Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

Abstract

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 lm)/Sn(65 lm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu 1-x Ni x ) 6 Sn 5 layer on the Ni side and the (Cu 1-y Ni y ) 6 Sn 5 layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu 1-x Ni x ) 6 Sn 5 layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu 1-y Ni y ) 6 Sn 5 layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu 3 Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.

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

“…The growth of (Cu,Ni) 6 Sn 5 at temperatures ranging from 150°C to 200°C follows approximately parabolic kinetics. 10,11 Upon current stressing, the Cu flux across the Sn (J Cu ) is determined by both the diffusion term due to the chemical potential gradient (J Cu chem ) and the drift term due to the electron momentum transfer effect (J Cu em ). It is clear that the direction and magnitude of the electric current play dominant roles in J Cu em ; thus affecting the growth of Cu-containing IMCs at the Ni-side.…”

“…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.…”

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

“…During joint structure fabrication or subsequent operation, an appreciable amount of Cu can diffuse through the entire solder to the Ni side, [1][2][3][4][5][6][7][8][9][10][11] where it nucleates as a very brittle (Cu,Ni) 6 Sn 5 /(Ni,Cu) 3 Sn 4 bilayer structure. 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].…”