Study On SnAgCu Solder Paste For Jet-Printing Technology

With the volume of electronic products shrinks,the traditional two-dimensional packaging cannot meet the requirements,and the three-dimensional packaging technology has emerged.However,the stencil printing method can only print the solder paste on a flat surface,which is not sufficient for some three-dimensional welded joints in the three-dimensional packaging.The problem of three-dimensional packaging can be effectively solved by jetting the solder paste onto the joints surface by using the jet printing method.The solder paste jet printing technology have different requirements on solder pastes,which is very important that study,the relationship between the composition of solder paste,the rheological properties and printing performance.In this thesis,Sn-0.3Ag-0.7Cu solder powder was used as the alloy powder,in combination with different lead-free solder pastes,the effect of flux composition on the rheological properties and printing performance of solder paste was studied systematically with a combined method including viscosity test,thixotropic loop test,oscillation stress sweep test and creep-recovery test and other test.In addition,the influences of viscosity,thixotropic loop area,storage modulus and loss modulus on rheological parameters and jet printing parameters such as temperature and jet printing height were explored.Lastly,the interrelationship and mechanism underlying was explored as well.Our results showed that the alcoholic solvents can have more viscosity than etheric solvents,and have shorter linear viscoelastic region,higher loss coefficient and a lower flow stress,hard to jet out.The low viscosity etheric solvent paste spray smooth,but the average point diameter is too large;the compounding of these two solvents can make the solder paste to maintain uniformity and stability,while reducing the average spot diameter solder paste printing,but have more tips.Polyamide thixotropic agent Crayvallac SUPER(CVS)could improve the viscosity and thixotropic of paste,has a positive effect on storage modulus G' and loss modulus G" and improve the flow stress,but the high modulus and viscosity will lead to obviously die-swell effect,that's not conducive to the jet printing of the solder paste.Castor oil thixotropic agent,such as Crayvallac cvp(CVP)and THIXATROL ST(ST),has smaller viscosity and good jet printing effect.The compounding of EBS with CVS or CVP reduces the average size of the dots and tips.Under room temperature,the loss coefficient of the solder paste containing succinic anhydride was high and its flow stress was small as well.Our conclusion after analysis is its structure is unstable and have a poor jet printing performance.At low shear rate,Dimethylol propionic acid(DMPA)had a positive effect on the viscosity of the solder paste,the thixotropic loop area and the unrecovered stress of the solder paste with DMPA were higher,G"/G' value of 0.41,the flow stress of 105 Pa,and superior performance,the better shape of the jet printing dot,the jet printing process was smooth without nozzle clogging phenomenon.When the value of G"/G' was between 0.4 and 0.5 and the flow stress was about 100 Pa,the jet printing condition of the solder paste was better,and there was less phenomenon of nozzle jam at room temperature.The values of G"/G ' had no effect on the jet printing of solder paste.Generally speaking,the long the range of the linear viscoelastic region on the solder paste oscillation stress sweep test curve,the better the result of solder paste jet printing.When the linear viscoelastic region was below 5 Pa,the jet printing effect was poor.In the creep recovery test,the magnitude of the applied shear stress had a very important effect on the recovery rate of the solder paste.The creep recovery rate,when shear stress was 5 Pa,was two times or more than the creep recovery rate when the shear stress was 50 Pa,the jet printing result was better.The appropriate nozzle temperature could be helpful to improve the quality of solder paste jet printing,and the increase of nozzle temperature would lead to bigger dot size.However,when the nozzle temperature was too high,the viscosity of the solder paste would be drastically reduced,and the structure of the solder paste would be damaged.Moreover,that would produce a large number of scatter dots.The higher the height of the jet printing,the worse the shape of the jet printing point was.