First-principles Study On Interfacial In Term Etallics Between Sn-based Lead-free Solder And Co Or Ni Substrate

In the field of electronic packaging,depositing engineering materials over the surface of printed circuit board has developed as an important technological method to improve the mechanical properties of solder joints.Among various of surface finishing materials,cobalt,nickel and their alloys have been widely researched and used due to their beneficial mechanical properties,well wettability and excellent corrosion resistance.During the production and service of electronic products,the interfacial intermetallic compound(IMC)would be formed due to the reaction between Sn-based solder and substrate or coating.Meanwhile,some alloying element within solder and substrate would diffuse and dissolve into IMC,which leads to the formation of doped phase.In this work,the crystal structure,mechanical and electronic properties of Sn-Co and Sn-Ni IMCs were studied via theoretical and experimental methods.Furthermore,we also investigated the effects of additional alloying element on the properties of those IMCs.CoSn2 and(Co,Ni)Sn2 IMCs were produced owing to the interfacial reaction between the Sn-based solder and Co substrate.The experimental results indicated that the Ni element within Sn-1Ni solder diffused and dissolved into the CoSn2,and its crystal structure of did not alter after Ni dissolution.By means of nano-indentation,it was found that the additional Ni led to the decrement of the mechanical properties of CoSn2,which was agreed to the results of first principles calculations.Furthermore,our theoretical results also demonstated that the additional Ni element led to the more thermodynamic stable phases for CoSn2.Among all studied IMC models,the(Co0.5,Ni0.5)Sn2 IMC was the most stable phase for CoSn2-based structures.Additionally,all studied CoSn2-based IMCs were anisotropic materials.The anisotropy of Ni-free structure was weaker than that of Ni-doped structures.Based on the analyses of electronic structures,it was found that the lose of bonding energies for covalent Co-Co bond was the origin for the degeneration of mechanical properties for CoSn2.The influences of various Ni contents on structural and mechanical properties of?-CoSn3 were researched in this work.The theoretical results showed that the phase stability of ?-(Co1-x,Nix)Sn3(x=0,0.125,0.25,0.375)was enhanced with the increasing of Ni content.The mechanical properties of built structures were calculated by using of a stress-strain approach.It was found that the doping of Ni weakened the mechanical properties of ?-CoSn3 IMC,which was consistent with the nano-indentation tests.Besides,the addition of Ni enhanced the ductility and anisotropy of ?-CoSn3 phase.The analyses of DOS for ?-CoSn3-based structures indicated that the lose of Co-d orbital peak was the main reason for the decreasing of mechanical properties of ?-CoSn3.The Sn/Ni and Sn-0.2Cu/Ni solder joints were prepared by dip soldering.After aging,it was found that the Ni3Sn4 and(N1,Cu)3Sn4 IMCs were formed at the interfaces of reaction couples.The EDX and XRD results demonstated that,similar to the dissolution of Ni in Co-Sn IMCs,the additional Cu only simply replaced the Ni sublattice in Ni3Sn4 crystal structures and thus 1ed to the formation of(Ni,Cu)3Sn4 solid solution.The mechanical properties of Cu-free and Co-doped phases were determined from nano-indentation.It was showed that the mechanical properties of Cu-doped phases were weakened that that of Cu-free phase.Our calculated results showed that the doping of Ni not only weakened the phase stability but also led to the decrement of mechanical properties of Ni3Sn4 IMC.Furthermore,all Ni3Sn4-based IMCs were anisotropic materials.The anisotropy of(Ni2.5,Cu0.5)Sn4(2a site)was strongest while that of(Ni2.5,Cu0.5)Sn4(4i site)was the weakest.Our theoretical results related to electronic structures indicated that the Sn-Ni bonding energy of Ni3Sn4 were weakened due to the additional Cu.On the other hand,after the doping of Cu,the previous Ni-Ni covalent bond of Ni3Sn4 were changed into new Ni-Cu ionic bond with the lower bonding energy.In summary,the addition of Cu weakened the bonding strength of Ni3Sn4,which was the determinant for the decrement of mechanical properties.