Research On The Liquid Structure Of Lead-free Solders And The Interfacial Reaction During Soldering And Their Correlation

At present,the research of lead-free solder alloys has been widely concerned around the world,and certain progress and achievement have been reached.Among the current solder alloy systems,Sn-Cu,Sn-Ag and Sn-Ag-Cu become the focus of scientific research and commercial development.In the application of lead-free solders,the interfacial reaction is one of the key issues that affect the quality of soldering and the reliability of the joint.During soldering,the solder must experience from the solid state to the liquid state then again to the solid state,while during the reaction stage the solder is always at the liquid state.Thus,to study the liquid structure of the solder alloys,to discover the correlations between the liquid structure and the interfacial reaction during soldering,will have a significant effect on understanding the growth mechanism of interracial intermetallic compound(IMC) and controlling the growth rate of IMC.In this dissertation,two kinds of lead-free solders,namely Sn-xCu(x=0.7,1.5,2) and Sn-3.5Ag-xCu(x=0,0.7,1.5),were chosen.First,the diffraction data of the solders were obtained by using a high temperature X-ray diffractometer,and then the liquid structure of the solders was calculated and analyzed.Only short rang order(SRO) structures were detected in liquid Sn-0.7Cu and Sn-1.5Cu solders,while there existed not only SRO structures but also medium rang order(MRO) structures in liquid Sn-2Cu solder at 260℃and 330℃.The MRO structures in Sn-2Cu melt were related to Cu₆Sn₅ ordered clusters.Also in Sn-3.5Ag melt only SRO structures were found.While MRO structures were detected in Sn-3.5Ag-0.7Cu melt at 260℃and 330℃besides the SRO.Such MRO structures were still related to Cu₆Sn₅ clusters indicating the size and amount of the ordered structures were increased with the addition of Cu.However with increasing temperature the before-mentioned Cu₆Sn₅ type correlative clusters were destroyed.As a result,the MRO structures disappeared.According to the previous results,it is clear that at the soldering temperature large ordered clusters even MRO structures will exist in lead-free solder melt,and the size or the amount will be changed with the variation of the alloying element in the solder,which can accordingly affect the soldering reaction.Secondly,the viscosity of Sn-xCu and Sn-3.5Ag-xCu solders was measured using a torsional oscillation viscometer.For all the melts the viscosity decreased with increasing temperature,however,there was obvious discontinuity.So two segments,namely a low temperature zone and a high temperature zone,were found correspondingly.The activation energyεand the unit volume of flow Vm were obtained according to Arrhenius equation.It was found that the anomalous variations of viscosity had a direct relation with the transition of the liquid structure,which is in good agreement with the results of high temperature X-ray diffraction.The difference in v_m of the solder melts will consequentially result in the different rate of interfacial reaction.At the same time,the surface tension calculated using the measured viscosity values was close to the reported value,and the results were also checked by performing the wetting test between the solders and the Cu substrates.This confirms the present method is quite reasonable to obtain surface tension values for solder alloys.Both viscosity and surface tension are closely related to the liquid structure of the solder alloy.Then the interfacial reactions of Sn-xCu/Cu(Ni) and Sn-3.5Ag-xCu/Cu joints during soldering were studied.When solders were reflowed with Cu substrates,the ripening process and the growth were accelerated by increasing Cu content.This was concerned with the increment of size and amount of Cu₆Sn₅ clusters in liquid solders.For Sn-xCu/Ni joints,the interfacial reaction product translated from(Ni_xCu_1-x)₃Sn₄ to(Cu_xNi_1-x)₆Sn₅ with increasing of Cu,and the growth rate of(Cu_xNi_1-x)₆Sn₅ IMC layer was controlled by the volume fraction of Cu₆Sn₅.Finally,the interfacial reactions between the lead-free solder containing elements and the metallization layers(Cu and Ni) are very complex,and the kind and the content of alloying elements have a great effect on the interfacial reaction.Furthermore,as the size of solder joint becomes smaller and smaller with the miniaturization of electronic products,the original composition of the solder joint can be changed easily by the introduction of extraneous elements,for example through the dissolution and the diffusion of the metallic layer,which leads to the interfacial reaction more complex and sometime even hard to control.Therefore from the viewpoint of liquid structure the effect of alloying elements on the type,morphology and growth behavior of the interfacial IMC was analyzed and predicted in this dissertation.