Modeling of the Electroplating of a Through‐Hole with Flow Reversal and Downstream Counterelectrode Configurations

Abstract

Two process configurations of periodic flow reversal with two equistant counterelectrodes and of unidirectional flow using only a downstream counterelectrode are analyzed using a general model of through-hole electroplating. Although plating rates greater than attained with an ohmic-limited process cannot be achieved with a flow reversal process, the flow required in the through-holes can be significantly reduced. The use of only a downstream counterelectrode can improve the uniformity of deposition and increase the plating rate over ohmic-limited plating under certain conditions. Our previous modeling efforts (1-3), as well as others (4-9), have indicated that higher plating rates than those currently achieved of 20-40 mA/cm 2 for through-hole aspect ratios of 6:1 (10) are attainable from plating under ohmic-limited conditions with the use of bath additives. The likely impediments to reaching these higher rates of deposition in industrial processes are insufficient flow in the through-holes or an uneven current distribution across the surface of the circuit board. If two counterelectrodes equidistant from the circuit board are used, the criterion to insure electroplating of the through-hole in the ohmiclimited regime was previously determined to be Pe/(~v~a ~ >-12 (1). If flow in the through-hole is provided that at least meets this requirement, the plating rate and the deposition uniformity for a through-hole of fixed dimensions only can be improved by increasing the conductivity of the plating solution or by using bath additives to decrease the dependence of the rate on the potential difference. As the ohmic-limited rate profile is symmetrical around the center of the through-hole, the plating uniformity is independent of the flow direction or periodic flow reversal.However, the rate under conditions of ohmic-mass transport limited plating is strongly dependent on the direction of flow. Accordingly, plating uniformities in this regime of control can be improved by different plating schemes. Plating in the ohmic-mass transport limited regime using periodic reversed flow or unidirectional flow with only a downstream counterelectrode may allow plating at higher rates than plating in the ohmic-limited regime. These types of plating processes are examined in this paper.In a previous paper (1) we developed a general twodimensional mathematical model of electroplating of high aspect ratio through-holes of multilayer printed circuit boards. This model included transport by electrical migration, diffusion, and convection in the through-hole. The solution of the model was used to examine the effects of plating variables, as applied to an acid copper plating bath with and without additives in the bath, on uniformity. Different regimes of plating conditions, particularly ohmiclimited control, were examined to determine criteria for selection of a model for through-hole plating.The flow reversal and downstream counterelectrode processes will be investigated using our general model of through-hole plating (1). The major ass...

Modeling of the Electroplating of a Through‐Hole Considering Additive Effects and Convection

“…Plating in the ohmic/mass transport-limited regime using reverse flow or only a downstream counterelectrode may allow plating at higher rates than plating in the ohmic-limited regime. These types of plating processes are discussed in another paper (44).…”

“…The ohmic/mass transport-limited rate profile indicates a strong dependence on the direction of flow. Therefore, plating uniformities in this regime of control can be improved by different plating schemes (44).…”

“…The optimal plating conditions predicted by solution of the models are for the conditions of ohmic-limited plating which will allow the maximum plating rates to be obtained, unless using a downstream counterelectrode process is feasible (44). The minimum flow rate required for ohmic-limited plating is given by Eq.…”

Electroplating apparatus and method considerations for high aspect-ratio through-hole copper electroplating process

“…The minimum diameter of through holes will decrease from 0.3 mm to 0.2 mm. Through hole (TH) copper plating, using conventional DC technologies, is capable of providing good throwing power and relaibility (2)(3)(4). While Pulse Reverse plating can be used on HAR boards, process complexity is greater and more careful process control is required to maintain product performance and relaibility.…”

Improved Uniformity of Conformal Through-Hole Copper Electrodeposition by Revision of Plating Cell Configuration

“…Recently, semi-analytical methods or numerical methods have been used for calculating current density and deposit thickness distribution with respect to diffusion, electrical migration and convection. [16][17][18][19][20][21] Most models of TH filling had been constructed on a bulk-diffusion approach which separated the electrolyte into a thin diffusion layer region and a bulk region. Concentration distribution of copper ions and velocity profile were analyzed in Hagen-Poiseuille flow mode by numerical calculations.…”