquid phase,the composite joints made of in-situ formed Cu₃Sn and residual microporous copper were quickly obtained by thermo-compression bonding.It provided a fast and efficient new connection method for the interconnection of third-generation semiconductor chips.The connection mechanism of Cu/Cu3Sn-Cu/Cu micro joints and the evolution law of interface microstructure were described by the experiment and theoretical analysis.The shear properties,in-situ indentation mechanical properties,electrical and thermal conductivity and aging properties of Cu/Cu3Sn-Cu/Cu micro joints were comprehensively investigated.The Cu/Cu₃Sn-Cu/Cu solder joints structure was designed according to the characteristics of the interconnection structure of the third generation semiconductor power device chip.The effects of bonding time,bonding temperature and bonding pressure on the microstructure and growth behavior were studied.It was concluded that the growth rate of Cu₃Sn was positively proportional to the bonding temperature and had a parabolic law with the bonding time roughly.The bonding pressure could not directly affect the growth rate of Cu₃Sn.In addition,this paper explained the influential mechanism of bonding time,bonding temperature and bonding pressure on the microstructure and growth behavior of solder joints.The microstructure evolution law of the composite joints was as follows:at initial bonding stage,with copper atoms dissolving into liquid tin,the scallop-like Cu6Sn5 formed.With the increase of bonding time,the liquid tin was gradually consumed.The gaps of the copper were filled by Cu6Sn5.Cu3Sn grew gradually at the interface of Cu6Sn5/copper.With the increase of bonding time,the Cu3Sn IMCs grew uniformly in the gaps of copper and the Cu3Sn-Cu microstructure formed.The orthogonal test was designed with the method of statistical analysis and the effects of bonding time,bonding temperature and bonding pressure on the microstructure and growth behavior of joints were analyzed by range analysis and variance analysis.Bonding temperature had the most significant effect on the growth of Cu₃Sn.The optimal bonding process parameters of Cu/Cu3Sn-Cu/Cu micro joints were derived according to the orthogonal test and practical needs.The optimal bonding process parameters of Cu/Cu3Sn-Cu/Cu micro joints were as follows:The volume fraction of Cu was 60%,the welding temperature was 340?360?,the welding time was 6?8 min,and the welding pressure was 0.3 MPa.In this paper,the effects of bonding time,bonding temperature and bonding pressure on the shear properties and in-situ indentation mechanical properties of Cu/Cu3Sn-Cu/Cu micro joints were studied.The shear strength of Cu/Cu3Sn-Cu/Cu micro joints reached about 170 MPa.The reasons why Cu/Cu3Sn-Cu/Cu micro joints had high shear strength were as follows:The interlocking structures between copper and Cu₃Sn increased the contact area,which could hinder the propagation of cracks.The grain size of Cu₃Sn was refined by the microporous copper in the composite joints.A large amount of copper with high shear strength were in the composite joints,which could improve shear strength and hinder the propagation of cracks.The effects of bonding time,bonding temperature and bonding pressure on the mechanical properties of in-situ indentation were analyzed variance analysis.The conclusion was that bonding pressure had a significant effect on the indentation modulus,creep rate and indentation hardness of microporous copper.And bonding pressure had a significant effect on the indentation modulus and creep rate of Cu₃Sn.The resistivity and thermal conductivity of Cu/Cu3Sn-Cu/Cu micro joints were measured and calculated by actual measurement and ANSYS finite element simulation.The resistivity of Cu/Cu₃Sn-Cu/Cu micro joints with copper volume fraction of 60%was 6.34??·cm,the conductivity was improved by 30%compared with the full Cu₃Sn joints.The thermal conductivity of Cu/Cu₃Sn-Cu/Cu micro joints was 130.5 W/(m·K),which was nearly twice that of full Cu₃Sn joints.The resistivity and thermal conductivity of Cu/Cu₃Sn-Cu/Cu micro joints calculated by ANSYS finite element simulation were 4.88??·cm and 141.82 W/(m·K)respectively.The thermal conductivity and resistivity of Cu/Cu₃Sn-Cu/Cu micro joints with different copper volume fraction were simulated by ANSYS.It was concluded that with the increase of copper volume fraction,the resistivity and thermal conductivity of Cu/Cu3Sn-Cu/Cu micro joints were improved.The aging test of Cu/Cu₃Sn-Cu/Cu micro joints was carried out at 300?.It was concluded that there were no phase transformation and macro defects in the microstructure during the aging test.The grain size of Cu₃Sn increased and voids at grain boundaries were found during the aging test.With the increase of aging time,the shear strength of Cu/Cu₃Sn-Cu/Cu micro joints decreased.The reason for the decrease of shear strength was the increase of Cu₃Sn grain size and the generation of holes at Cu₃Sn grain boundaries.With the increase of aging time,indentation modulus and indentation hardness of Cu/Cu3Sn-Cu/Cu micro joints decreased,the creep rate increased.