Study On The Viscoplasticity Of Sn3.0Ag0.5Cu And Reliability Of The POP Package Under Thermal-Vibration Load

In recent years,all kinds of electronic packaging products were developing in the direction of miniaturization,high integration and high reliability.Package On Package Stack(POP for short)has been more and more widely used in various fields including smart mobile phones,smart tablet computers,on-board chips and aerospace.Due to the structural characteristics of POP stack package with high integration,it was sensitive to temperature change and vibration load,and the package often faced the simultaneous loading of temperature cycle and vibration load under most practical working conditions.Therefore,the reliability of POP stack package under thermal vibration needed to be studied and discussed urgently.In this paper,the Anand viscoplastic constitutive model parameters of SAC305 solder were determined by constant temperature compression test,and the finite element model of three-dimensional POP stack package was established.through the direct coupling of thermal cycle and random vibration,the influence of random vibration under different load conditions and random vibration after thermal cycle on the reliability of POP stack package was analyzed,which provided theoretical basis and scientific guidance for further optimization and improvement of POP stack package.Firstly,the viscoplastic constitutive model of SAC305 solder was determined by constant temperature compression test: the stress-strain curve of SAC305 was obtained by constant temperature compression test,and the Anand constitutive model parameters of SAC305 solder at different temperatures and strain rates were calculated and determined.Under the same conditions,the yield stress decreased with the increase of temperature and increased with the increase of strain rate,and the temperature effect was more significant than the strain rate effect.Secondly,the thermal cycle reliability of POP stacked package was studied: according to Anand viscoplastic constitutive model obtained above,the finite element model of POP stacked package was established by ANSYS software,and the thermal cycle-structure coupling analysis was carried out,and the distribution of accumulated plastic strain and equivalent stress in POP stacked package after thermal cycle was analyzed.The results show that Von Mises stress at the key position of solder joint in thermal cycle shows opposite direction and was changed synchronously with the change of temperature,while the equivalent plastic strain increased with the change of temperature.Thirdly,study on thermal vibration reliability of POP stack package was carried out: it was analyzed that the influence of high and low temperature,holding time,temperature change rate and thermal cycle on random vibration of POP stack package.When vibrating randomly at constant temperature,the maximum stress at solder joint was changed with temperature,and the maximum stress at solder joint increased with temperature.The maximum stress at high temperature can reach 22.335 MPa,which was much higher than17.425 MPa at low temperature.The maximum stress at solder joint decreased with the increase of holding time.When holding time increased from 300 s to1500s,the maximum stress increase at high temperature was 0.114% higher than 0.003% that at low temperature,and the holding time at high temperature has more obvious influence on the maximum stress at solder joint.The increase of temperature change rate would lead to the decrease of the maximum stress value of solder joint under transient constant temperature random vibration.When the temperature rise and fall rate was changed from0.3K/s to 0.9K/s,the maximum stress reduction at high temperature was0.076%,which was greater than that at low temperature by 0.054%,and it was more sensitive to the change of temperature rise and fall rate at high temperature.The maximum stress of solder joint varies greatly with temperature when vibrating randomly at constant temperature in thermal cycle,and the maximum stress at high temperature was 25.641 MPa,which was much higher than that at low temperature,which was 5.304 MPa.The stress distribution of solder joint at high temperature was affected by thermal stress and thermal deformation during thermal cycle,and the whole stress shifted from the middle part of inner ring solder joint to outer ring solder joint,and the stress distribution was changed.