Investigation of Agitation Effects on Electroplated Copper in Multilayer Board Plated‐Through Holes in a Forced‐Flow Plating Cell

Abstract

The effect of electrolyte agitation in p l a t e d -t h r o u g h holes (PTH's) of multilayer boards (MLB's) is investigated for high conductivity CuSO4/H2SO4 electrolytes with and without brightener additives. The e x p e r i m e n t s were carried out in an 80 liter forced-flow plating system capable of agitation by air sparging also. PTH quality is assessed in terms of the PTH electrical resistance, the PTH deposit uniformity, and the deposit morphology. Correlations are made with plating current density, e l e c t r o l y t e v e l o c i t y in the PTH's, and two dimensionless plating parameters, ~T~ and j/j~ir~. The results show that for additive-free plating, the agitation requirements to prevent m a s s transport-limited plating conditions increase rapidly with increasing current densities and decreasing cupric ion concentrations. At the current densities common in MLB production, the low levels of agitation produced by air sparging alone provide only for marginal plating conditions particularly at the board edges where the test coupons are located. The plating criteria for good PTH deposits, ~TK ~ 1 and J-/Jlim ~ 0.25 are confirmed by the experimental results. Plating with Cubath| additives is shown to be considerably less sensitive to variations in the plating parameters which are lil~ely to occur in production. The criteria for good PTH quality can be extended to ~T~ ~ 2 and j/jlim ~ 0.25, which allows for more latitude for plating current density variations.Among the m a n y critical process steps that a multilayer printed wiring board (MLB) undergoes during its production, the electroplating of copper is one of the most important. MLB's contain p l a t e d -t h r o u g h holes (PTH's) which provide electrical connections between the various circuit layers of the MLB (see Fig. 1). During the electroplating step, copper is deposited in the drilled PTH's as well as on the outer MLB surfaces. Since the plating of PTH's in MLB's places much more complex demands on the plating process as compared to standard electroplated product, a better understanding of the plating process and its controlling parameters is necessary (1-3). Efforts have been reported which were designed to gain understanding either through analysis of the existing plating phenomenology (I, 5-12) or through analytical modeling (2-4, 13). P a r a m e t e r s were defined which provide a gauge for the relative importance of the three mechanisms (electrical characteristics of the plating solution, electrochemical kinetics of the plating reaction, and fluid dynamic conditions at the cathode) whose interaction determines the performance of an electroplating process. Different mechanisms become dominant for the copper distribution at the different scales important in the plating of MLB's: macroscale, miniscale, and microscale (3).T h e macroscale is characteristic of plating of complete panels. Plating at this scale is dominated by the p r i m a r y current distribution as long as sufficient electrolyte agitation at the cathode surface prevents...

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Effects of Brighteners in a Copper Plating Bath on Throwing Power and Thermal Reliability of Plated Through Holes

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