Effect Of Ce On Properties And Growth Behaviour Of Cu₆Sn₅ Layer Of Sn-3.8Ag-0.7Cu

The process of soldering is ubiquitously employed during manufacturing electronics devices in modern life.Traditional Sn-37Pb solder is well known as an excellent solder in the electronic packaging due to its excellent soldering capability.However,with lead classified as hazardous to health and environment,the majority of solder alloys have changed composition to avoid the presence of lead.As a result,many studies have been done to develop lead-free solder alloys with capabilities comparable or better than lead solder alloy.One of such promising alloys is Sn-Ag-Cu alloy.This work investigates the microstructure,microhardness,wettability,shear strength and Cu6Sn5 layer growth behaviour of Sn-3.8Ag-0.7Cu and Sn-3.8Ag-0.7Cu-0.2Ce lead-free solders compared with traditional Sn-37Pb.The main contents and conclusions are summarized as follows:With 0.2wt%Ce addition,microstructure including β-Sn,Cu6Sn5 and Ag3Sn was remarkably refined.It is known that the standard Gibbs free energy of formation of Sn-RE IMCs is lower than those for Ag-RE and Cu-RE.Accordingly,in this work,Ce has a higher affinity for Sn,and this explains the refinement of P-Sn.Addition of Ce could reduce the nucleation energy and facilitate a high nucleation density of Ag3Sn and Cu6Sn5 in the eutectic of Sn-3.8Ag-0.7Cu-0.2Ce.The microhardness of Sn-3.8Ag-0.7Cu-0.2Ce is 22.2 HV which is slightly decreased by 1.0 HV compared with Sn-3.8Ag-0.7Cu.Because the atom radius of Ce is bigger and bond energy of CeSn3 is lower.However,the microhardness is still higher than traditional Sn-37Pb.The wettability of Sn-3.8Ag-0.7Cu-0.2Ce was enhanced by 11%compared with Sn-3.8Ag-0.7Cu.Ce is an active element.With 0.2wt%Ce addition,the surface tension of liquid solder and flux is reduced.Thus,the spreading area was increased and the contact angle was decreased.Nevertheless,the wettability is still poorer than Sn-37Pb.With 0.2wt%Ce addition,the shear strength of solder joint fabricated by Sn-3.8Ag-0.7Cu-0.2Ce was strengthened by 10%compared with Sn-3.8Ag-0.7Cu which is very close to the traditional Pb-37Pb.This enhancement is due to,firstly,the refinement of microstructure;secondly,the grain boundary could be pinned by CeSn3 and imped the movement of grain boundary.The Cu6Sn5 layer,which is integral,formed during soldering is essential for the fabricating of a functional and reliable solder joint.It is important to have a clear understanding on the growth and ripening of intermetallic compounds layer which plays a vital role in determination of properties of lead-free solders.With 0.2wt%Ce addition,the mean thickness of Cu6Sn5 layer formed on the interface of Sn-3.8Ag-0.7Cu-0.2Ce solder and Cu substrate was decreased and the mean radius of Cu6Sn5 grain was increased.So the mean thickness-to-radius ratio of Cu6Sn5 was decreased.This phenomenon maybe due to that Ce could retard the diffusion of Cu atoms into liquid solder deeply.The shear strength relates to mean radius of Cu6Sn5,the shear area is enlarged by the increasing of the mean radius,which leads to a higher shear force F.This is another reason for the enhancement of shear strength of Sn-3.8Ag-0.7Cu-0.2Ce.