Research On Interfacial Reaction Of Sn-35Bi-1Ag Lead-free Solder Joints

With the development of micro-interconnect solder joints and lead-free interconnect materials in integrated circuits,microelectronics industry is paying more and more attention to the reliability of interconnect solder joints.The critical factor affecting the reliability of interconnect solder joints is the interface reaction,so it is of great significance to conduct in-depth research on the interfacial reaction of lead-free solder.Sn-Bi lead-free solders are considered to be ideal substitutes for traditional Sn-Pb solders and have been used in industry because of their low melting point and good wettability.In this paper,the Sn-35Bi-1Ag lead-free solder with the closest melting point to Sn-37Pb is used as the research carrier.The Cu and Ni-P/Cu,which are widely used in the electronic industry,are selected as the pad metal layer.The interfacial reactions of two typical Sn-35Bi-1Ag lead-free solder joints are studied.According to the defects formed during the service process of solder joints,a solution to replace the traditional Ni-P diffusion barrier with Ni-Co-P modified layer was proposed,and the influence of Ni-Co-P plating on the interface reaction of solder joints was explored.Related studies are as follows:?1?The microstructure evolution of solder joints in Sn-35Bi-1Ag/Cu,growth behavior of interface IMC and grain coarsening are studied.The results show that the interface after reflow is scallop-like Cu₆Sn₅ layer,and the thickness of Cu₆Sn₅ layer continuously increases during aging.A very thin Cu₃Sn layer appears at the Cu₆Sn₅/Cu interface,and the final interface consists of a thicker layer of Cu₆Sn₅ and a thinner Cu₃n layer.In process of Cu₃n growth,Bi segregations and Kirkendall voids appear at the interface,which weakens the reliability of solder joints.The growth kinetics of the IMC layer showed that the growth rate constants of IMC at the aging temperatures of 130°C and 150°C were 0.0993 um/h^0.5 and 0.1413 um/h^0.5,respectively.It can be seen that the growth rate of IMC varies with the service temperature of the solder joints.When the aging temperature is 150°C,the growth indices of Cu₆Sn₅ and Ag₃Sn grains are 0.2388 and 0.3370,respectively.It can be seen that the grain coarsening behavior depends on the grain properties.?2?The interface reaction of Sn-35Bi-1Ag/Ni-P/Cu solder joint was studied.The influence of the diffusion barrier thickness on the interface reaction was studied by preparing three Ni-P coatings with thicknesses of 1,5,and 10 um,respectively.The results show that after reflow and aging,the final product of the thin-film solder joint interface is?Cu,Ni?₆Sn₅+Ni₂SnP+?Cu,Ni?₆Sn₅,and the final product of the middle and thick plated solder joint interfaces is?Ni,Cu?₃Sn4+Ni₂SnP+Ni₃P.During the aging process,the amorphous Ni-P coating gradually transforms into a Ni3P crystallized layer due to consumption.With the thickness of the Ni₃P layer increasing,a large number of voids or micro-cracks occur inside.Once the Ni-P layer is completely converted to Ni₃P,a more stable Ni₂SnP compound will gradually form as Ni₃P continuously consumed.The growth kinetics results showed that the growth rate constants of the IMC layer at the interface with thin,medium,and thick coating layers were 0.4040 um/h^0.5,0.1137 um/h^0.5,and 0.4399 um/h^0.5,respectively.?3?The interface reaction of Sn-35Bi-1Ag/Ni-Co-P/Cu solder joint was studied.The microstructure of the interface and the growth behavior of IMC during the service of the solder joint were studied,and the results were compared with the Cu,Ni-P solder joints in the same conditions.The results show that the interfacial IMC of Sn-35Bi-1Ag/Ni-Co-P/Cu solder joints at the aging stage is always?Ni,Co?₃Sn₄,and no solid solution of Cu is found.With the thickness of?Ni,Co?₃Sn4 increasing,?Ni,Co?₃P is generated due to continuous consumption of Ni-Co-P coating,but the coating always maintains good resistance to diffusion,and there are no voids,micro-cracks,Bi segregation in the interface layer.Further growth kinetics results shows that the growth rate constant of the IMC layer at the interface of Ni-Co-P solder joints at 150°C is 0.09843 um/h^0.5,which was lower than that of Cu and Ni-P interface IMC layers under the same conditions.Co element can effectively slow down the crystallization behavior of Ni-P coating during welding and service,thus improving the diffusion barrier effect of Ni-Co-P coating and inhibiting the growth of IMC in solder joints.