ETCHING PROCESS AND TECHNOLOGIES
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One of the major steps in the chemical processing of subtractive printed boards is etching, or removal of copper, to achieve the desired circuit patterns. Etching is also used for surface preparation with minimal metal removal (microetching) during innerlayer oxide coating and electroless or electrolytic plating. Technical, economic, and environmental needs for practical process control have brought about major improvements in etching techniques. Batch-type operations, with their variable etching rates and long downtimes, have been replaced completely with continuous, constant-etch-rate processes. In addition, the need for continuous processing has led to extensive automation along with complete, integrated systems. The most common etching systems are based on alkaline ammonia and cupric chloride. Other systems include peroxide sulfuric acid, persulfates, and ferric chloride. Process steps include resist stripping, precleaning, etching, neutralizing, water rinsing, and drying.This chapter describes the technology for etching high-quality, fine-line (0.003 to 0.005 in) circuits in high volume at a practical cost, as well as continuous processing, constant-etch rates, and control at high dissolved copper capacities. Increasingly, the production of uniform and constant feature geometries calls for precision and statistically robust control of the circuitization processes and materials. There remains the ever increasing need to balance the process selection according to the factors of cost, environmental and regulatory compliance, stable factory productivity, low worker intervention requirements, compatibility with board design, and construction innovations. It is significant that environmental concern has eliminated the use of chromic sulfuric acid and ammonium persulfate etchants and chlorinated solvents. Limitations on chlorine gas and volatile organic emissions loom as a significant factor in determining future process choices, and pressure to eliminate brominated compounds and lead content may have consequences for processing mandated by material developments. More than ever, the simplification of process selection and vendor support appear to drive process selection in the industry. Typical procedures are given for etching organic (i.e., dry film) and metal-resist boards, and for innerlayers. Strippers and procedures for resist removal are described based on resist selection, cost, and pollution problems. The properties of available etchants are also described in terms of resist compatibility, control methods, ease of control, and equipment maintenance. Other considerations include chemical and etchant effects on dielectric laminates, etching of
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PRINTED CIRCUITS HANDBOOK
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thin-clad copper and semiadditive boards, solder mask on bare copper (SMOBC), equipment selection techniques, production capabilities, quality attained, and facilities.
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GENERAL ETCHING CONSIDERATIONS AND PROCEDURES
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Good etching results depend on proper image formation in both organic innerlayer print-andetch and plated-metal etch resists. Etch personnel must be familiar with screened, photosensitive, and plated resists commonly used. The etching of printed boards must begin with suitable cleaning, inspection, and pre-etch steps to ensure acceptable products. Increasingly, metal foil or plating uniformity in structure thickness and freedom from coatings and defects must be maintained to achieve uniform small-feature definition. Plated boards also require careful and complete resist removal. The steps after etching are important because they remove surface contamination and yield sound surfaces. This discussion considers the various types of resists and outlines typical procedures used to etch printed boards using organic and plated resist patterns.
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Screened Resists Screen printing is a common method for producing standard copper-printed circuitry on metalclad dielectric and other substrates. The etch-resist material is printed with a positive pattern (circuitry only) for copper etch-only boards or with a negative image (field only) when plated through-holes and metal resist are present. The type of resist material used must meet the requirements for proper image transfer demanded by the printer and for stripping chemistry compatibility. From the metal etcher s point of view, the material needs to provide good adhesion and etch-solution resistance; be free of pinholes, oil, or resin bleed-out; and be readily removable without damage to substrate or circuitry. Typical problems are excessive undercutting, slivers, unetched areas, and innerlayer shorts in multilayer boards. In addition, conductor line lifting may occur when the copper-to-laminate peel strength (or surface contamination) is below specification.
Hole Plugging Plugged-hole, copper-only boards use alkaline-soluble screen resists in a unique manner. The technique, called hole plugging, makes the SMOBC board possible. This technique can be used with screened images and to augment tent-and-etch processing for small annular ring structures.
UV-Cured Screen Resists Ultraviolet-cured solventless systems are available for print-and-etch and plating applications. These products are resistant to commonly used acidic plating and etching solutions. Stripping must be evaluated carefully.
Photoresists Dry-film and liquid photoresist materials are capable of yielding the fine-line (0.003 to 0.005 in) circuits needed for production of surface-mount circuit boards. Like screened resists,