Influence Of Nanoparticles On Interfacial Reaction Of Sn/Cu Solder Joint

With the development of electronic packaging technology,the miniaturized direction makes a sharp reduction of the solder joint size,which results in the increase of undercooling degree,promotion in the interaction and diffusion of elements at the solder joint interface,and an enhancement in the size of the solder joint.Therefore,the thickness of the intermetallic compounds(IMCs)layer increases at the interface of the solder joint.In addition,thick IMC layer can be a severe challenge due to its brittleness for improving the reliability of solder joint.Therefore,to effectively suppress the overgrowth of IMC layer at the interface of solder joint becomes the key to improve the solder joint reliability and develop miniaturization of advanced packaging technology.In this study,Cu nanoparticles(NPs)and TiO₂ NPs were respectively introduced at the interface of Sn/Cu solder joint.Combined with high-pressure air purging liquid solder experiment,the effect of NPs on the growth behavior of Cu₆Sn₅ at the interface of Sn/Cu solder joint during the heat preservation stage and the cooling stage of soldering has been studied,respectively.Moreover,the growth mechanism of Cu₆Sn₅ with different NPs has been analyzed.In industry,the electro-deposition technology is widely employed to prepare the micro bumps,so the optimized double-pulse electro-deposition method is managed to prepare composite solder joint with evenly dispersed NPs.Accordingly,the growth mechanism of Cu₆Sn₅ at the interface of the NPs composite solder joint is studied.This research can provide technical support and data reference for the application of NPs composite solder joint in industry,proposing an effective strategy to improve the reliability of solder joint.The main results of this study are listed as follows:(1)The interfacial reaction and Cu₆Sn₅ growth behavior of Sn/Cu solder joint with the adoption of Cu NPs indicates that the thickness of Cu₆Sn₅ decreases firstly and then increases with the increase of the amount of Cu NPs.At the beginning of heat preservation stage of soldering,Cu NPs not only provide heterogeneous nucleation sites but also partially dissolve in the liquid solder resulting in the Cu concentration increase,which lead to the increase of the nuclei number of Cu₆Sn₅ grains and sharply decrease in the grain size.With the ongoing soldering process,the effect of Cu NPs on growth of Cu₆Sn₅ is governed by two competing mechanisms.On the one hand,the increase of Cu concentration enhances the ripening growth of Cu₆Sn₅,which promotes the growth of Cu₆Sn₅;On the other hand,the reaction between Cu NPs and Sn lead to the formation of Cu₆Sn₅ NPs,which adsorb on the surface of Cu₆Sn₅ grains,reduce in Cu₆Sn₅ surface energy and slow down the growth rate.With the addition of Cu NPs at the cooling stage of soldering,the spreading area of the liquid solder is increased on the surface of substrate,which enlarge the radial temperature difference and enhance turbulence in the liquid solder.Consequently,enlargement of the radial temperature difference and enhancement of turbulence in the liquid solder slowdown the diffusion speed of Cu atom from the liquid solder to the interface of solder joint.Therefore,the growth of prismatic Cu₆Sn₅ is suppressed.At the same time,the Cu₆Sn₅ particles can pin to the edge of dimensional nucleation growth steps and screw dislocation growth steps,which can inhibit the expansion of the steps and inhibit the growth of prismatic Cu₆Sn₅.(2)Studying the interfacial reaction and Cu₆Sn₅ growth behavior of Sn/Cu solder joint with the adoption of TiO₂ NPs,it is observed that the inhibition of Cu₆Sn₅ grain growth enhanced with the increase in TiO₂ NPs.At the initial stage of soldering,the number of Cu₆Sn₅ grains increases and the grain size decreases at the interface of solder joint,because TiO₂ NPs can provide heterogeneous nucleation cites for the nucleation of Cu₆Sn₅ grains.With the ongoing heat preservation stage,the adsorption of TiO₂ NPs on the surface and grain boundaries of Cu₆Sn₅ grains can decrease the Cu₆Sn₅ crystal grain surface energy and hinder the diffusion of Cu atoms,which suppress the Cu₆Sn₅ crystal grain growth.At the cooling stage,pinning of TiO₂ NPs on the edge of dimensional nucleation growth steps and screw dislocation growth steps inhibit the expansion of the steps and then hinder the growth of prismatic Cu₆Sn₅.For analyzing the effect of different size TiO₂ NPs with the same additional amount,5 nm TiO₂ NPs have a stronger adsorption on Cu₆Sn₅ surface than 50 nm TiO₂ NPs,so the thickness of Cu₆Sn₅ layer of solder joint with 5 nm TiO₂ NPs is thinner than solder joint with 50 nm TiO₂ NPs.(3)Comparing the growth behavior of Cu₆Sn₅ of solder joint with the addition of Cu NPs and TiO₂ NPs,both can hinder the growth of Cu₆Sn₅.Even though,during the heat preservation stage,the addition of Cu NPs can promote the Cu flux in the liquid solder,thereby,the Cu₆Sn₅ growth is enhanced.For TiO₂ NPs,the adsorption of TiO₂ NPs on the surface and boundary of Cu₆Sn₅ grains increase by increasing the amount of TiO₂ NPs,which decreases the surface energy of Cu₆Sn₅ grains and suppresses the growth of Cu₆Sn₅ grains.Compared with Cu NPs,TiO₂ NPs have a stronger inhibitory effect on Cu₆Sn₅ grain growth.(4)The Cu/Sn-TiO₂/Cu solder joint with TiO₂ NPs uniformly doped successfully prepared by the double-pulse composite electro-deposition technology.The Cu₆Sn₅ growth behavior of Cu/Sn-TiO₂/Cu solder joint revealed the main conclusions as follows:with TiO₂ NPs at the interface of Sn-TiO₂ composite solder joint,the fraction of Cu₆Sn₅ volume of the solder joint is decreasing.The mechanism for TiO₂ NPs hindering the growth of Cu₆Sn₅ grains as follows:1)TiO₂ NPs adsorb and pin on the surface of Cu₆Sn₅,which can reduce the surface free energy of Cu₆Sn₅ grains and hinder the migration of Cu₆Sn₅ grain boundary;2)TiO₂ NPs gather at the Cu₆Sn₅ grain boundary,block the diffusion of Cu atoms from substrate into the liquid solder and suppress the ripening growth of Cu₆Sn₅.