Study On Interfacial Reactions In Ni/Sn-xZn/Ni And Ni/Sn-xCu/Ni Micro Solder Joints Under Thermomigration

Microelectronic packaging technology is moving towards high density and high realibity to meet the portable electronics demands of ultra-thin,ultra-light,high performance and low power consumption.Under this circumstance,3D microelectronic packaging emerged and opened up a new dimension for the semiconductor industry to maintain Moore's law with a much lower cost.The micro solder joints in 3D packaging are quite small,resulting in serious Joule heat,which may cause solder joint melting and induce thermomigration(TM).The present work studied the interfacial reactions of Ni/Sn-x Zn/Ni and Ni/Sn-xCu/Ni micro solder joints at 265 oC under a temperature gradient of 1304 oC/cm.The combined influence of TM and Zn and Cu contents on the evolution of interfacial intermetallic compounds(IMCs)were uncovered.The following conclusions are drawn in the present work:(1)In all the Ni/Sn-xZn/Ni(x=1,5,9 wt.%)solder joints,asymmetrical growth and evolution of intermetallic compounds(IMCs)and dissolution of Ni substrate between the cold and hot ends occurred.The main diffusion species into IMC layers changed from Zn atoms at the early stage to Sn and Ni atoms at the later stage.As a result,the IMC evolution followed Ni5Zn21 ?? phase ?(Ni,Zn)3Sn4 ? Ni3Sn4 along with reflow time.The extent of IMC evolution significantly depended on the Zn content in the solder.Faster IMC growth at the cold end and more prompt IMC transformation at the hot end were identified due to the mass thermomigration of Zn and Ni atoms towards the cold end.A higher Zn content(higher than 5 wt.%)could effectively restrain the growth rate of the cold end IMCs and inhibit the dissolution of the hot end Ni substrate.(2)In Ni/Sn-0.3Cu/Ni micro solder joints,though the interfacial IMCs remained as the initial(Ni,Cu)3Sn4,asymmetrical IMC growth occurred between the cold and hot ends,i.e.,the(Ni,Cu)3Sn4 IMC at the cold end was always obviously thicker than that at the hot end.In Ni/Sn-0.7Cu/Ni and Ni/Sn-1.5Cu/Ni micro solder joints,the interfacial IMCs gradually transformed from the initial(Cu,Ni)6Sn5 into(Ni,Cu)3Sn4.Meanwhile,the transformation at the cold end lagged behind at the hot end,namely asymmetrical transformation phenomenon occurred.Moreover,the transformation at the cold and hot ends in the Ni/Sn-1.5Cu/Ni micro solder joints both lagged behind those in the Ni/Sn-0.7Cu/Ni micro solder joints.Based on the analysis of the Cu and Ni atomic fluxes for the IMC growth at both cold and hot ends,the TM direction of Cu and Ni atoms was confirmed to be from the hot end towards the cold end.The Cu concentration in the micro solder joints had a significant effect on the dominant TM element,and thus affected the growth and transformation behavior of the interfacial IMCs at the two ends.In addition,TM promoted the diffusion of Ni atoms into liquid solder at the hot end,which accelerated the dissolution of the hot end Ni substrate.Most of the dissolved Ni atoms migrated to the cold end and participated in interfacial reaction locally.On the contrary,TM inhibited the diffusion of Ni atoms at the hot end,and thus no obvious dissolution of the cold end Ni substrate was observed.(3)During isothermal reflow process,the change of Cu and Zn contents caused similar IMC evolution as that during TM process.However,the growth of IMC was symmetrical at the two interfaces,and both the growth and evolution rates were much slower than those during the TM process.During TM process,the IMC growth followed a linear law with reflow time at the cold end,indicating a reaction controlled mechanism;while the IMCs grew parabolically with reflow time,indicating a diffusion controlled mechanism.