Study On Preparation And Rapid Laser Sintering Process Of Nano Silver Pastes

Recent years,nano silver pastes have emerged as the most promising electronic packaging materials,with excellent electrical conductivity,thermal conductivity and high temperature mechanical properties,can be used for high frequency,high density and high power applications,which provide an effective lead-free electronic packaging solution instead of high-lead solers.However,the applications of nano silver pastes are still limited by long sintering time(20-30min).Therefore,this paper probed into the rapid laser sintering of nano silver pastes,and the sintered joints with shear strength of about 12-38 MPa can be obtained in 5-15 s.In addition,the mixed silver pastes are designed and prepared to achieve high-intensity interconnection at low laser power.Firstly,nano silver particles(Ag NPs)with particle diameters of 62 nm and19nm were prepared by chemical reduction method,and the parameters of laser sintering were investigated with the large diameter Ag NPs.It is found that the laser power and the sintering time have a significant effect on the shear strength of the joints.With the increase of power and the sintering time,the shear strength is greatly improved.The effect of pressure on the shear strength of the joint is not obvious at low power but the shear strength is improved as the applied pressure increases at high power.Neverthelesss,the high power and long sintering time can produce high sintering temperature,which will cause damage to the chip,so the best sintering parameter is 80 W,10s,5MPa,and the shear strength of the sintered joints is 23 MPa.Furthermore,to avoid the damage caused by laser direct exposure to the chip,it also explored the effect of the back sintering of the substrate.The shear strength of the joints obtained by the back sintering is more higher than the frontal sintering.The increase of the defocus amount can reduce the surface temperature of the chip.However,the reduction of power density will lead to the decrease in the shear strength of the joints.Then,the mechanism of rapid laser sintering was proposed by exploring the microstructure evolution of the sintered joints and the simulation of the sintering temperature field.Based on the results,the sintering process consists of four stages.In the first stage,the decomposition of the organics promotes the direct contact and close arrangement of the Ag NPs.Then,local melting of the Ag NPs accelerates the diffusion of atoms,promoting the growth of the sintering necks.Afterwards,the deformed particles are segmented to fill large pores.Finally,the disappearance of the microdefects accelerates the densification of the sintered joints.Finally,the high-intensity interconnection under the sintering parameter of60 W,15s,3MPawas explored.The shear strength of the sintered joints obtained by the large size Ag NPs is only 17 MPa.As the sintering driving force of the small size Ag NPs is bigger,the sintered joints with shear strength of 25 MPa can be obtained under the same parameter.The different size Ag NPs mixed pastes and doping Ag NWs pastes can also improve the shear performance,the shear strength is 36 MPa and 24 MPa,respectively.But the enhancement mechanism is different.The former is because the Ag NPs with two different sizes can fill the gap with each other in the process of sintering.It is easier to achieve densification through atoms diffusion.The latter is because the Ag NWs in the sintered joints act as a reinforcing phase to improve the strength by absorbing the strain energy consumption during loading.