Influence Of Nano-modifided Solder On Interfacial Reaction And Mechanical Property Of Lead-free Solder Joints

With the trend towards miniaturization of microelectronic products andimplementation of lead-free soldering, the size of the microelectronic components and thusthe solder joints has been scaled down. As a result, the percentage of the volume of thebrittle intermetallic compounds （IMC） layer in solder joints trends to become higher, andpossibly only a few IMC grains is interconnected with solder joint interface, resulting inmore brittle fracture and affect the reliability of microelectronic products. Focusing on thekey issues of the reliability of microelectronic packaging, the influence of TiO₂nanoparticles dopant on the IMC growth, interconnection interface microstructure, andmechanical properties in reflow and aging processes has been investigated. The mechanismof effective inhibition of IMC growth and the doping scheme for the improvement of theinterconnect reliability have been explored. The interface reaction kinetics betweenlead-free nano-composite solder and Cu substrate, the effect of interface microstructureevolution on the reliability of solder joints during aging process, and the effect of strainrate and temperature on the mechanical properties of composite solder alloys have beensystematically investigated. The nano-composite solder has been optimized. The resultsachieved in the thesis can provide the reference in aspects of the solder joint reliabilitydesign, process and materials. The main conclusions obtained are as follows:The influence of TiO₂nanoparticles dopant on the melting point, wettability andmicrostructure in Sn-3.0Ag-0.5Cu-xTiO₂（x=0,0.02,0.05,0.1,0.3, and0.6wt.%）composite solders have been investigated. Results show that the melting points of theTiO₂-containing composite solders have no obvious change compared with TiO₂-freesolders. However, with an increase in the proportion of TiO₂nanoparticles, the wettabilityof the composite solders slightly increase and the microstructure of composite soldermatrix has obviously been changed. When the weight percentage of TiO₂nanoparticles is0.1wt.%, the wettability of composite solder is the best, the surface contact angle has aminimum value of8.40, while the spreading area has a maximum value of155.24mm².SEM photos show that dopant significantly refine the microstructure of the solder matrix. This is because the adsorption effect on the surface of TiO₂nanoparticles can reduce thesurface energy of Ag₃Sn grains in the composite solder matrix and inhibit the Ag₃Sn grainsgrowth.The influence of TiO₂nanoparticles dopant on liquid-solid interface reaction inSn-3.0Ag-0.5Cu-xTiO₂solder joints in reflow process has been investigated. Results showthat some of TiO₂nanoparticles are dissolved in the Sn-rich phase, some of themparticipate in the surface of Ag₃Sn phase, and the rest dissolves in surface of the Cu₆Sn₅IMC layer. Both of the thickness and grain size of IMC decrease when TiO₂nanoparticlesis added into the Sn-3.0Ag-0.5Cu solder system. According to the results of the IMC layergrowth exponents, it can be found that the growth kinetics of interfacical IMC layer is amixed growth mechanism. The wetting reaction process may contain three stages: thereaction diffusion stage, the grain boundary diffusion controlled IMC growth stage, and thevolume diffusion controlled IMC growth stage. The results of the IMC grain exponentsshow that the growth of IMC grains is controlled by atomic interdiffusion and grainmaturity. The data also show that Sn-3.0Ag-0.5Cu with about0.1wt.%TiO₂nanoparticlessolder system exhibits the smallest growth rate and gives the most prominent effect inretarding IMC growth and refining IMC grain size. In order to better understand theinterface reaction and IMC growth mechanism, a dual phase lag diffusion model and awave model based on the solid-state diffusion theory are presented for predicting the IMClayer growth in reflow process. A flux-driven ripening of copper-tin scallops is presentedfor predicting the IMC grain growth in reflow process. Based on the observation of themicrostructural evolution of the solder joints, a heterogeneous nucleation mechanism andOstwald grain ripening mechanism for retarding the IMC layer growth and refining theIMC grains due to TiO₂nanoparticles addition is proposed.The influence of TiO₂nanoparticles dopant on solid-solid interface reaction inSn-3.0Ag-0.5Cu-xTiO₂solder joints in120℃,150℃, and190℃aging process have beeninvestigated. Results show that the thickness of IMC decreases when TiO₂nanoparticlesare added into Sn-3.0Ag-0.5Cu solder system. There is a significant drop in IMC thickness when the weight percentage of TiO₂nanoparticles reaches0.05-0.1wt.%. When takenCu₆Sn₅and Cu₃Sn as a single entity, the IMC layer growth may be mainly controlled by aninterdiffusion-controlled mechanism. When taken Cu₆Sn₅and Cu₃Sn as independententities, at190℃higher aging temperature, the growth for both the Cu₆Sn₅and Cu₃SnIMC layers might be controlled by diffusion. At120℃lower aging temperature, thegrowth for both the Cu₆Sn₅and Cu₃Sn IMC layers are not compoletely diffusion controlled,bus also affected by chemical reaction. Results also show that adding TiO₂nanoparticles inSn-3.0Ag-0.5Cu solder system can increase the activation energy, and thus reduce theatomic diffusion rate, so as to inhibit the excessive growth of the IMC. Compared withCu₆Sn₅IMC layer, Cu₃Sn IMC layer has much higher activation energy. Based on theobservation of the microstructural evolution of the solder joints, a grain boundary pinningmechanism for inhibition of the IMC growth in aging process due to TiO₂nanoparticles isproposed.The influence of TiO₂nanoparticles dopant on mechanical properties inSn-3.0Ag-0.5Cu-xTiO₂composite solder alloys and solder joints have been investigated.Results show that the microhardness enhancement of these TiO₂-containing compositesolders is19%to37%compared with TiO₂-free noncomposite solder. The existence ofTiO₂nanoparticles in the solder matrix can apparently enhance the microhardness of thecomposite solder. This might be attributed to the reduction of the size and spacing betweenAg₃Sn grains in the solder matrix. The ultimate tensile strength （UTS） ofSn-3.0Ag-0.5Cu-xTiO₂composite solder alloys has a logarithmically linear increaserelation with strain rate and has a linear decrease with temperature. At higher strain rate,more and more dislocations are created. Increasing strain rate is accompanied by anincrease in the dislocation density. The increase in dislocation density will in turn increasethe interaction between dislocations and result in improving the composite solder alloysresistance to deformation. Tensile strength of solder joints drops with the aging time. Withan increase in aging time, the growth of interfacial IMC layer leads to change the fracturepattern in solder joints. The fracture pattern can be classified into three types with the increase in ageing time: ductile fracture in the solder at the begining, and then mixedfracture type containing ductile fracture and brittle fracture, and lately the brittle fracturealong the IMC interface for the long aging time. TiO₂-containing composite solder alloysand solder joints have higher UTS than TiO₂-free solder alloy and solder joint due to solidhardening and paricle hardening.