Tuesday, September 28, 2010 | Mark Lefebvre, Dow Electronic Materials, The Dow Chemical Company
Maintaining electroplated copper deposit physical properties within specified ranges is critical to ensuring the reliability of PCBs. From thick-panel, high-aspect ratio backplanes to fine line HDI via fill applications, modern acid copper electroplating systems have been optimized to offer a variety of customized performance capabilities. Irrespective of the specific application,to allow consistent, predictable and reliable operation, the electroplating process parameters must be controlled within specifications. Perhaps the most critical aspect of copper electroplating bath control is the monitoring of organic additives. Failure to properly control the organic additives can quickly result in inconsistent performance and reduced deposit quality.
This paper provides an overview of methods used for copper electroplating bath control, including approaches to organic additive analysis, appropriate for use in PCB manufacturing.
The vast majority of copper electroplating baths used in the PCB industry today are based on electrolytes consisting of copper sulfate and sulfuric acid. Combining low cost and convenient operation, such acid sulfate based systems have now been used in the PCB industry for over 50 years. This article will therefore focus on methods used with this class of bath.
Analysis of Inorganic Components
A typical acid sulfate system contains copper sulfate (the primary source of cupric ions), sulfuric acid (for solution conductivity) and chloride ion (as a co-suppressor). Baths designed for high PCB through hole throwing power applications will typically have copper sulfate concentrations below 75 g/L and sulfuric acid concentrations above 160 g/L (to provide high solution conductivity to minimize the potential drop across holes). In contrast, baths designed for high current densities or for via filling will typically have much higher copper concentrations and lower acid concentrations. High copper concentrations favor via filling, due to the lower mass transport rate in blind microvias.
Analysis of inorganic components is typically done using the following techniques:
Table 1: Inorganic component analytical methods.
Analysis of Organic Components
An acid copper sulfate system operated without organic additives yields deposits of poor physical properties. Proprietary organic additives are used to modify deposit characteristics, with the particular type(s) and concentration(s) of these components selected to enhance specific deposit attributes. Organic additives are employed to improve grain refinement and increase throwing power, leveling and brightening of the deposit. Generally, there are three basic types of additives used in acid copper plating: Carriers, brighteners and levelers.
Carriers, also referred to as suppressors, are typically large molecular weight polyoxy-alkyl type compounds. Carriers adsorb at the surface of the cathode and, in concert with chloride ion, act to suppress the plating rate.
Brighteners, also referred to as accelerators, are typically organo-sulfur compounds that increase the plating reaction by displacing adsorbed carrier. Brightener compounds may exist in several forms in electrolytic working baths.
Levelers are typically nitrogen-bearing heterocyclic or non-heterocyclic aromatic compounds that act by displacing brightener species in high current density sites (for example, at protrusions). Adsorption of these additives at protrusions reduces the plating rate in those regions.