Preparation Of Sn-58Bi Solder Preform For Low Temperature Soldering And Properties Of The Reflow-assembled Preform Solder Joints

With the continuously increased component packaging density in electronics,substrate pad shape,accurate amount of the solder required for soldering interconnect and thermal failure caused by hierarchical packaging have recently drawn much attention.Sn–58Bi solder preform has attracted much attention due to its low porosity and precise shape.Therefore,it is necessary to find a process with an uncomplicated process flow and high production efficiency to fabricate Sn–58Bi alloy solder preform.In this thesis study,the effects of solidification conditions including air-cooled,water-cooled,chill and aging at 100oC for 200 h on microstructure and mechanical properties of the Sn–58Bi alloy was investigated firstly.Then the relationship between ductility and deformation conditions of Sn–58Bi alloy was obtained by changing the strain rate from 10^-1s^-1 to 10^-4 s^-1,and the effect of strain rate sensitivity on mechanical properties of Sn–58Bi alloy was concluded.On this basis,an attempt is proposed to prepare Sn–58Bi solder preforms by using rolling process.The parameters of the rolling process were optimized by studying the effects of homogenization annealing temperature and time,rolling temperature,and rolling direction on the Sn–58Bi alloy.Optimized soldering fluxes were selected for the Sn–58Bi solder preform for reflow soldering,and then the effect of the reflow parameters on the Sn–58Bi solder joint was observed by changing peak temperature,reflow time,and thickness of solder preforms.The results demonstrate that as the cooling rate of the alloy solidification increases,the microstructure of the Sn–58Bi alloy becomes much finer.After tensile test,it can be noted that the ductility of the Sn–58Bi alloy slightly increases.As the aging time increases,the elongation of the Sn–58Bi alloy decreases and the ductility degrades gradually.Due to the strain rate sensitivity,the effect of strain rate on ductility of the alloy is greater than heat treatment.Compared to the samples loaded at high strain rate,the elongation of the Sn–58Bi alloy increases 334.13%when deformed at a low strain rate.Homogenization annealing can effectively reduce the component segregation which produced in the casting process,and 60%rolling reduction can be achieved in this condition.With increasing rolling temperature from room temperature to 75oC and 100oC,the effect of recrystallization softening makes the alloy much easier to deform under large reduction rates.It can maintain good rolling quality with a maximum reduction of 98%.However,with the increase of the number of rolling process,the microstructure of the alloy exhibits texture characteristics along the rolling direction.Therefore,it is necessary to change the rolling direction to obtain a homogenous microstructure,which leads to lower hardness and springback.The solder preform is reflowed by using the modified soldering flux.After reflow soldering,the solder joints with shiny surface were obtained.By increasing the peak temperature and prolonging reflow time,both the spreading area and thickness of intermetallic compound（IMC）layer of the solder joints increase,resulting in the increase of reliability.While increasing the thickness of the solder preform from 0.1 mm to 1.0 mm,the shear strength of joints decreases by 46.94%,leading to deteriotation of mechanical properties of joints.