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The decomposition is catalyzed by many metals, including Cu, Ni, and Fe. If traces of metal or metal salts are allowed to contaminate piping runs, the piping may also rupture explosively. Facilities should be constructed with pressure relief capabilities. Electrolytic Regeneration. The electrochemical reversal of the etching of copper shown in Eq. 34.9 and 34.10 is claimed to be effective and economical. Descriptions of this system are given in the literature.15 On a large scale, electrolytic regeneration requires a high investment in equipment and materials, as well as high power consumption. The chemical formulation is relatively high in acid content and low in copper, a condition not optimized for etching effectiveness or plating efficiency. The etchant is a solution of cupric chloride and hydrochloric acid (Table 34.1, no. 1). Etchant flows continuously between spray-etching machines and a plating tank. In the plating machine, two processes take place simultaneously: copper is plated at the cathode, and regeneration of the spent etchant occurs at the anodes. Copper recovery may not return copper value and may be expensive, inconvenient, and cause difficulty with recycling. 34.4.2.4 Problems with Cupric Chloride Systems
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1. Slow etch rate. Depending on chemical formulation, cupric chloride is inherently slower than ammonia and must be properly evaluated before production expectations are established. Check to ensure no resist residues or chromate coatings remain on the panel surface before etching. A slower than usual rate is frequently due to low temperatures, insufficient sump mixing, or poor solution chemical control. If temperature and agitation are under control, slow etching with a dark green solution may result from a low cupric ion content, indicating insufficient oxidizer. Acid must also be added to clarify cloudy solutions. (Before acid addition, make sure excess oxidizer is not present.) In regenerative systems, the source of oxidation may be depleted. It is advisable to analyze specific gravity, free acid, and total chloride on a regular schedule and maintain control charts to ensure that bath controllers are consistent and operating properly. 2. Sludging. This occurs if acid is low or if water dilution occurs. 3. Etcher goop. This is a common floating accumulation of photoresist compounds leached into and precipitated from etch baths. Accumulations appear to increase with higher acid formulations. Formation can be limited by proper exposure of photoresists and continuous recirculation of the etch bath through carbon filtration. Periodic, complete drain-down of the etching machinery and thorough mechanical and chemical equipment cleaning with a sulfamic acid type commercial cleaner can minimize problems. 4. Yellow or white residues on copper surface. Yellow residue is usually cuprous hydroxide. It is water insoluble and is left when boards are etched and alkali-cleaned. A white precipitate will probably be cuprous chloride, which can remain after etching in solutions that are low in chloride ion and acid. To eliminate both conditions, the solution in which the board is rinsed just before final water-spray rinsing should be 5 percent by volume hydrochloric acid. 5. Waste disposal. Spent or by-product etchant is usually sent off-site for copper reclamation. There is usually a fee for this service depending on copper content and distance to the reclamation facility. The solutions must be free of unreacted oxidizer (see previous chlorate system discussion). Etchant can contain traces of zinc, chromium, and arsenic from the foil treatments.
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Sulfuric Acid Hydrogen Sulfuric-peroxide systems are used extensively for copper surface preparation by microetching the surface to provide texture and active surfaces. These formulations have been used in
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the past for primary circuitization etching but are no longer favored because of slow etch rate and needs for accompanying control of exotherm and copper crystallization. However, the increasing need for fine-line etching has renewed use possibilities for this and other etchants (see Sec. 34.7.4). In essence, these call for etching foils thinner than 1 2 oz and the control precision offered by slower etchants. They are compatible with metal resists and many organics. 34.4.3.1 Chemistry. Typical constituents of both immersion and spray etchants and their functions are as follows: 1. 2. 3. 4. 5. 6. Hydrogen peroxide is an oxidizing agent that reacts with and dissolves metallic copper. Sulfuric acid makes copper soluble and holds copper sulfate in solution. Copper sulfate helps to stabilize etch and recovery rates. Molybdenum ion is an oxidizing agent and rate exaltant.22 Aryl sulfonic acids are peroxide stabilizers.23 Thiosulfates are rate exaltants and chloride ion controllers that permit lower peroxide content.24 7. Phosphoric acid retains clean solder traces and plated through-holes.25,26 The etching reaction is as follows: Cu + H2O2 + H2SO4 CuSO4 + 2H2O (34.11)
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34.4.3.2 Properties and Control. The earlier technical problems of slow etch rates, peroxide decomposition, and foaming in spray systems have been solved, but critical concerns still remain. Among these are process overheating, etchant composition balance with by-product recovery, etchant contamination, and the dangers of handling concentrated peroxide solutions. The specific problem of decomposition of peroxide during idle hours has caused disastrous equipment meltdowns, and thus requires thermal management while equipment is otherwise not in operation. 34.4.3.3 Closed-Loop Systems. Production facilities require continuous recirculation of the etchant through the etching tank or machine and the copper sulfate recovery operation. Etchant replenishment is controlled by chemical analysis and by additions of concentrates. Copper sulfate recovery is based on lowering the solubility of CuSO4 5H2O by decreasing the etchant temperature to 50 to 70 F. Crystallizer equipment and conveyorized discharge has been available for this process. 34.4.3.4 Problems Encountered with Peroxide Systems
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1. Reduced etch rates. This problem can be caused by operating conditions, solution imbalance, or chloride contamination. 2. Under- and overetching. A review of etching conditions, solution control, and the resist stripping process may show deviations from normal. In the case of immersion etching, the solution and panel agitation may need to be increased.When spray-etching, a check of nozzles and line clogging is indicated. 3. Temperature changes. Recirculating water rates and thermostats need to be examined regularly. Overheating may be due to high copper content, contamination, or rapid peroxide decomposition. 4. Copper sulfate recovery stoppage. Examine solution balance, heat exchanger, and other recovery equipment. Exit conveyor jamming difficulties from thermal excursions and varying crystal size and liquid content cause major shop floor problems.
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