Study On Lead-free Solder Paste With Fine-pitch And Nano-reinforced Solder Paste

The size of electronic packaging substrate is decreasing,the number of pins is increasing,and the pin width/pin spacing is finer.In order to adapt to this trend,the research on lead-free solder paste with fine-pitch has become a hot spot in solder paste research.The decrease of spacing means that the cross section size of solder joint is reduced and the demand of mechanical properties of solder joint is higher.The reliability of solder joint is improved by nano-reinforced composite solder paste,which is also a hot spot in the research of solder paste at present.In this paper,based on Sn0.3Ag0.7Cu lead-free solder powder,the effects of film forming agent,solvent,thixotropic agent and solder powder content on the weldability,thixotropy and anti-hot slump of solder paste were investigated.By optimizing film forming agent,solvent,thixotropic agent and solder powder content,it is hoped that the lead-free solder paste with fine distance and even ultra-fine distance is obtained.The effects of TiO₂ nanoparticles on the microstructure and the mechanical properties of solder joint of composite solder paste were investigated to improve the reliability of solder joint.The main conclusions are as follows:（1）The lead-free solder paste with fine-pitch has been developed in this topic,the flux formula including 40%film-former（KE-604:Foral AX-E=2:3）,35%solvent（tetrahydrofurfuryl alcohol:methoxypolyethylene glycols=1:1）,7.5%active（adipic acid:anchoic acid=1:3）,7%thixotropic agent（modified hydrogenated castor oil:fatty acid amide wax 6500=1:1）,margin additive（polyisobutene）.Finally,when the ratio of the flux and the solder powder is 10.5:89.5,the non-bridge spacing of solder paste is greater than or equal to 0.06mm at 150℃ and 180℃,the solder paste can meet more than or equal to 0.06mm fine distance and even ultra-fine distance of welding requirements,and the fine distance welding performance is better than the Super-flex commercial lead-free solder paste.（2）The addition of TiO₂ nanoparticle effectively inhibit the growth of intermetallic components（IMC interface）of solder joints,and increase the wettability,the microhardness and shear strength of solder joints.And the optimum addition of TiO₂ nanoparticle is 0.10wt.%.At the same time,the composite solder paste still meet more than or equal to 0.06mm fine distance and even ultra-fine distance of welding requirements.（3）With the increase of aging time,the thickness of the IMC layer increase gradually,the morphology of the IMC layer change from scalloped to lamellar,and the shear strength of solder joints decrease gradually at 150℃ isothermal aging.And the fracture form of the solder joints change from the ductile fracture to the quasi-cleavage fracture.