Study On The Reaction Mechanism At Sn/Cu Liquid-solid Interface During The Cooling Process

The miniaturization of electronic devices is seriously strict with the reliability of soldering interfaces.Therefore,it is of great significance to study the growth mechanism of intermetallic compounds(IMC)at the soldering interface.In the existing research,soldering is regarded as a continuous process to study,and the interface reaction mechanism established on the basis of this has a large deviation from the actual interface reaction measurement data.Our results show that the cooling stage of soldering has a significant effect on the interface IMC growth.Thus,the synchronous radiation real-time imaging technology of Shanghai source was utilized to in-situ observe the whole IMC(Cu6Sn5)growth behavior of Sn/Cu soldering interface in this paper.Meantime,high-pressure air method used to prevent subsequent reactions as the setting soldering time to obtain the IMC growth data at the heat preservation stage.Comparing with complete soldering date,the IMC growth data at the cooling stage was obtained.Then,combined with the Cu concentration and temperature field distribution simulation in the solder joint,the diffusion behaviour of the elements and control factors of the cooling stage were revealed,and the IMC growth model of the liquid-solid interface at the cooling stage of soldering was established.The growth process of typical Cu6Sn5 grain morphology and the effect of alloying elements added into Sn solder on the growth behavior of IMC at the cooling stage were also studied.Conclusions were listed as below:1.It was found that the growth mechanism of IMC at the cooling stage is different from the heat preservation stage of Sn/Cu soldering.The thickness of IMC growing at the cooling stage follows the law of h=ktn(n=1),which is the control mechanism of interface reaction.The precipitation flux of Cu plays a controlling role in this IMC growth mechanism.The interface Cu6Sn5 layer growth model at the cooling stage-the interfacical reaction theoretical model of precipitation flux control(PFC)was established.Based on this model,the growth mechanism of continuous growth at rough surface,two-dimensional nucleation and growth,and screw dislocation were proposed to explain the growth behaviors of three typical Cu6Sn5 morphologies for example,scallops,facets and prisms.2.The liquid channel between two Cu6Sns grains was firstly in situ observed at Sn-3.5Ag/Cu liquid solid soldering interface by using synchrotron radiation imaging technology.The liquid channel improves the grain boundary diffusion flux of Cu and decreases the growth activation energy of Cu6Sn5 at the interface.It was indicated that the addition of Ag can hinder the transverse annexation of Cu6Sn5 grains boundary at the heat preservation stage and promote the longitudinal growth of Cu6Sn5 grains,which leaded to a smaller aspect ratio of Cu6Sn5 grains obtained at Sn-3.5Ag/Cu interface than at Sn/Cu interface.3.The growth of interfacial Cu6Sn5 at the cooling stage of the tin-based solder with Ag element/Cu soldering was affected by the pinning effect of Ag3Sn nanoparticles on the interfacial Cu6Sn5 grains and the Cu precipitation flux.The main controlling factor is the Cu precipitation flux for IMC growth of solder with low Ag concentration/Cu interface,while the main controlling factor for IMC growth of solder with high Ag concentration/Cu interface is the pinning effect of Ag3Sn nanoparticles.4.After soldering for a short isothermal time at Sn-xCu/Cu(x=0.5,0.7,1.5wt.%)soldering interfaces,the morphologies of interface IMC grains at the cooling stage were not obviously affected by the initial Cu concentration,and all followed the three grain growth mechanisms of the continuous growth at rough sourface,nucleation and growth and screw dislocation to grow up.After solder for a long isothermal time,the initial Cu concentration of solder had a great influence on the grain growth mode of Cu6Sn5 at the cooling stage.At low Cu concentration,Cu6Sn5 growth at the cooling stage followed the three grain growth mechanisms of the continuous growth at rough sourface,nucleation and growth and screw dislocation to grow up.At high Cu concentration,Cu6Sn5 growth at the cooling stage only followed the grain growth mechanisms of the continuous growth at rough sourface,which grew faster in the direction perpendicular to the Cu substrate.