Design And Reliability Study On Sn-Bi Base Lead-Free Solders

Modern electronic products are becoming needing higher miniaturization,higher density,higher reliability and versatile characteristics,which require more stringent performance for solder joints used in electronic packaging.As people's awareness of environmental protection becomes stronger,the traditional Sn-Pb solder has withdrawn from the historical stage because of the harmful effects of Pb on the human body and the environment.Now the lead-free solder used in electronic packaging is mainly Sn based solder,including Sn-Ag-Cu,Sn-Cu,Sn-Zn,Sn-Cu-Ni,Sn-Bi.The Sn-Bi base solders have the advantages of low melting point,good wettability and low cost,which have been paid much attention by many researchers.This paper studied the eutectic Sn-58 Bi of Sn-Bi solders.For electronic packaging,the lower the encapsulation temperature,the lower the thermal damage to each component.It is of important influence on the life of components.Therefore,the melting point of solder plays an important role in electronic packaging.The eutectic melting point of Sn-58 Bi solder is 139 ?,far below the melting point of other solders.So Sn-58 Bi solder has a good application prospect.But Sn-58 Bi also has shortcomings such poor mechanical properties and the coarsening of microstructure during service and brittle,especially the Bi phase.It easily leads to solder joint holes,stripping and other problems.These factors restrict the application of Sn-58 Bi solder.Thus improving the performance and microstructure of Sn-58 Bi solder has become a focus in electronic packagingThis paper studied that adding memory alloy Cu ZnAl particles in Sn-58 Bi solder to improve the properties of Sn-58 Bi solder,including melting point,wettability,mechanical properties,microstructure and interface structure.Through the thermal cycling and aging test.It studied the effects of CuZnAl particles on mechanical properties of Sn-58 Bi solder alloy,the microstructure and interface structure.The influence of the addition of CuZnAl particles on the wettability of Sn-58 Bi solder was studied by the spreading area.It was found that the wettability of Sn-58 Bi solder could be improved significantly by adding the appropriate amount of CuZnAl particles.When the content of CuZnAl particles was 0.2%,the spreading area was the largest and the wetting property was the best.At the same time,the influence of CuZnAl particles on the wettability of Sn-58 Bi solder at different temperatures was also studied.The result was that the temperature was high,the solder had the good wetting property.At different temperatures,the wettability of Sn-58 Bi solder with 0.2%CuZnAl particles was still the best.DSC was used to study the influence of CuZnAl particles on the melting characteristics of Sn-58 Bi solder.After adding CuZnAl particles into Sn-58 Bi solder,the melting point and undercooling degree did not change much,which had no obvious effect on the melting characteristics of Sn-58 Bi solder.The mechanical properties of Sn-58 Bi solder with CuZnAl particles were tested by STR-1000 solder joint shear and tensile tester.It was found that the shear strength and tensile strength of Sn-58Bi-0.2CuZnAl composite solder were best,which was 6.8% and 6.1% higher than that of Sn-58 Bi solder,respectively.When the content was excessive,the mechanical properties began to decrease.By observing the microstructure of the solder and the interface structure,it was found that the microstructure of Sn-58 Bi was refined well when the content of CuZnAl particles was 0.2%.The original coarse Bi phase was refined and distributed in ?-Sn phase uniformly.The interface morphology changed from scallop shape into lamellar shape.The thickness of the interfacial layer was reduced by 36%.It was important to improve the mechanical properties and reliability of solder joints.It was important to study the effects of CuZnAl particles on the microstructure and mechanical properties of Sn-58 Bi solder during thermal cycling and aging.It was found that no matter at thermal cycling or aging test,the addition of 0.2%CuZnAl particles slowed the coarsening of Sn-58 Bi,reduced the growth rate of the interfacial layer,mainly because of reducing the diffusion coefficient between Sn and Cu.The shear force and tensile force of solder joints decreased with the increase of high and low temperature thermal cycling and aging test time,but the Sn-58Bi-0.2CuZnAl composite solder still had the largest mechanical properties.After 900 h thermal cycling time,shear strength and tensile strength of Sn-58Bi-0.2Cu ZnAl and Sn-58 Bi solders decreased by 31.9%?44.8% and 28.9%?43.2%,respectively.After 900 h aging time,these decreased by 21.7%?30.2% and19.9%?28.1%.It could find that the addition of CuZnAl particles slowed down the decline of the shear stress and tensile strength of Sn-58 Bi solder and the influence of thermal cycling test on the mechanical properties of Sn-58 Bi solder was greater than the aging test.