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FIGURE 34.1
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34.4.1.3 Continuous Systems. A practical method of maintaining a constant etch rate with minimal pollution uses automatic feeding controlled by specific gravity or density.6 The process, which is generally referred to as bleed and feed, is illustrated in Fig. 34.1.As the printed boards are etched, copper is dissolved, and the density of the etching solution increases. The density of the etchant in the etcher sump is sensed to determine the amount of copper in solution. When the density sensor records an upper limit, a switch activates a pump that automatically feeds replenishing solution to the etcher and simultaneously removes etchant until a lower density is reached. An elegant by-product of this technique is the introduction of the replenisher into the first rinse stage following the etcher. The rinse captures the copper from the panel and reintroduces it to the etcher, where it is eventually eliminated to the by-product tank with the rest of the copper. When used with aqueous processing photoresists, it has been found that low pH (7.9 to 8.1) aids etch reliability. However, maintaining this pH by adjustment of exhaust extraction has been difficult. Measurement of pH and controlled addition of anhydrous ammonia into the etchant has improved consistency. It has been established that control of free ammonia as well as ammonium chloride and oxygen levels are needed to stabilize etching rates. Direct injection of oxygen has also been attempted to stabilize the etching rates between low and high copper etching demand process utilization periods. Typical operating conditions are as follows:* Temperature pH Specific gravity at 120 F (49 C) Baum , Be Copper concentration, oz/gal Etch rate, 0.001 in/mm Chloride level 120 to 130 F (49 to 50 C) 8.0 to 8.8 1.207 to 1.227 25 to 27 20 to 22 1.4 to 2.0 4.9 to 5.7 mol/L
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* PA Hunt Chemical Corporation, West Paterson, NJ.
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ETCHING PROCESS AND TECHNOLOGIES
A study of the etching rate vs. dissolved copper content shows the following effects: 0 to 11 oz/gal 11 to 16 oz/gal 18 to 22 oz/gal 22 to 30 oz/gal Long etching times Lower etch times but solution control difficult Etch rates high and solution stable Solution unstable and tends toward sludging
All work must be thoroughly rinsed immediately upon leaving the etching chamber. Techniques such as replenisher rinsing and multiple cascade water rinsing assist in thorough rinsing with controlled effluent and limited water use.8 Do not allow the boards to dry before rinsing. Etched circuitry with plated tin/lead solder resist also requires an acid application of solder brightener for reflow of the coating. Thiourea-containing brighteners have largely been eliminated due to use of sacrificial plating resist (SMOBC) and environmental pressures. Rinsing must be sufficient to remove etchant from under circuit edges and to completely clean the circuit surfaces and plated through-holes.8 Multiple-stage cascading water rinsing and air knife drying result in clean, stain-free surfaces (see Fig. 34.1). Modern processes often follow the etching rinse with metal-resist stripping and rinsing chambers before exiting the machine conveyor. 34.4.1.4 Closed-Loop Regeneration. True regeneration requires the following:
1. Removal of portions of the spent etching solution from the etcher sump under controlled conditions, according to the amount of copper added to the etch bath. 2. Chemical restoration of spent etchant (i.e., removal of excess by-products and adjustment of solution parameters for reuse). 3. Replenishing of etchant in the etching machine, balancing the actual production use demands. Constant etching conditions are achieved when regeneration is continuous. Regeneration by these methods is expensive and is thus limited to large printed-circuit facilities. The principal methods of regeneration are crystallization, liquid liquid extraction, and electrolytic recovery.
Crystallization reduces the copper level in the etchant by chilling and filtering precipitated salts. This is followed by refortification and adjustment of operating conditions. Liquid liquid extraction9,10 is gaining acceptance because of its continuous and generally safe nature. The process involves mixing spent etchant with an organic solvent (i.e., hydroxy oximes) capable of extracting copper. The organic layer containing copper is subsequently mixed with aqueous sulfuric acid, which extracts copper to form copper sulfate. The copper-free etchant is restored, and the copper sulfate is available for acid copper electrowinning. Closed-loop regeneration systems reduce chemical costs, sewer contamination, and production downtime, but require floor space, resources, and technical attention. Economics are directly affected by copper market prices. Electrolytic recovery of copper directly and through membrane cells from an ammoniacomplexed copper sulfate etchant yields possible benefits such as reduced waste shipments and cost savings.11,12 These processes are usually employed by the large-scale vendor/recycling plants that offer a spent removal and replenisher replacement service as part of the product contract. This practice allows for economies of scale and environmental responsibility to be removed from the fabrication plant site.
34.4.1.5 Special Problems Encountered During Etching13 1. Low etch rate with low pH, <8.0. This is caused by excessive ventilation, heating, downtime, and spraying when the solution is hot, under conditions of adequate replenishment or low ammonia. The pH must be raised with anhydrous ammonia. Automatic replenishment equipment must be checked.
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