Wetting Behavior And Interfacial Structure Of Sn-based Solders On Cu₆Sn₅ And Cu Surface

As health and safety awareness caused more and more attention in the field of electronic packaging,the replacement of traditional lead-containing solders by lead-free solders has become an inevitable trend.However,the wettability of most lead-free solders is not so better as the conventional tin-lead eutectic solders,and the quality of the solder joints could not be guaranteed once the brittle phase segregation or the dissolution of substrate occurs at the soldering interface.On the other hand,interfacial metallurgical reaction accompanied with intermetallic compounds（IMCs）precipitation will take place in the most brazing or soldering processes.And the role of IMCs is still a controversial topic:On one hand,the precipitation of the IMCs on the interface removes the oxide film of the substrate surface,which promotes that the substrate could be wetted by solder very well.On the other hand,the brittleness of the IMC layer would decrease the strength of joints.And then,some questions were raised:if the formation of IMC layer is necessary for soldering?How is the wettability of the intermetallic compound itself?This is the main research motivation of this thesis.Firstly,a modified sensible drop method was used for the study on the wetting of solders on surface of Cu and IMCs（pre-formed Sn-Cu intermetallic compounds based on the copper surface）substrates,respectively.By comparing the wettability and spreading dynamics of different tin-based solders on the surface of both kinds of substrates,the wetting mechanism and spreading driving force were analyzed.The main research conclusions were drawn,as following:（1）At 623K-723K,in the Sn/Cu and Sn/IMCs systems,the contact angles depended on the temperature weakly.A thin passivation Au film on the substrate surface can be used as a method to improve the wettability.At the same time,the pre-formed IMC layer also can prevent the further dissolution of Cu,so that it can be used as a method to control the thickness of the interface IMC.For the Sn/Cu system,without the oxide film effect,the precipitation of IMC would never be a driving force for spreading due to the intrinsic wettability of Sn/Cu.In fact,the wetting process of both systems was limited by the reduction reaction between Sn and the surface oxide film rather than the precipitation of IMC at the interface,and thus RPC model was not suitable for predicting the final wettability.The wetting activation energies of the two systems,Sn/Cu and Sn/Sn-Cu IMCs are 22.270 kJ/mol and 20.469 kJ/mol,respectively.（2）At 623K,in the Sn-Bi/Cu and Sn-Bi/Sn-Cu IMCs systems,the chemical composition of the droplet material has a significant effect on the wettability of Sn-Bi/Cu,theorderofwhichfromsuperiortoinferioris:Bi30Sn70>Bi58Sn42>Sn>Bi90Sn10>Bi.The segregation phenomenon of Bi is particularly obvious at the interface of Sn-Bi/Cu,which was resulted from the saturated precipitation from solid solution rather than diffusion from liquid,and the higher the value of Bi:Sn,the more severe the segregation.However,in Sn-Bi/IMCs,the segregation of Bi was rarely found.The pre-formed IMC can also control the thickness of IMC as well as inhibit the segregation of Bi.The interfacial structure of the two systems was in good agreement with the Sn-Bi-Cu ternary phase diagram.The wetting and spreading mechanism are the same both of Sn-Bi/Cu and Sn-Bi/IMCs,the driving force of wetting comes from the reduction reaction[Sn]+2Cu₂O→SnO₂+4[Cu]at the triple line.The difference of reactivity in Sn-Bi/Cu and Sn-Bi/IMCs systems may cause the different spreading characteristics.（3）At 673K,the wetting test of Sn-0.7Cu/Cu,Sn0.3Ag0.7Cu/Cu and Sn-0.7Cu,Sn0.3Ag0.7Cu/IMCs was carried out,and the results were compared with pure Sn solder under the same experimental conditions.The comparative experimental results shows that the addition of trace amounts of Ag and Cu in the pure tin solder can effectively inhibit the formation of interfacial intermetallic compounds and shorten the spreading time of the droplets.The driving force of Sn-0.7Cu/Cu and Sn0.3Ag0.7Cu/Cu system is mainly controlled by the oxide film broken reaction[Sn]+2Cu₂O→SnO₂+4[Cu].In Sn/Cu system,the influence of the dissolution of the substrate at the interface and the diffusion of Cu atoms on the spreading rate cannot be ignored.Sn0.7Cu and Sn0.3Ag0.7Cu spread on the surface of Cu₆Sn₅ very well.Sn-0.7Cu/IMCs,Sn0.3Ag0.7Cu/IMCs and Sn/IMCs have the same wetting mechanism,which was also controlled by the reaction of breaking the oxide film,Ag and Cu elements have little effect on the wetting mechanism.In conclusion,the results of this study are expected to enrich the relevant basic theory of wettability during soleringor brazing,and provide relevant experimental basis for the further development of lead-free solder.