Numerical Simulation Of The Effects Of Sn Orientation And Thickness On Thermomigration In Cu/Sn/Cu Solder Joints

With the continuous innovation and development of integrated circuit(IC)manufacturing and electronic packaging technology,market requirements for miniaturization,low cost,multifunction,portability and reliability of electronic products have been continuously increasing,and the characteristic size of IC has entered the nanometer level.Solder joints for connecting IC chips and package substrates are important parts of in a package interconnection structure and mainly play the roles of electrical connection,mechanical connection and heat dissipation.Due to the increasing requirements for packaging density and performance of electronic products,solder joints are gradually being miniaturized.When electronic products are in service,Joule heat is generated on chip side due to the action of current.Thus,temperature difference may be generated between the chip side and the substrate side of micro solder joints who serve as heat transfer channel,resulting in the thermomigration(TM)of metal atoms.Under TM,overgrowth of intermetallic compound(IMC)at the cold end interface of micro solder joints is prone to occur,which will cause potential reliability problems due to the brittleness of IMC.The main factors affecting the growth of interfacial IMC in micro solder joints are Sn grain orientation,joint size,etc.Therefore,this study focuses on the influences of Sn grain orientation and Sn layer thickness on the TM behavior of Cu atoms and the growth of IMC at the interface by numerical simulation of temperature gradient(TG)in Cu/Sn/Cu linear micro solder joints.The major findings are as follows.(1)The model of Cu atomic diffusion in solder joints under TG was established.The calculation program for IMC growth at the cold end interface was edited based on finite element software,and the reasonableness of the calculation program was checked by setting different initial concentrations.Using the experimental data of IMC growth in the literature,a reasonable range of Cu atom concentration at the hot end in the solder joint was calculated and obtained.(2)The effect of Sn grain orientation on the interfacial response of solder joints under TG was simulated.The Cu atom concentration fields in the solder joints with different Sn grain orientations were solved for solder joints with single Sn grain having different θ angles,and the TM diffusion fluxes that maintained the IMC growth at the cold end interface were also solved.The results show that the Sn grain orientation of the solder joints had no effect on the concentration field at the steady state,but affected the time to reach the steady state.However,the Sn grain orientation in the solder joints had a significant effect on the IMC growth at the cold end interface,and the smaller the θ angle was,the larger the Cu atomic TM flux and the faster the IMC growth at the cold end interface.(3)The effect of Sn layer thickness on the interfacial reaction of solder joints under TG was simulated.The relationship between Sn layer thickness and interfacial IMC growth was obtained by solving the Cu atom concentration fields in solder joints with different Sn grain orientations for single Sn grain solder joints of different Sn layer thickness.The results show that as the Sn layer thickness increased,the diffusion flux of Cu atoms that maintained the IMC growth at the cold end interface also increased.But this increase trend was gradually weakened.When the Sn layer thickness and grain orientation changed at the same time,the smaller the θangle was,the greater effect of the Sn layer thickness on the IMC growth rate at the cold end interface.When θ = 90°,the effect of Sn layer thickness on IMC growth was negligible.