Research On Fabrication And Bonding Performance Of Micropatterns Of Nanoparticles For Connections Of Micro Solder Joints

With the rapid development of packaging technology,the diameters of solder joints have been reduced from hundreds of microns to tens of microns.There are many problems when using Sn alloys for interconnections.Nanoparticles are promising to replace Sn alloys as solder for interconnections of solder joints because of its low sintering temperature and excellent electrical conductivity.There have been many studies on synthesizing nanoparticles,but applying nanoparticles to bonding interfaces in mass production is still difficult,especially when diameters of Cu pads are smaller than 10 ?m.It will impede the applications of nanoparticles in bonding technology.This thesis focuses on the patterning methods for applying nanoparticles to Cu pads on demand.The specific contents are as follows.(1)Patterning methods were studied,and the methods for applying nanoparticles in different situations were proposed.The theories and processes of transfer molding by a liquid bridge and selective wetting were researched,and the patterns of Ag nanoparticles with diameters of 35 ?m were fabricated successfully.The thicknesses of the Ag nanoparticles layers were 650 nm and 80 nm respectively.Besides,selective wetting with shallow wells assisted was proposed by combining the advantages of transfer molding by a liquid bridge and selective wetting.The thickness of the Ag nanoparticles layer could reach 1.1 ?m.(2)The factors that affected the coating process were studied,which helped guide the formulation of ink and the design of Cu pads.The receding contact angle of Ag ink on the surface of the silicon wafer with FOTS deposited increased significantly,which was good for coating.Moreover,the ratio of pitches to diameters of Cu pads had effects on the coating process and could decrease to 3/2 when diameters were larger than 10 ?m.With diameters of Cu pads decreasing,the volume of ink left on Cu pads would be reduced,but the thickness and the coverage of the Ag nanoparticles layer could be raised by repeating coating.(3)The bonding test by using Ag nanoparticles as solder was performed,which proved that patterning nanoparticles by selective wetting was feasible,and the process parameters for a good bonding were found.The top of Cu pillars electroplated by the electroplating solution of Atotech was flat and the tolerances were small,which was good for the bonding of Cu pads and Cu pillars with thin solder.With the bonding temperature and the bonding time increasing,the shear strength was raised markedly.And a good bonding could be achieved when the bonding temperature was higher than 225 ? and the bonding pressure was larger than 20 MPa.Besides,the shear strength could reach 22.92 MPa after sintering for 5 min at 250 ? with a bonding pressure of 20 MPa.The average series resistance of 128 solder joints was 63.88 ?.In this thesis,a lot of studies about the methods of patterning nanoparticles and the bonding process have been done,which will further promote the applications of nanoparticles in bonding technology.