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 :

The Cu/solder/Ni structure is a common joint configuration used in microelectronic packages today. In high-temperature operation of this joint structure, an appreciable amount of Cu can readily diffuse across the entire solder to the Ni side, where it might grow into a brittle bilayer structure of (Cu,Ni) 6 Sn 5 /(Ni,Cu) 3 Sn 4 . The driving force for Cu diffusion is the chemical potential gradient. To counterbalance this chemical-force-induced Cu flux (J chem Cu ), an attempt to apply a reverse electric current into a Cu/Sn(50 lm)/Ni structure was made in this study. Ten current densities (j = 0 A/cm 2 to 2 9 10 4 A/cm 2 ) were examined at 150°C upon current stressing. The results indicated that, under current stressing of <10 4 A/cm 2 , Cu atoms were still driven to the Ni side, resulting in noticeable increase in the amount of Cu (N Cu ) or (Cu,Ni) 6 Sn 5 at the Sn/Ni interface. In contrast, N Cu decreased significantly, and the (Cu,Ni) 6 Sn 5 was further converted into another lowCu-content phase, (Ni,Cu) 3 Sn 4 , when j exceeded 1.25 9 10 4 A/cm 2 . Under $10 4 A/cm 2 , the (Cu,Ni) 6 Sn 5 thickness and N Cu remained relatively unchanged over time, suggesting that 10 4 A/cm 2 is close to the critical current density (j crit ) that can counterbalance J chem Cu . Growth of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 under the conditions (I) j < j crit , (II) j > j crit , and (III) j % j crit were also examined in this study.

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