Study On Flux Of Sn-58Bi Solder Paste And The Reflow Process

Sn-Bi lead-free solder paste is widely used in the assembly of devices with poor heat resistance such as microelectronic sensors and flexible plates because of its low melting point,and has become a research hot spot in the field of cryogenic packaging.The reduction in welding temperature means that the original flux formulation and process are no longer suitable for Sn-Bi solder paste.Therefore,in the development trend of package cryogenic,the research on flux and reflow process of Sn-Bi solder paste is particularly important.In this paper,based on Sn-58Bi lead-free solder powder,the effects of active agent,film-formering agent and thixotropic agent on the solder paste welding performance and printing performance were investigated by single variable method,in order to obtain excellent Sn-58Bi cryogenic lead-free solder paste.The effects of reflow process on solder paste soldering performance,Sn-58Bi/Cu intermetallic compound(IMC)layer and solder joint shear strength was discussed.The research results show that:(1)The active agent is compounded with salicylic acid and suberic acid at a mass ratio of 4:1,and the active agent content is 15%of the flux quality,the Sn-58Bi low-temperature solder paste is excellent in soldering performance and the solder joint spreading rate can be up to 84.89%.(2)The film-forming agent is compounded with water white rosin and acrylic modified rosin(KE-604)at a mass ratio of 2:3,and the film-forming agent content is 35%of the flux quality.The solder paste has excellent soldering performance,moderate viscosity.For fine pitch printing,the minimum unbridged pitch can be up to 0.2mm.(3)The thixotropic agent is compounded with fatty acid amide and hydrogenated castor oil at a mass ratio of 2:3,and the thixotropic agent content is 8%of the flux quality.The prepared solder paste can meet the minimum spacing of 0.06mm pad printing.(4)When the holding time is 140s,the solder paste has good wettability and the solder joints are not retracted.Too long holding time will cause fine spacing pad retraction.(5)When the reflow time is 40s and the reflow temperature is 180?,the solder paste has good soldering performance.The thickness of the IMC layer is moderate,and the solder joint shear strength is high.(6)As the thickness of IMC layer increases,the fracture position of solder joint is transferred from the inside of solder to IMC layer.