Study On Mechanism And Kinetics Of Interfacial Reactions In Sn-based Solder Joints During Multiple Reflows

Driven by the miniaturization of electronic products,electronic packaging technology is now in a trasition of 2D to 3D and multiple reflows are generally required in the current generation of 3D pakaging.Most of the research works hither to,simply have been regarding the multiple reflow process as a long time soldering reaction consequently resulting in deduction of some inaccurate or impractical mechanisms for interfacial reaction.The present work has further studied the effects of solder alloy,reflow parameter,joint size and bubble defect on the mechanism and kinetics of liquid/solid interfacial IMC growth in every stage of multiple reflows,utilizing numerical simulation and real-time imaging technology of synchrotron radiation as well as experimental method of high pressure air blowing.Finally,the precise interfacial reaction in Sn/Cu solders was revealed clearly and a model for interfacial IMC growth in multiple reflows was developed.The main conclusions are drawn as follows:1.In isothermal heating of Sn/Cu multiple reflows,the growth of interfacial IMC followed the grain annexation behavior,and its controlled mechanism was gradually changed from boundary to volume diffusion while the time exponent transformed from 1/3 to 1/2;in cooling,the interfacial IMC grew rapidly following the atom deposition behavior and the time exponent kept as 1;in heating,the interfacial IMC tended to dissolve partly induced by Cu solubility factor and the time exponent remained as 1,which was an inverse of IMC evolution in cooling;based on these analysis,a model for interfacial IMC growth in multiple reflows was developed.In Sn-xCu/Cu solders,the increase of Cu concentration resulted in the raising of interfacial IMC growth rate and declining of the contribution of boundary diffusion to IMC growth.In Sn-xAg/Cu solders,the adsorption of Ag3Sn nano particles on the surface of interfacial IMC grains enhanced the contribution of boundary diffusion to IMC growth;besides,the smaller the particle size,the stronger the inhibition effect.The addition of TiO2 nano particle in Sn-xTiO2/Cu solders suppressed the growth of interfacial IMC during multiple reflows,and this effect strengthened with the higher of soldering temperature or TiO2 content.2.The higher the temperature or the longer the holding time,the larger the time exponent and interfacial IMC size.With the increase of reflow cycle,the time exponent and interfacial IMC size increased but the IMC growth rate decresed.However,when the cooling rate got faster,the IMC growth rate increased but both the final size and time exponent decreased.3.The interactional growth of interfacial bubble and IMC during multiple reflows:the evolution of IMC phase,including growth,dissolution and thermal expansion or condensation phenomenon,can influence the bubble size;meanwhile,the interfacial bubble could restrain the growth of IMC grains in turn by stopping the contact between liquid solder and Cu substrate.4.For Sn/Cu and Sn-0.7Cu/Cu solders,the downsizing of solder joint tended to generate bigger IMC grains through the increase of Cu concentration in liquid solder.For Sn-3.SAg/Cu solder,because of the effects of both Cu concentration and Ag3Sn particles,the decrease of solder size resulted in the first increase and then decrease of IMC thickness.