Influence Of TiO₂ Nanoparticles Addition And Joint Size On Interfacial Reaction Of Lead-free Microscale Solder Joints

With the development trend towards functionality and miniaturization of consumer electronics products,the size of solder joints has been continuously scaled down.The reliability of solder joints will become more and more important to the performance of these electronic products.One of the crucial factors which is known to affect solder joint reliability is the IMC layer formed between the substrate and the solder.The appropriate thickness of interfacial IMC is necessary to guarantee reliable interconnections.However,due to its brittle nature and thermal mismatch with the solder and the substrate,excessive IMC formation at the interface will potentially cause weakening in joint strength and eventually fatigue failure.In addition to that,with the size and volume of solder joints become miniature continuously,the proportion of the brittle IMC to the whole solder joint will increase,which can cause the higher probability of brittle interface failure of solder joint interconnections.Hence,improvement of reliability of the microscale solder joints becomes the key scientific problem for the microelectronic products.Focusing on the key issues of the reliability of microelectronic packaging,the influence of TiO₂ addition on the interfacial reaction in microscale solder joints has been systematically investigated in reflow and aging processes.The influence of joint size on the interfacial reaction,IMC growth and interface microstructure evolution in Sn-3.0Ag-0.5Cu-0.1TiO₂ solder joints have been explored.The results achieved in the thesis can provide theoretical and experimental basis for improving the solder joint reliability.The main conclusions obtained are as follows:The influence of TiO₂ nanoparticles addition on liquid-solid interface reaction in microscale solder joints in reflow process has been investigated.Results show that both the thickness and grain size of the IMC decreases when TiO₂ is added.The Sn-3.0Ag-0.5Cu-0.1TiO₂ solder system gives the smallest growth rate and exhibits the most prominent effect in retarding IMC growth and refining IMC grain size.The morphology of IMC grain might be related to Cu concentration near the IMC/solder interface.When the composition is far away from the eutectic composition,the interfacial energy is more anisotropic and makes the scallop morphology of the IMC highly faceted.As the composition gradually close to the eutectic composition,the interfacial energy of scallops tends to be more isotropic,and then the scallop morphology became more rounded.According to the results of the IMC layer and IMC grain growth exponents,it can be found that the growth kinetics of IMC is a mixed growth mechanism.The reaction process may contain the reaction diffusion stage and the volume diffusion and the grain boundary diffusion controlled IMC growth stage.The growth of IMC grains is affected by element diffusion and interface reaction.Based on the microstructure and kinetics of IMC growth in microscale solder joints,the adsorption and heterogeneous nucleation could be the main mechanisms for retarding the IMC layer growth and refining the IMC grains due to TiO₂ nanoparticles addition.The influence of TiO₂ nanoparticles addition on solid-solid interface reaction in microscale solder joints in 100?,120? and 150? aging process has been investigated.Results show that the IMC growth rate decreases when TiO₂ nanoparticles are added.Composite solder Sn-3.0Ag-0.5Cu with about 0.1wt% TiO₂ nanoparticles solder system exhibits the lowest IMC growth rate.Because adding TiO₂ nanoparticles in Sn-3.0Ag-0.5Cu solder system can increase the activation energy,and reduce the atomic diffusion rate,so as to inhibit the excessive growth of the IMC.Based on the observation of the microstructural evolution of microscale solder joints,the inhibition of IMC growth mechanism due to TiO₂ nanoparticles addition might be grain boundary pinning mechanism in aging process.The existence of small particles between the Cu₆Sn₅ grain boundaries effectively obstructs grain boundary diffusion of the Cu and Sn via the IMC layer,leading to inhibition of the growth of the IMC.The influence of joint size on liquid-solid interface reaction in Sn-3.0Ag-0.5Cu-0.1TiO₂ solder joints in reflow process has been investigated.Results show that both the thickness and grain size of IMC increases with the increase in joint size.For the solder joint with bigger pad,the Cu concentration in the solder rises faster than those with smaller ones.So the solder joint with bigger pad could experience the effect of the higher Cu concentration for longer period of time,which makes the thickness and grain size of IMC become larger.It is also shown that the elongated scallop-like Cu₆Sn₅ grains appear earlier in larger solder joint.The formation of the elongated shape Cu₆Sn₅ IMC towards the matrix solder emerging from the scallop tops is related to the higher local Cu concentration reacting with tin in solder.Finite element simulate results show that the Cu concentrations are basically consistent with the experiment results,which further confirms that the interfacial IMC growth is highly correlated with the Cu concentration near the interface.The influence of joint size on the growth of IMC between composite solder and the Cu pad in isothermal aging at temperatures of 100?,120?,and 150? has been investigated in this study.Under the same aging conditions,the thickness of the IMC layers in solder joints with bigger pad is thicker than that of smaller ones in solder joint.IMC growth rate decrease,whereas the activation energy increases with the decrease in joint size for all aging temperature.The difference of Cu concentration at the reaction sites which caused by the difference of stress magnitude at the interface might be the reason of the joint size effect during thermal aging.It is reported that stress caused by the mismatch in CTE of materials involved in the assembly could increase the rate of interfacial IMC growth.As the stress could cause to form crystal defects in bulk solder and IMC layer leading to the fast element diffusion,Cu concentration near the interface might be affected.Three-dimensional finite-element simulations for the solder joint show that the stress magnitude increases with the increase of the aging temperature.In addition to that,the stress magnitude decreases with the decrease of the Cu pad size during thermal aging.The higher stress in big solder joints may cause the relatively higher Cu concentration near the interface of the solder joints during thermal aging,which might ultimately lead to the fast growth of the IMC at the interface of the solder joints.