Research On Size Effect Of Interfacial Reaction At Sn-xCu/Cu Under Multiple Reflow

With the development of the miniaturization and the integration of electronic products,the size of solder joints have gradually decreased in electronic packaging.In addition,due to the development of the packaging process,micro solder joints often need to be reflowed during the process of package interconnection,which greatly reduces the reliability of solder joints.The size of solder joints,the number of reflow and the composition of solder joints are all factors that cannot be ignored in order to study the reliability of solder joints.In this study,Sn-xCu?x=0,0.7,2.0 wt.%?solder balls with diameter of 200?m,500?m,and 800?m were employed to react with Cu pads and reflowed 1 to 20 times.The holding time was set to 1 min,3 min and 5 min.The soldering temperature was set to 250°C,275°C and 300°C.The evolution of the interface IMC provides reasonable data support for reliability analysis in the actual packaging process.The main research results of the paper are as follows:?1?The study found that Cu₆Sn₅ intermetallic compounds are generated at the interface reaction between the solder balls and the Cu substrate.The Sn-xCu/Cu soldering interface reaction has obvious size effect under multiple reflows.In the Sn/Cu interface,after one reflow,the thickness of the IMC in the brazing interface with different diameters is:500?m>800?m>200?m.In addition,the smaller the diameter of the solder ball,the larger the average grain diameter of the Cu₆Sn₅ grains.In the Sn-0.7Cu/Cu interface,the larger the solder ball size,the thinner the thickness of interface IMC layer,and the smaller the diameter of the solder ball,the larger the average grain diameter of the Cu₆Sn₅ grains.?2?With the increase of reflow numbers,the thickness of IMC layer gradually becomes thicker,the average diameter of the Cu₆Sn₅ grains increases,and the growth rate is gradually reduced.At the initial stage of reflow,the Cu concentration in the solder layer mainly comes from grain boundary diffusion and bulk diffusion.As the number of reflow increases,the IMC becomes thicker,and Cu atom diffusion needs to go through the thicker IMC layer,which lead to the decreases of diffusion rate.However,the ripen-reaction is continues,which lead to the growth rate of the grains along the direction perpendicular to the substrate is smaller than that in the direction parallel to the substrate.In addition,multiple reflow have little effect on IMC growth which produced by large solder balls.?3?The addition of Cu does not have a significant effect on the thickness of the IMC,but it promote the ripen-reaction of the grains result in the increase of the Cu₆Sn₅ grain size.?4?With the increase of holding time and soldering temperature,the thickness of interface IMC and the average diameter of interfacial Cu₆Sn₅ are increased.At different holding time and soldering temperature,the thickness of the IMC in the brazing interface with different diameters is:500?m>800?m>200?m.The smaller the solder ball diameter,the larger the average grain diameter of Cu₆Sn₅.In the initial stage of soldering,the Cu concentration gradient in the small-sized solder layer is small,so that the grain size is large,the number of grain boundaries is less,which suppressed the diffusion of Cu into the solder layer.The Cu atoms in the substrate dissolve into the IMC layer,which promotes the ripen-reaction of Cu₆Sn₅grains.Therefore the small-sized solder balls have a thinner IMC layer thickness and a larger grain diameter.In addition,the increase of the interface IMC thickness is not only related to the diffusion flux of Cu atoms,but also related to the amount of Sn atoms in the solder.With the increase of soldering temperature,the time for cooling to room temperature becomes longer.Due to the sufficient amount of Sn atoms in the large-sized solder balls,the interface reaction in the cooling stage is sufficient,so the IMC at the interface of the large-sized solder balls grow thicker.