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40 The PCB Magazine • January 2017 The same can then be said about current flow to the plating cell. Current is dependent on resistance and voltage. If resistances increase, the flow of current to the plating cell is reduced. Based on this analogy, the author prefers to design the cell with minimal distance of the ca- bles from the power supply to the plating cell. In addition, the current-carrying capacity of the cables is critical to achieve uniform current flow and to minimize current loss. If the cabling from the power supply (rectifier) to the cell feels hot to the touch, this is an indication that there is a loss of current reaching the cathode (circuit board to be electroplated). It is wise to replace these cables on a regular basis to insure quality plating. Finally, do not overlook the quality and ma- terial of construction of the plating racks. While stainless steel is a rugged material for rack con- struction, it does not have the conductivity of copper. A well-designed copper plating rack will deliver 88−92% of the current supplied by the rectifier to the cathode. Conversely, stainless steel is only 50−55% effective! Filtration The need for filtration cannot be emphasized enough, especially when plating in through- holes and blind vias. Any void in the plating in the hole caused by small pieces of contami- nant causes a reduction in the area available to carry the electric current. Rejects would also be caused when insoluble debris is co-deposited on the surface or in the hole. Flow rates are the only means of carrying solids to a filter or bringing fresh solution into contact with the particulate matter. The rate of flow is referred to as the turnover—total gallons pumped per hour in relation to the size of the tank (for example, 200 gal/hr on a 100-gallon tank equals two turnovers per hour). Dirt hold- ing capacity is essential and can be attained with throw-away paper, or cartridges of differ- ent porosities, or filter surfaces coated with filter aid. Porosities of 100 microns down to less than one micron are typical. In practice, the average plating solution is turned over once per hour. The recommended flow rates should provide at least two complete tank volume turnovers per hour. However, to achieve the ultimate in clar- ity, turnovers of up to ten times per hour may be necessary. Keep in mind that the initial flow rate is not the average flow rate. In other words, if one started at 1000 gal/hr, and cleaned or re- placed the filter when the flow was reduced to 200 gal/hr, the actual average flow would prob- ably be about 600 gal/hr, depending upon the type of filter media used. Developing Developing photoresist is one of the simpler chemical processes in the making of a PCB, yet one of the more crucial, as it defines what the circuitry will look like when the PCB is finished. Because developing is apparently so simple, it is easy to overlook its significance to the produc- tion of high quality PCBs, and to overlook the details necessary to produce high quality devel- oping. Incomplete developing, over-develop- ing, positive or negative foot, etc. will plague even the most experienced plating engineer. The standard developer today is a 1% solution of either sodium carbonate monohydrate (Na- 2CO3.H2O), or potassium carbonate (K2CO3). There is not much to choose from between the two, although the use of potassium carbonate has been reported to give slightly better quality developing, and have a wider process window. Potassium carbonate can be purchased in con - centrated liquid form, allowing convenient feed and bleed application, without large feed tanks. In the days when 6-mil lines and spaces were considered fine lines, using developer solutions to the point where the developing slowed dra- matically and then dumping them, was an ac- ceptable scenario. However, this modus operan- di is no longer effective. Today's higher circuit density creates a washboard effect, making it more challenging to deliver sufficient amounts of developer solution to the circuit board. Therefore, manufacturers insure that tightly pH-controlled developing processes are in use. While understanding that feed and bleed sys- tems allowing for steady pH control are criti- cal for optimum performance, there are several other aspects of developing that are often over- looked as to their importance. One such aspect is the spray nozzles used in the developing and rinsing chambers. Full cone and high impingement spray nozzles are most popular in conveyorized spray ACID COPPER PLATING—UNDERSTANDING WHAT'S OFTEN TAKEN FOR GRANTED