Effect Of Zn Addition In Cu Substrate On Interfacial Reactions During Soldering

During a soldering process for electronic packaging,an intermetallic compound?IMC?layer will form at the interface between molten Sn-based solders and Cu substrate to achieve interconnection in electronic devices.However,thick IMC layer is usually obtained at the interface of a solder joint after high temperature reflow or during long term aging in service.The thicker interfacial IMC means the poorer mechanical properties of the solder joints the natural brittleness of IMC.Adding a small amount of alloying elements to the system provides a reliable solution to improve interface structure and mechanical properties of the solder joints,and Zn is one of the commonly used alloying elements.In recent years,many researches have been carried out on the interfacial reactions between Zn-containing solders and Cu or Ni substrates at home and abroad,but few of them focus on the interfacial reactions between Cu-Zn substrates and Sn-based solders during a soldering process.Eutectic Sn-0.7Cu solder is mainly used in board-level packaging technology.Therefore,choosing Sn/Cu-xZn and Sn-0.7Cu/Cu-xZn system as solder joints,conducting a comparative study,focusing on wetting behavior and the evolution of IMC type,thickness and morphology at the solder joint interfaces during reflow,will have practical significance for evaluating the application prospect of Cu-Zn substrates.To this end,this research studied the Sn/Cu-xZn and Sn-0.7Cu/Cu-xZn?x=0,5,10,20,30 wt.%?solder joints from the points of wetting and spread behavior,growth behavior of interfacial IMCs and evolution of grain morphology after reflowed at 260°C.The main conclusions are drawn as follows:?1?The Zn element in the Cu-xZn substrates has certain adverse effect on the wettability.On the same composition Cu-Zn substrate,the Sn-0.7Cu solder exhibits better wettability than the pure Sn solder.From the standpoint of wettability,the amount of Zn addition to the Cu substrate should not exceed 5 wt.%.?2?Scallop-like Cu₆Sn₅ and laminar Cu₃Sn IMCs are formed at the interface of the Sn/Cu solder joints after isothermal reflowing at 260°C.The Zn element in the Cu-Zn substrates can effectively inhibit the formation of Cu₃Sn and the growth of Cu₆Sn₅-type IMC.When the Zn concentration in the Cu-Zn substrate reaches 30%,a laminar Cu?Zn,Sn?phase is formed at the interface between the Cu₆Sn₅-type IMC and the Cu-30Zn substrate.The Cu?Zn,Sn?phase will overgrow after extending isothermal reflow.?3?Scalloped-like Cu₆Sn₅ and thin Cu₃Sn are found at the interface of Sn-0.7Cu/Cu solder joints after isothermal reflowing at 260°C.As the reflowing time increases,the many Cu₆Sn₅ scallops gradually grow into long hexagonal prisms.The Zn element in the Cu-Zn substrates can inhibit the formation of Cu₃Sn and the growth of Cu₆Sn₅ in the form of long hexagonal prisms.Compared with the Sn/Cu solder joints,the interfacial Cu₆Sn₅ is thicker in the Sn-0.7Cu/Cu solder joints,but the Cu₃Sn is thinner.The thickness of the interfacial IMCs in the Sn-0.7Cu/Cu-xZn solder joints is thicker than that in Sn-0.7Cu/Cu solder joints.?4?In the Sn/Cu-xZn solder joints,the Zn element in the Cu-xZn substrates has a strong effect on refining interfacial IMC grains.In addition,when the Zn concentration in the Cu-xZn substrates is lower than or equal to 20%,the interfacial Cu₆?Sn,Zn?₅ grains show a cobblestone-like morphology;when the Zn concentration reaches 30%,many interfacial Cu₆?Sn,Zn?₅ grains become short hexagonal prisms.In the Sn-0.7Cu/Cu-xZn solder joints,the grain radius of the interfacial IMC in the Sn-0.7Cu/Cu-30Zn solder joint is higher than that in the other solder joints.