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January 2017 • The PCB Magazine 37 ACID COPPER PLATING—UNDERSTANDING WHAT'S OFTEN TAKEN FOR GRANTED Monitoring and Controlling Carbon Content Engineers fully understand the need for or- ganic addition agents in the plating solution. Whether it be for acid copper or etch-resistant tin, organic chemicals are required to enhance the physical properties of the deposit, effect leveling and help minimize overplating in high current density areas. However, these organic materials, even when controlled under the most diligent protocols, do form break-down by-products over time. Essentially these break- down products accumulate in the plating solu- tion. There is also the potential for additional organic materials entering the plating electro- lyte from photoresist leachants and pre-plate cleaner drag-in. Negative effects of the accu- mulated organics include rough copper-plated deposits, reduced ductility of the copper, and concerns over long-term resistance to thermal and mechanical excursions that the printed circuit device functions reliably in service over long periods of time. Higher-than-normal levels of organic break-down products will also lead to cosmetic issues. One tool that is used to monitor organic content in plating solutions is TOC—total or- ganic carbon analysis. Since a TOC analyzer can be used to manage the concentrations of organic material in plating solution, it can be utilized for quality management of electroplat- ed products and the overall health of the plat- ing solution. While it is not expected that every printed circuit board manufacturer would have its own TOC unit in-house, most chemical sup- pliers provide this service. What is critical for the manufacturer is to monitor the TOC con- tent over a time interval and equate these val- ues with end-product performance. It should be noted that not all acid copper plating additives are created equal. Therefore, one should not try to draw conclusions on organic content with values of TOC from two different suppliers. Anode Length and Placement The placement of the anodes in a plating cell is critical with respect to plating distribu- tion. One should look at the anode as where the lines of flux or current is distributed into the cell and onto the cathode (the circuit board). Elec- tricity follows the path of least resistance. That is precisely why it is often difficult to achieve optimum surface plating distribution across a printed circuit board panel. Typically, those iso- lated circuit features and the top of the circuit board attracts more current than other areas. Thus, these areas tend to plate to higher thick- nesses than the lower current areas. The engi- neer's job is to find ways to reduce these cur- rent variations. Certainly, chemical and other operational parameters can be adjusted to aid distribution. However, one should not overlook the anode placement and length. As a rule of thumb, distribution and throwing power are im - proved if the cell design allows for a longer dis- tance between the anode and cathode (ideally, 10–12 inches is recommended). In addition, an- ode length should be 4–6 inches above the low- est level of the circuit board in the plating cell. Finally, there are additional techniques that can be used to enhance plating uniformity. One such method is the use of non-conductive shields. These shields are strategically placed in the cell to redirect current away from the per- ceived higher-current density areas of the cir- cuit board. The shields can always be removed. Electrical Resistance in the Plating Cell While most engineers work diligently to control the chemical aspects of the electroplat- ing process to insure optimum throwing pow- er (surface-to-hole ratio), they often overlook other critical parameters of the process. These include resistance through the cabling lead- ing from the power supply to the plating cell, plating rack current-carrying capacity, and ad- ditional resistances within the cell. As it stands, electroplating is governed by Ohm's law. The greater the resistances within the plating cell, the more difficult it will be to achieve uniform plating thickness distribution. If the author can use this analogy: • Current flow (analogy) is likened to a flow of water through a hose • Flow of water: – GPM = pressure divided by resistance • In other words, the longer the hose or the smaller the diameter of the hose opening, the less water you will get