Study On Liquid-Solid Interfacial Reactions In Cu/Sn/Cu-xZn Mircro Solder Joints

In electronic packaging,Sn-based solder and Cu pads undergo a soldering interfacial reaction to generate intermetallic compounds(IMCs)to form micro solder joints.In order to meet the development needs of miniaturization and integration of electronic devices,the size of micro solder joints is gradually reduced,which will increase the proportion of interfacial IMC.Since Cu-Sn type IMCs are brittle phases,an excessively thick IMC layer will reduce the mechanical properties and reliability of the micro solder joints.Therefore,it is necessary to effectively control the growth of interfacial IMC.Because the liquid-solid interfacial reaction for forming micro solder joints occur during reflow process,the diffusion rate of metal atoms in the liquid solder is fast.Meanwhile,temperature gradient(TG)across micro solder joints is easy generated during the reflow process,which makes the metal atoms migrate oriented.These all will significantly affect the growth behavior of IMC.It has been reported that adding Zn to Cu substrate can inhibit the growth of IMC.This study focused on the liquid-solid interfacial reactions of Cu/Sn/Cu-xZn(x=0,5,10,20)micro solder joints under isothermal reflow at 260 ^? and reflow at a TG of 200 ^?/cm for different time.The main conclusions are as follows:(1)After immersion soldering at 260 ^? for 15 s,only a layer of scallop-shaped IMC was formed on both sides of the Cu/Sn/Cu-xZn micro solder joints.The IMC at the interfaces of the Cu/Sn/Cu micro solder joint was Cu₆Sn₅;while that at the interfaces of the Cu/Sn/Cu-xZn(x=5,10,20)micro solder joints was Cu₆(Sn,Zn)₅in which Zn atoms occupied part of the sublattice positions of Sn atoms in Cu₆Sn₅.(2)During isothermal reflow at 260 ^?,the IMCs on both sides of the Cu/Sn/Cu-xZn micro solder joints with the same composition grew symmetrically.That is,the IMCs on both sides in same micro solder joint had similar thickness and morphology.With the increase of Zn content in the micro solder joints,the IMC thickness at both interfaces decreased slightly,indicating that the addition of Zn into the Cu substrate did not significantly inhibit the growth of the interfacial IMC during isothermal reflow.This is because that Cu₆(Sn,Zn)₅had a large solubility of Zn under the reflow temperature,so that it was hard to form an effective Zn-rich phase layer at the interfaces as a diffusion barrier for the diffusion of Cu and Zn atoms.(3)When reflowed under a TG of 200 ^?/cm,the IMCs on the cold and hot sides of the Cu/Sn/Cu-xZn micro solder joints exhibited obvious asymmetric growth.That is,the cold end interfacial IMC grew rapidly,while the hot end interfacial IMC grews slowly and the hot end Cu-xZn substrates was dissolved seriously.The thickness of the cold end interfacial IMC of the Zn-containing micro solder joints was reduced,indicating that the addition of Zn had a significant inhibitory effect on the growth of the cold end interfacial IMC.This is because that the addition of Zn reduced the dissolution of the substrates,thereby reducing the atomic flux that diffused from the hot end to the cold end for the cold end interfacial reaction under thermal migration.In addition,it was found that the Cu(Sn,Zn)layer formed at the hot end interface of the Cu/Sn/Cu-20Zn micro solder joint more effectively inhibited the dissolution of the hot end substrate,so that the IMC growth rate at the cold end interface of the micro solder joints was also sharply reduced.(4)Based on the growth law of interfacial IMCs with reflow time,the growth kinetics of IMCs on both sides of the Cu/Sn/Cu-xZn micro solder joints during isothermal reflow and under TG were obtained.