Study On The Preparation And Reliability Of Fine-Pitch Microbumps

As the core technology in the three-dimensional package,the copper pillar bumps,due to their excellent electrical and mechanical properties,can prevent short circuits issues and achieve high-precision interconnection.Nowadays,as bump size shrinks to a few microns,its limited solder volume can be easily depleted by the growth of intermetallic compounds?IMCs?and voids,which make interconnection reliability issues precarious.In this paper,microbumps of?6-20?m in diameter were prepared by electrodeposition.The growth of interfacial IMC and voids,with the shear strength change of IMC during reflow and aging process,were investigated.The influences of bump diameter,side wall wetting reaction,Ni barrier layer and Ag on interfacial reaction were discussed.The main conclusions of this paper were as follows:By comparing the influence of different additives in CuSO₄ bath,the optimum ratio of accelerator to leveler was 5:7.Four kinds of Sn-3.5Ag/Ni/Cu bumps with different diameters?6,8,9 and 11?m?were prepared.The growth rate of interfacial IMC after reflow increased from 0.45 to 0.58?m/min as bump diameters decreased from 11?m to 6?m.IMC could be divided into two layers:the?Cu,Ni?₆Sn₅ and Ni₃Sn₄ layer.Due to the migration of Ni atoms,many voids were left between two IMC layers.In the edge of Cu pillar,side wall wetting reaction was detected,which caused the formation of voids and?Cu,Ni?₆Sn₅ phase.The sidewall wetting reaction provided an additional diffusion channel for Cu and Ni atoms.As the bump diameters decreased,the increase of sidewall diffusion induced the acceleration of interfacial IMC growth and increase of side wall IMC length/diameter ratio.The presence of Ni layer could effectively inhibit the growth of interfacial IMC.During aging process for 100h,the IMC of?6?m Sn/Cu microbumps were Cu₆Sn₅ and Cu₃Sn.In Sn/0.4?m Ni/Cu bumps,?Cu,Ni?₆Sn₅ were formed initially and then turned into Cu₆Sn₅ and Cu₃Sn after Ni layer was depleted.In Sn/1.12?m Ni/Cu bumps,the initial IMCs were Ni₃Sn₂ and Ni₃Sn₄,and then turned into?Cu,Ni?₆Sn₅ and Ni₃Sn₄.The growth rate of Sn/1.12?m Ni/Cu was the slowest one,whose reaction constant was 1.71×10^-17 m²/s,due to the barrier effect of thick Ni layer.The IMC growth in Sn/Cu could be divided into two stages.In Sn/0.4?m Ni/Cu,the formation of?Cu,Ni?₆Sn₅ slowed down the growth of IMC.The presence of Ag₃Sn inhibited the growth of IMC and affected the shear strength of Cu₆Sn₅.During aging process for 960h,in?20?m Sn/Cu bump,the growth of IMC thickness was proportional to the square root of time,and the reaction coefficient was 2.55×10^-17 m²/s,which was higher than 1.19×10^-17m²/s for Sn-3.5Ag/Cu bump.It was because that Ag₃Sn grain in Cu₆Sn₅ grain boundary inhibited the grain boundary diffusion of Cu,and thereby inhibited the growth of interfacial IMC.In the shear strength test of IMC,with the aging process,the fracture surfaces of Cu₆Sn₅ and Cu₃Sn changed from ductile to brittle one.The shear strength results showed that for Sn/Cu bump,the shear strength of Cu₆Sn₅ rose and then stabilized within 105 MPa,but the one of Cu₃Sn increased gradually.For Sn-3.5Ag/Cu bump,affected by the pinning effect of Ag₃Sn and the growth of Cu₆Sn₅,the shear strength of Cu₆Sn₅decreased and then rose to 130.2 MPa,while the one of unaffected Cu₃Sn rose with the aging time.