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Electrolytic (Cathode) (Anode) Cu+ + e Cu Cu+ Cu2+ + e (34.9) (34.10)
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Electrolytic systems of direct and membrane cell design have been employed. However, this technology is not a major factor in current manufacturing practice. 34.4.2.2 Properties and Control. Early cupric chloride formations had slow etch rates and low copper capacity and were limited to batch operation.15 21 Regenerated continuous operation using modified formulations has brought useful improvements. Etch rates as high as 55 s for 1 oz copper are obtained from cupric chloride sodium chloride HCl systems operated at 130 F with conventional spray-etching equipment. Copper capacities are maintained at 20 oz/ gal and above. However, more recently, higher copper contents and adjusted chemistries at 125 F typically etch at 75 to 90 s for 1 oz copper. 34.4.2.3 Continuous Etching and Regeneration. Systems in use include chlorination, sodium chlorate, hydrogen peroxide, and electrolysis. Chlorination. Direct chlorination has been the preferred technique for regeneration of cupric etchant because of its historically low cost, high rate, efficiency in recovery of copper, and pollution control. The cupric chloride sodium chloride system (Table 34.1, no. 3) is suitable. Figure 34.2 shows a generalized process.17 Chlorine, hydrochloric acid, and sodium chloride solutions are automatically fed into the system as required. Sensing devices include oxidation-reduction instruments (Cu oxidation state), density (Cu concentration), level sensors,
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Control Valves
Chloride Additive Tank (HCl, NaCl)
FIGURE 34.2
Cupric chloride chlorination regeneration system.
ETCHING PROCESS AND TECHNOLOGIES
and thermostats. Chlorination is reliable and controllable. Other factors are safety and solution control. Optical colorimetric sensors can be used to detect Cu(I) levels. However, these cells are subject to fogging by organic contamination and crystalline buildup with the hazardous defect of continuing to add chemicals past the proper control points and liberating chlorine into the work area. 1. Safety. Use of chlorine gas requires adequate ventilation, tank and cylinder storage, leak-detection equipment, emergency protocols, personal protective equipment, operator training, and fire department approval and inspection of installations. 2. Solution control. An increase in pH will cause the copper colorimeter to give erroneous readings caused by turbidity in the solution. Organic deposits can also foul controls. Excess NaCl at 18 to 20 oz/gal copper causes coprecipitation of salts when the solution is cooled. Solution filtration and etch tank cleanliness must be maintained. Replacement of electrodes, instruments, and metering devices should be scheduled regularly. Chlorate Regeneration. This method, currently widely supported, uses sodium chlorate, sodium chloride, and hydrochloric acid and is an alternate method similar to chlorination. A control sequence similar to the one shown in Fig. 34.2 may be used. Sodium chlorate is available in commercial solutions of several strengths with some degree of formulated agents (usually sodium chloride) added to the mixture. These solutions are added with hydrochloric acid either as a coordinated addition or in a special control logic sequence that allows very low free acid. This control system uses a multiple sensor color or turbidimetric optical system. The addition of one component (e.g., oxidizer) to a flowing sample cell occurs when the primary cell senses are triggered. A second cell allows this addition so long as there is a change from the first cell input. When there is no longer a change response and the first cell is still in triggered condition, the second component (e.g., HCl) is added. The logic continues to flip-flop until the first cell is satisfied. The etch sump conditions seem to have a partially delayed response to the chemical addition reaction such that oxidizing seems to occur over a longer period. Therefore, proper addition and mixing in the sump must be considered for stable control operation. 1. Safety. Sodium chlorate is a potent oxidizing agent that can support combustion. It is necessary to ensure that solution spills are completely cleaned up and that the rags or other materials are not allowed to dry out. Excess addition of reagents above amounts to support the actual copper etched may form chlorine gas that will be liberated into the etch machine and environment. Proper instrumentation and training for this eventuality must be implemented. 2. Solution control. If there is insufficient free acid content in the etch sump to react with the oxidizer and copper (Eq. 34.8), a dangerous imbalance can occur. First, the instruments and controls may not respond properly, and the etch reaction may slow down. Second, any subsequent addition of acid or mixing with acid-containing wastes can liberate chlorine gas in an uncontrolled and dangerous manner. Proper chemical balance is another issue. Water is added in all of the chemical streams and is produced in the chemical reaction. Therefore, any excess chemical addition can dilute the chemistry and change the copper-holding ability of the etchant. Hydrogen Peroxide Regeneration. Hydrogen peroxide uses a two-part chemistry much like sodium chlorate. ORP instrumentation is identical in the control of adding proportional amounts of oxidizer and hydrochloric acid. Peroxide tends to decompose on sitting so that the spills are not as dangerous. Older application of colorimetric cells for the control of dosing has largely been eliminated. 1. Safety. Hydrogen peroxide solutions can become unstable and disassociate into oxygen and water with the liberation of heat.This can occur with explosive force in a confined vessel such as a drum stored in sunlight or heat. It can also occur in plumbing with closed valving.
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