Molecular Dynamics Simulation Study On Mechanical Properties Of Polycrystalline Cu₆Sn₅

Intermetallic compounds（IMCs）are the critical components for forming reliable mechanical and electrical connections of electronic products,therofore IMCs affect the reliability of solder joints a lot.Polycrystalline Cu₆Sn₅ is the most common and dominant component of IMCs,and its mechanical properties are very important to solder joint reliability.Experiment is the main method to study the mechanical properties of polycrystalline Cu₆Sn₅,but the experimental data are limited.In addition,the internal mechanism that causes the mechanical properties of the solder joint IMC（polycrystalline Cu₆Sn₅）to change during the service is not clear at present.In this paper,molecular dynamics（MD）simulation was used to study the mechanical properties of polycrystalline Cu₆Sn₅.The influences of strain rate,temperature,grain size and geometric size of Cu₆Sn₅ layer on the mechanical properties of polycrystalline Cu₆Sn₅were analyzed,and the internal mechanism of mechanical properties changes was explored from the atomic scale.The main conclusions are as follows.（1）The influence law of strain rate on mechanical properties of polycrystalline Cu₆Sn₅.Strain rate is an important factor affecting the mechanical properties of polycrystalline Cu₆Sn₅.According to the research conclusions,mechanical properties and deformation characteristics of polycrystalline Cu₆Sn₅ are quite different in the whole strain rate ranges.The results show that:in the high strain rate range(0.0005～0.1 ps^-1)Young’s modulus,yield strength and ultimate tensile strength of polycrystalline Cu₆Sn₅ increased approximately linearly with logarithm of strain rate.And in the low strain rate range(less than 0.0005 ps^-1),the above three parameters show an approximately linear increase trend with strain rate itself.In the whole strain rate range,the internal deformation characteristics of polycrystal are more complex.The polycrystal is uniformly elongated at the strain rates of 0.05～0.1 ps^-1.The atomic shear strains at the grain boundary are gradually larger than those inside the grain at the strain rate of 0.0005～0.1 ps^-1,and cracks begin to appear at the grain boundary.However,at the low strain rate the atomic shear strain at the grain boundary is still larger than that in the grain,but no cracks appear in the polycrystal.（2）The influence law of temperature on mechanical properties of polycrystalline Cu₆Sn₅.Temperature is another key factor affecting the mechanical properties of polycrystalline Cu₆Sn₅.According to the typical working temperature of electronic products,the simulation temperature range of this project is set to 200～450 K.The results show that the Young’s modulus,yield strength and ultimate tensile strength of Cu₆Sn₅decrease with the increase of temperature.it is found that the total energy of the system at the state of equilibrium increases approximately linearly with the increase of temperature.And the energy absorbed when stretching the polycrystals under the same loading conditions at different temperatures are almost the same.Therefore,it is inferred that the mechanical properties of Cu₆Sn₅ are mainly related to its initial energy state（temperature）.（3）The influence law of grain size and geometrical size of Cu₆Sn₅ layer on mechanical properties of the polycrystal.Since the electronic products usually service at room temperature and the different mechanical properties of polycrystalline Cu₆Sn₅ at the different strain rates,the loading temperature was set to 300 K and the loading strain rate range was 0.0001～0.1 ps^-1 when the effect of grain size was studied.The results show that Young’s modulus and yield strength of polycrystalline Cu₆Sn₅ increase with increase of the grain size.And the change of ultimate tensile strength is related to the strain rate.At the high strain rate range(0.001～0.1 ps^-1),the ultimate tensile strength increases at first and then decreases with the increase of grain size.At low strain rate(0.0001 ps^-1),the ultimate tensile strength always increases with the increase of grain size.The effects of width and thickness of Cu₆Sn₅ layer on its mechanical properties of were studied,and it was found that the ultimate tensile strength was almost unaffected by the changes of width and thickness at high strain rate(0.01 ps^-1).However,the ultimate tensile strength decreases with increase of width and thickness at low strain rate(0.0001 ps^-1).（4）Experimental study on mechanical properties of Cu₆Sn₅.The results of molecular dynamics simulation were partially verified through nano-indentation test.The hardness and Young’s modulus of Cu₆Sn₅ were measured,and the evolution law was studied.The results show that the average hardness of Cu₆Sn₅ increases at the loading rate ranges of 0.02～0.2 s^-1,and the hardness value decreases slightly when the loading rate increases to 0.5 s^-1.The average Young’s modulus of Cu₆Sn₅ increases in the whole loading range.According to the indentation curve andΠtheorem,the average yield strength of Cu₆Sn₅ is calculated,and it is found that the yield strength increases with increase of loading rate.The Young’s modulus and yield strength of Cu₆Sn₅ obtained by nano-indentation experiment are included in the low strain rate range of MD simulation,and the variation trend of Young’s modulus and yield strength obtained by the two methods are consistent with that of strain rate.In this paper,polycrystalline Cu₆Sn₅is taken as the research object,which is closer to the engineering.The relevant conclusions can enrich the research results on the mechanical properties of Cu₆Sn_5,and a foundation for exploring the failure mechanism of micro-nano scale solder joints was built.