Investigation On Rheological Behavior And Materials Design Of Solid-Liquid Two-Phase Flow Based On Jet Printing Technology

Jet printing technology is more suitable for Surface Mount Technology?SMT?to develop in high density and flexible production compared with traditional stencil printing.In recent years,domestic and foreign scholars have carried out a lot of research work in the design of lead-free solder paste components,jet printing equipment and process,and made some progress.However,due to the lack of rheological research of related materials,the control theory of jet printing performance has not yet been established,which has affected the popularization and application of jet printing technology in the electronic assembly field.Investigation of solder paste and isotropic conductive adhesives used for jet printing technology and establishing the relationship between materials composition of solid-liquid two-phase flow materials,rheological behavior and jet printing performance will provide a reference for the design of a wider range of solid-liquid two-phase flow materials suitable for jet printing technology.In this work,new solid-liquid two-phase flow materials containing Sn-58Bi solder pastes,Sn-3.0Ag-0.5Cu solder pastes and isotropic conductive adhesives were chosen and the effects of temperature,solid particle size distribution and solid phase contents on the rheological properties of viscosity,thixotropy,viscoelasticity,creep recovery,storage modulus and loss modulus of solid-liquid two-phase flow were studied systematically.On the basis of this,combined with the jet printing process tracking and the shape of the jet printing droplets,the conditions of the droplets pitch-off during the jetting process and methods of satellite droplet formation and control were discussed.A certain relationship between rheology and jet printing performance of solid-liquid two-phase flow was established.The viscosity of solder paste decreases significantly with the decrease of solid contents,and solder paste has shear thinning property.The thixotropic coefficient Ti is significantly improved after addition of HCO to the auxiliary.Solder paste with lower solid content usually has a larger Ti value.The solder pastes show viscoelasticity and LVR region under oscillating stress,and have G'>G”.The viscosity measured in steady shear flow field is close to the dynamic composite viscosity modulus calculated in SAOS flow field,and the rheology of solder paste accords with Cox-Merz empirical formula.The viscosity of solder paste in steady shear flow field conforms to Herschel model.The loss coefficient and flow stress of solder paste in LVR region will be reduced by increasing temperature and decreasing solid content,which is conducive to the flow of solder paste.Solder paste with low viscosity usually has larger creep compliance J_C and recovery compliance J_R values.Solder paste exhibits shear thickening properties when low-speed shearing occurs between0.01s^-1 and 0.0161s^-1.With the increase of shear rate,when the shear stress exceeds the flow stress,solder paste changes from shear thickening to shear thinning.There is a lag between the upstream and downstream lines of thixotropic loop,which indicates that solder paste has obvious thixotropy.The thixotropic loop area and unrecoverable stress decrease with the increase of temperature.Sn-3Ag-0.5Cu solder paste has larger thixotropic hysteresis than Sn-58Bi solder paste.The solder paste jet printing process includes four periods:initial expansion,necking,pitch-off and formation of satellite droplets.The collapse behavior of solder paste in jet printing process was observed at 35?,45?and 55?.It was found that the spatter originated from the delayed fracture process in the early stage of jet process,and the upward shrinkage of residual droplet pitch-off to form a second satellite droplet.The ratio of height to diameter is proportional to the square root of viscosity and to the nozzle diameter d.The ratio of height to diameter of solder paste droplets decreases with the increase of temperature.The ratio of height to diameter of high solids content P10 solder paste and low solids content P12 solder paste reaches the closest ratio to 0.5 at 55?and 50?,respectively.The critical length of satellite droplets formation during jet printing is7)^*₍?7?=0.317mm.The main droplet length of solder paste exceeded this critical length,so satellite droplets appeared in the jet printing process.The ratio of height to diameter of high-viscosity solder paste is always more than 1 and the diameter of jetted solder joint is smaller,which is prone to collapse.The ratio of height to diameter of low-viscosity solder paste is relatively small,while the pitch-off position and tip of satellite droplet will also affect the jet printing quality.The P3 solder paste with apparent viscosity of 95.2Pa.s had the best ratio and jetting solder joint morphology.The viscosity of Isotropic conductive adhesives in steady shear flow field conforms to Cross model and the rheological behavior accords to Cox-Merz empirical formula.In SAOS flow field,the ICAs has G”>G'in LVR region,which shows the viscous component is dominant and ICAs represent viscoelastic fluid characteristics.With the increase of temperature,the length of LVR region of ICAs decreased,the distance between G'and G”curve tended to decrease,and the phase angle in LVR region declined rapidly.The creep recovery of ICAs decreases with the increase of temperature and the decrease of solid contents.The creep recovery rate of ICAs A1 is 0.21%under 5Pa stress and 0.007%under 50 Pa stress at 55?.ICAs have shear thickening at low shear rate when temperature is lower than 45?.In the shear rate range of 0.01s^-1 to 0.053s^-1,the viscosity of A1 increases gradually,and the maximum value is 505.9Pa.s.When the shear stress exceeds the flow stress,the ICAs colloid shear thinning behavior.Only shear thinning exists in ICAs temperature more than 45?.In the case of lower temperature and higher solid content,the Pitch-off of ICAs has longer duration with position close to the nozzle,which in turn causes longer satellite droplets leading to more tips and bridges.ICAs A1 with low solid content has the best jet printing performance at 50?,which the ratio of height to diameter of jet printing ICAs droplets is 0.35.