The Influence of Current Direction on the Cu-Ni Cross-Interaction in Cu/Sn/Ni Diffusion Couples

Abstract

The influence of current direction on the Cu-Ni cross-interaction in the Cu/Sn/Ni joint configuration was investigated in this study. During current stressing, an electric current towards or away from the Ni-side of Cu/Sn/Ni was imposed at 150°C. It was observed that the (Cu,Ni) 6 Sn 5 ternary compound was the dominant reaction product at both interfaces, and its growth at the Ni-side strongly depended upon the direction and magnitude of the electron flow. When the electron flow was towards the Ni-side, more Cu was found to be driven to the Ni-side, resulting in an increase in the thickness of (Cu,Ni) 6 Sn 5 . This is due to the chemical-potential-induced Cu flux (J Cu chem ) that was enhanced by the electromigration (J Cu em ). In the case of electron flow away from the Ni-side, the supply of Cu to the Ni-side was retarded due to the fact that J Cu em was in the opposite direction to J Cu chem : The results of this study revealed that the Ni-side (Cu,Ni) 6 Sn 5 thickness remained almost unchanged under current stressing of 10 4 A/cm 2 at 150°C, which suggests the inward Cu flux is approximately equal to the outward flux, i.e., J Cu chem % J Cu em :

Critical Current Density for Inhibiting (Cu,Ni)6Sn5 Formation on the Ni Side of Cu/Solder/Ni Joints

“…14 We hypothesized that this increase resulted from more Ni being capable of migration to the Cu side during soldering because the distance between Cu and Ni was reduced by half, to 50 lm, in this present study. Additionally, there were (Cu,Ni) 6 Sn 5 compounds uniformly scattered throughout the entire Sn matrix, as shown in Fig.…”

“…Once J em Cu is comparable to J chem Cu , they will cancel each other, resulting in the amount of Cu on the Ni side (N Cu ) reaching a so-called dynamic equilibrium. From our preliminary study, 14 N Cu [or (Cu,Ni) 6 Sn 5 ] was found to remain nearly unchanged over time under 10 4 A/cm 2 stressing at 150°C. This finding suggested that J chem Cu is completely inhibited by the J em Cu induced by 10 4 A/cm 2 current stress.…”

“…1a). This thickness is half of that applied in our previous study, 14 and corresponds to the diameter of microbumps utilized in current C4 packages. More details of the joint fabrication process can be found in Ref.…”

“…8,12 The cross-diffusion of Cu to the opposite Ni is known as an up-hill diffusion process, in which Cu diffuses from a region with lower Cu concentration (C Cu ) (less than 0.1 wt.% for solid solder at 150°C 13 ) to one with higher C Cu [more than 20 wt.% for the (Cu,Ni) 6 Sn 5 phase formed on the Ni side]. [2][3][4][5][6][7][8][9][10][11]14 Such a process can occur within hours at elevated temperature, e.g., 150°C, because Cu is a well-known ''fast diffuser'' in tin-based alloys. 15 The driving force is the chemical potential gradient between the Cu 6 Sn 5 with a lower Ni content (Cu side) and that with a higher Ni content (Ni side).…”

“…15 The driving force is the chemical potential gradient between the Cu 6 Sn 5 with a lower Ni content (Cu side) and that with a higher Ni content (Ni side). 2,7,10,11,14 Applying an electric current to the Cu/solder/Ni joint structure complicates the chemical crossinteraction because Cu and Ni in the conducting media (i.e., the solder matrix) are affected by the electron wind force, and they are accelerated/decelerated according to the current direction and magnitude. 14 In terms of Cu diffusion, the coupling effect of the chemical and electrical forces induces a net Cu flux (J Cu ) of 16 J Cu ¼ J Cu chem þ J Cu em ¼ C D kT À @l @x respectively; C is the Cu concentration in the solder; D is the diffusivity of Cu in solder; k is Boltzmann's constant; T is the temperature in Kelvin; ¶l/ ¶x is the chemical potential gradient; Z * is the effective charge number of electromigration; e is the electron charge; q is the resistivity of the solder; and j is the current density in the conducting medium, i.e., the solder.…”