Research On Interfacial Reactions Between Sn And Electrodeposited Nanocrystalline Copper

Soldering is a key process in the industrial manufacture of electronic devices.As the size of the micro-connection continues to decrease,the volume fraction of intermetallic compound?IMC?layer formed at the substrate/solder interface increases significantly.For the thermodynamic reasons,the interfacial IMCs continue to grow during storage and exploitation of the manufactured electronic devices.In order to inhibit the growth of IMC,nano-plating was used as pad to form the Cu/Sn joint which interfacial behaviors were investigated.After electrodeposition,the plating was characterized by the scanning electron microscopy?SEM?and X-ray diffractometry?XRD?to study the influence of the plating parameters on the coating.Nano-Cu and HP-Cu were reflowed for several times before their IMC being characterized.The interfaces reaction of Sn/Nano-Cu and Sn/HP-Cu and its kinetics ware investigated.Different layers of Cu₆Sn₅ intermetallic compound layer was soldered as pads after Pre-tinned?PT?,and the influence of Sn/Cu₆Sn₅/Cu was studied.Electroplating results show that:Phosphor copper anode plate plating is beneficial to improve the flatness and uniformity of the coating,while failing to refine the grain at low current density?3ASD?.Pulse plating usually can refine the grains and overcome the defects of DC plating like plating instability at high current density?10ASD?.However,the plating morphology is rough coated at low current density.The plating grains grows into faceted geometry when the current density is low?<3ASD?,and becomes unstable when too high?12ASD?.The coating electroplated at10ASD is uniform and its grains are fine,with the average grain size of 45.8nm measured by XRD,meeting the test requirements.The result of soldering of Sn/Nano-Cu and Sn/HP-Cu and aging test shows that:in the liquid phase reaction of Nano-Cu/Sn and HP-Cu/Sn,the thickness of IMCs increased with the increase of the number of reflow,while the growth rate slowing down.The IMCs formed on the Nano-Cu/Sn interface was always thicker than that of HP-Cu/Sn interface compounds.After reflow 5 times,a large number of cracks appear in the IMC grains of the Nano-Cu/Sn interface.During the solid-phase reaction,the Cu₃Sn phase at the Nano-Cu/Sn interface is thinner than that at the HP-Cu/Sn interface.With the extension of thermal aging time,the Cu₃Sn phases are connected into a whole layer,and the cracks in the Cu/Cu₃Sn interface of Nano-Cu/Sn joints disappear gradually,the number of defects largely reduced,which means self-healing occurs.After reflow for 2⁴ times,the newly-added IMC at the Nano-Cu/Sn interface after aging was always thinner than that at the HP-Cu/Sn interface.Nano-Cu/Sn joints effectively inhibited IMCs growth during thermal aging process.During liquid phase reaction of soldering of PT-Nano-Cu/Sn and PT-HP-Cu,the new IMC grains remain relatively small size.After PT for sereval times,the morphology of IMC at the PT-Nano-Cu/Sn interface is closer to that at the PT-HP-Cu/Sn interface,showing a scalloped shape.PT treatment significantly inhibited the growth of IMCs of Nano-Cu/Sn interface.The thickness of IMCs was only half that of non-PT Nano-Cu/Sn after reflow 5 times.The prefabricated Cu₆Sn₅ layer has the most significant inhibitory effect on the liquid phase reaction after PT for twice.During solid-state reaction of PT-Cu/Sn,Kirkendall voids disappear gradually and the number of cracks in IMCs decreases with the increase of aging time.PT treatment did not inhibit the growth of PT-Cu/Sn interfacial compounds,but significantly inhibited the growth of Cu₆Sn₅ phase.The Cu₆Sn₅ phase of PT-Nano-Cu/Sn interface had better inhibitory effect on the growth of Cu₆Sn₅than PT-HP-Cu/Sn after PT for 1² times.However,when the aging time reaches 72h,PT and non-PT no longer affect the thickness of IMCs.The thickness of IMCs is only related to the heat aging temperature and time.