Research On Preparation Process And Mechanism Of Nanoparticle Reinforced Low Melting Point Lead-free Solder

With the development of electronic packaging industry, lead-free solders are required to have better performance. So the urgent task is how to improve the overall performance of lead-free solder. There are mainly two methods to prepared lea-free solder, alloying and composite particles strengthening in which the base alloy systems are Sn-Cu, Sn-Ag, Sn-Bi, Sn-Zn, Sn-Ag-Cu（SAC）,etc.. In this paper, nano TiO₂ and carbon nanotubes（CNTs） was adopted to prepare Sn-Ag-Cu composites lead-free solders. The melting point, wettability, microstructure, mechanical properties and thermal stability were studied. The main research contents and results are as follows:1. Surface modification was carried out by chemical deposition technology to avoid the aggregation of nano TiO₂ and carbon nanotubes. The experimental results show that the surface modified nanometer TiO₂ particles and carbon nanotubes have good wettability with solder.2. Composite solders with different contents of TiO₂ and CNTs were prepared by ball milling and powder metallurgy. The results of composition, melting point and wettability analysis show that the chemical composition and phase constitution of base solder keep unchanged; The melting point of composite solder is lower than that of SAC solder, which means that the current soldering process and soldering equipment can be used; Compared to SAC, composite solder has better wettability on copper plate, especially for the case of0.5% TiO₂ and 0.05% CNTs added composite solder, for which the wetting angle decreases by 9.5% and 6.1% respectively and the spreading area increases by 5.3% and 3.9%respectively.3. Microstructure analysis shows that: compared to SAC solder, the microstructure of composite solder is finer and the thickness of intermetallic compound layer and mean grain size of IMC decrease. The thickness and particle size of IMC layer and of SAC-0.5TiO₂ composite solder and SAC-0.05 CNTs composite solder were 4.88μm, 5.16μm, and 3.90μm,4.05μm which decrease by 45.8%, 42.7% and 56.3%, 45.8% respectively.4. The experimental results of mechanical properties and service reliability of composite solder joint show that the microhardness, ultimate tensile strength and ultimate shear strength of composite solder is significantly higher than that of SAC solder, especially for SAC-0.5TiO₂ composite solder and SAC-0.05 CNTs composite solder. The microhardness is about 38 HV and 37 HV increasing by 58% and 54%. The ultimate tensile strength and ultimate shear strength of SAC-0.5TiO₂ composite solder are 72 MPa and 56 MPa, increasing by 27.3% and 26.6% and that of SAC-0.05 CNTs composite solder are69 MPa and 53 MPa increasing by 25.5% and 17.7%. The pull-out force of QFP device pins of composite solder are 30.4% and 22.5% higher than that of SAC solder. The maximum shear stress of chip resistant solder joints are 64.08 N and 60.83 N, increasing by13.9% and 8.1%.5. The experimental results of thermal fatigue resistance of composite solder show that after experiencing several times reflow soldering or isothermal aging treatment, the performance of SAC solder joint and composite solder joint decreases But the composite solder have higher thermal stability than that of SAC.