Ni기 및 Co기 3원계 전기도금 확산방지층의 열적 안정성과 솔더링 특성 연구

Abstract

Electro-deposited nickel- and cobalt-based ternary alloys were investigated systematically to clarify the effect of refractory metal and phosphorus in the diffusion behavior of copper and the soldering reliability with Pb-free solders. Nickel- and cobalt-based ternary alloys were electro-deposited on copper substrate. The influence of different factors on the chemical composition, surface morphology and microstructure were examined. It is well known that tungsten cannot be electro-deposited from an aqueous solution of sodium tungstate (Na2WO4) or any other soluble compound containing this element. However, if a suitable nickel compound is added to the plating bath, "induced codeposition" can take place. A similar phenomenon is observed during electrodeposition of W, Mo or Re with Ni, Co or Fe. The effect of thermal treatment on these alloys characteristics was investigated. Crystallization temperature for the deposits increases with increasing refractory heavy metal content of the deposits, which improves thermal stability of binary alloy films. The amorphous nickel-based ternary films is more stable against heat treatment than the binary films, because the formation of phosphide compound (such as Ni3P) is suppressed by the codeposition of refractory atoms. The amorphous nickel-based ternary films are more effective diffusion barrier than the binary films, because the nickel-based ternary films maintain amorphous with increasing annealing temperature, which eliminates the short-circuit paths of the grain boundaries and improves the barrier properties. Unlike Ni-W-P coatings, it was shown that lowering the tungsten concentration improved diffusion barrier properties in Co-W-P coatings. Because the solubility of W and P in Co is considerably lower than their concentration in the films, it is evident that the diffusion barrier property of electro-deposited Co-W-P films was deteriorated by the codeposition of a large amount of W and P. The thermal soldering reliability of the electro-deposited Co-W-P coatings for different aging times were examined. The electroless Ni-W-P ternary coatings were prepared to compare the soldering properties of the cobalt-based ternary coatings to those of the nickel-based ternary coatings. Also the soldering properties of crystalline was investigated in addition to amorphous. After reflow, all samples of Ni-W-P have similar interfacial microstructure and chemistry, consisting of coarse chunky-shape Cu rich ((Cu,Ni)6Sn5) and dense needle-shape Ni rich ((Ni,Cu)3Sn4) intermetallic compound formed at the Sn-3.0Ag-0.5Cu/barrier layer interface during the reflow. After annealing at 200℃, the thickness of IMC of all samples of Ni-W-P coatings increased as the aging time increased. And IMC grew thicker with increasing W content. The remaining diffusion barrier layer thickness was reduced with increasing W content. This result indicates that the presence of W affects the formation and suppression of Ni(W)3P phase, which prevents the diffusion of Ni from Ni layer. Therefore, it maybe concluded that Ni-P containing thicker crystallized Ni3P phase hinders the growth of IMC layer. Also the diffused Sn atoms come in contact with the remaining Ni-W-P layer and then Cu pads at a faster rate through this barrier and reacts to form Cu-Sn phase. This Cu-Sn phase in the solder joint may induce brittle fracture at an interface, and have detrimental effects on joint reliability. Unlike nickel, only (Cu,Co)6Sn5 was detected in IMC layer for electro-deposited Co-W-P coatings. And cobalt phosphide compounds was not detected at the interface between Co alloy electro-deposition layer and solder layer. Regardless of structure of electro-deposited Co-W-P coatings, (Cu,Co)6Sn5 in all samples grew thicker as the aging time increased. In the case of crystalline Co-W-P coatings, (Cu,Co)6Sn5 layer grew thicker than Cu6Sn5 and Cu3Sn phase as the aging time increased. (Cu,Co)6Sn5 layer grew thicker with increasing W content of films. The order by (Cu,Co)6Sn5 thickness is crystalline Co-W-P > amorphous Co-W-P > Co-P. The columnar grain structure of crystalline Co-W-P coatings may assist Cu diffusion from Cu substrate to the solder layer.