Performance Regulation Study Of Residue-free Sn-Bi Solder Solder Joints Under Formic Acid Atmosphere

As the solder joint density increases and the size and spacing decreases,flux removal becomes more and more difficult.Therefore,helpless flux soldering technology has attracted extensive attention in the industry.Formic acid(FA)atmosphere soldering is one of the most promising helpless flux welding technologies.On the other hand,after the use of Sn-Pb solder has been restricted,lead-free Sn base solder has been rapidly developed.Due to the wide application of polymer materials in electronic devices,increasing the need for lowtemperature welding,Sn-58 Bi solder has the advantages of low melting point(138 °C)and low cost,making it the most commonly used commercial low-temperature solder for packaging.Therefore,based on FA technology,this paper explores the wettability,mechanical properties and long-term reliability of low-temperature Sn-58 Bi solder joint with commonly used industrial Cu pad and ENIG pad,determines the action principle and influence of formic acid atmosphere on the change of interface state between pads,and explores the failure mechanism of solder joints under different test conditions.In order to adapt to the development of high density three-dimensional packaging structure,but also has good economic low pollution helpless flux connection technology to provide theoretical support and technical reserve.The results obtained in this study are as follows:The wettability of Sn-58Bi/Cu solder joints under an FA atmosphere is similar to that of the flux process.The microstructures of Sn-58Bi/Cu after reflow and aging under the two methods are identical,but there are holes.On the premise that Sn-58Bi/Cu has good solderability under FA atmosphere,to ascertain the microscopic evolution law of Sn-58Bi/Cu solder joints under FA atmosphere,an oil bath(100 °C)accelerated thermal aging was carried out Test to observe the microstructure changes of solder joints after reflow and aging under FA atmosphere.The solder joints are mainly composed of a gray Sn-rich phase,white Bi-rich phase,and interfacial IMC,and as the aging time prolongs,the IMC layer thickens from about 1 μm to 3 μm at the beginning,and the Bi-rich phase gradually coarsens.Follows bulk diffusion and Ostwald Ripening mechanisms.The reflow shear strength of solder joints under FA atmosphere(40.91 MPa)is lower than that of the Sn-Bi unique flux process(79.29 MPa).Local hole stress concentration,the fracture mode is a ductile fracture caused by solder hole stress concentration.After oil bath aging,the shear strength of Sn-58 Bi solder joints in both processes will decrease with the prolongation of aging time.After aging for 30 days,the shear strength of solder joints in the FA atmosphere decreased to 33.18 MPa(18% decrease),which showed better thermal fatigue resistance compared with 39.09 MPa(50% decrease)in the flux process.The above results show that the mechanical properties of Sn-58Bi/Cu in the FA atmosphere are low,and there is much room for optimization.Based on previous research and experimental results,we tried three methods to improve the mechanical properties of Sn-58 Bi solder joints under FA atmosphere from the perspective of technology and materials.The results are as follows:Using Pt catalysis to improve the FA atmosphere process,it was found that although the porosity of the solder joints decreased,the larger pores formed still affected the mechanical properties of the solder joints;by prolonging the reflow preheating time,the FA atmosphere can more fully restore the Cu pads,can significantly reduce the porosity and increase the shear strength of solder joints;in addition,using ENIG pads with excellent oxidation resistance and wettability in Pt-catalyzed FA atmosphere to replace easily oxidizable Cu pads can further improve the FA atmosphere soldering quality.