Electroless Copper Plating Process By Applying Alternating One-Side Air Stirring Method for High-Aspect-Ratio through-Holes

Abstract

A new electroless copper plating process was studied to improve throwing power (ratio of thickness of deposited films at the center of through-hole to that of the films at the surface of the board) of high-aspect-ratio through-holes in multilayer printed circuit boards. An alternating one-side air stirring method was devised to supply adequate plating solution into through-holes. In this method, a board was put between peripheral plates via spacers. Plating solution was stirred through supplying air to one side of a narrow flow channel composed of the board and the plate by air-supplying pipes arranged at the bottom of the flow channel, while the solution in the other channel was kept static. Air was alternately supplied for each channel. This operation was expected to generate differential-pressure between the front and the back of a board, leading to supplying the solution into through-holes. Plating tests were conducted to verify the effect of this method on throwing power of the high-aspect through-holes. A plating test by applying a conventional method (both-side air stirring) was also conducted as a reference. Electroless copper plating solution was used that contained copper sulfate, formaldehyde, sodium hydroxide, ethylenediaminetetraacetic acid, dipyridyl and surfactant. Sodium sulfate, sodium formate, and sodium carbonate were also added into the solution as salts accumulated as plating time increased. The plating rate of the solution was approximately 2 to 3 μm/h. Copper sulfate, formaldehyde, and sodium hydroxide were periodically supplied by using the concentrated solution. Temperature and pH were 73 °C and 12.4. Six-mm-thick printed circuit boards with through-holes of 0.25-, 0.20-, and 0.15-mm diameter (aspect ratio: 24, 30 and 40, respectively) were used as substrates. Flow velocity of the solution was controlled by adjusting the flow rate of supplied air. Studied flow velocity was preliminary calculated by applying the Bernoulli and Hagen-Poiseuille equations on the basis of consideration that the supplying rate of reactants, particularly copper ion, needed to be higher than the consumption rate of the ion in a through-hole to improve throwing power. Plating tests revealed that throwing power indicated approximately 0.4 for the through-holes of a 24 aspect ratio in the conventional method. On the other hand, throwing power indicated nearly 1 even for through-holes of a 40 aspect ratio under the conditions in which flow velocity of the solution was more than 7 cm/s by applying our alternating one-side air stirring method. Throwing power was also found to increase as flow velocity of the solution increased at one side for any through-hole (24, 30, or 40 aspect ratio). Throwing power decreased remarkably as flow velocity of the solution decreased, implying that flow velocity of the solution significantly affected throwing power as the aspect ratio of through-hole increased. Moreover, flow velocity theoretically estimated by fluid equation reflected behavior of throwing power measured in the plating tests. Therefore, it is concluded that the alternating one-side air stirring method efficiently improves throwing power of high-aspect-ratio through-holes.