Cross-Interaction between Ni and Cu across Sn Layers with Different Thickness

Abstract

The Ni/solder/Cu material sequence is one of the most common material sequences in the solder joints of electronic packages. In this study, the Ni/Sn/ Cu ternary diffusion couples were used to investigate the solder volume effect on the cross-interaction between Ni and Cu. Experimentally, a pure Sn layer with the thickness of 100-400 lm was electroplated over Cu foils. A pure Ni layer (20 lm) was then deposited over the as-deposited Sn surface. The diffusion couples were aged at 160°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the cross-interaction could occur in as short as 30 min. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 25-40 times higher than the Ni flux. Moreover, it was found that (Cu 1-x Ni x ) 6 Sn 5 on the Ni side reduced the consumption rate of the Ni layer, and the cross-interaction also reduced the Cu 3 Sn thickness on the Cu side.

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

“…One of these reliability concerns results from the so-called crossinteraction between Cu and Ni. [1][2][3][4][5][6][7][8][9][10][11] It was recently reported that both Cu and Ni atoms can diffuse across the entire solder (hundreds of microns) to opposite sides of the sandwich structure in a very short time (a few seconds) during soldering. [1][2][3][4][5][6][7][8] The diffusion of Cu to the Ni-side can form a single layer of (Cu,Ni) 6 Sn 5 or a bilayer of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 over the Ni pad, 1-8 resulting in a brittle solder/pad interface.…”

“…7,[14][15][16][17][18] The Cu-Ni interaction continued but at a relatively low rate during subsequent solid-state annealing. 2,[7][8][9][10][11] It is clear that cross-interaction refers to an up-hill diffusion process wherein both Cu and Ni diffuse from regions of low concentration to those of high concentration. The driving force for the process is the chemical potential gradient or the chemical force.…”

Effects of Electromigration on Interfacial Reactions in the Ni/Sn-Zn/Cu Solder Interconnect

“…The interfacial reactions in the Ni/Snbased solder/Cu combination after annealing were reported and attributed to the difference in the Cu or Ni concentrations between the Ni and the Cu pads. [1][2][3][4][5][6] Under such conditions, the interfacial reactions were greatly accelerated and the thermal stability of the Cu UBM degraded due to the formation of (Cu,Ni) 6 Sn 5 intermetallic compounds (IMCs). 1,2,5,6 Electromigration behavior of the Ni/Sn-based solder/Cu combination has also been studied.…”

Advances in Pb-free Solder Microstructure Control and Interconnect Design

“…[33] These results agreed well with research that has been conducted in the area of Ni-doped Cu- 6 Sn 5 by a number of different research groups. [34][35][36][37][38][39][40] The research has documented the strong cross-interaction of Cu and Ni in solder joints. Typically, Ni is observed to substitute on the Cu 6 Sn 5 IMC lattice for Cu atoms, thus creating (Cu, Ni) 6 Sn 5 , which enhances the impact resistance of the solder IMC interfacial layer by stabilizing the hexagonal form of Cu 6 Sn 5 (g) down to room temperature, negating its transition to the monoclinic structure (g¢) at 186°C.…”

“…[37][38][39][40] Ni additions were also shown to enhance the planarization of solder IMC (Cu 3 Sn and (Cu, Ni) 6 Sn 5 ) interfacial layers, and the cross-interaction of Cu and Ni has been shown to reduce the thickness of the Cu 3 Sn IMC layer during thermal aging. [33,35] …”