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followed by nitric acid. In either case, feed-and-bleed solution management with filtration and periodic chamber cleanout has been successful.
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This section is a survey of the technology and chemistry of the copper etching systems in common use. Selection of practically available etchants has been limited by economic, operational, and (environmental) regulatory concerns. Fabricators have been forced into practical trade-off decisions to suit situations. Two etchants in particular, chromic sulfuric acid and ammonium persulfate, are no longer practical considerations due to environmental pressures. Other formulations and choices have been modified to suit these pressures. There are two basic etchant needs to be met. The first is traditional foil etching for print and etch, plate/tent and etch, and pattern plate and etch. Virtually all processes in the United States and Europe use constant-rate systems for alkaline ammonia or cupric chloride etchants for this purpose. The second need is developing technology for specific precision very-fineline etching including foil thinning and thin metallization clearout for HDI constructions and fine features. (See Sec. 34.7 for additional discussion and mention of additional chemistries that may be useful for these applications.) Continuous constant-rate systems with process automation represent current practice in production etching. These systems feature feed and bleed of replenishment chemicals under control of process instrumentation that monitors and responds to real-time changes in properties of the working solution. The bleed stream of the working composition etchant is usually returned to suppliers for copper reclamation and chemical recycle. The resulting degree of constant etching rate allows for stable and repeatable performance to achieve practical manufacturing processes. However, as demands for higher precision increase, further sophistication in controls will be necessary. Key factors for selecting an etching chemistry include:
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Board design requirements High yield Compatibility with resist Etch rate (speed) Equipment required for process control of etch rate, regeneration, and replenishment Ease of equipment maintenance By-products disposal and pollution control Operator and environmental protection
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The preceding factors serve to evaluate copper etchants to be used. Introduction, chemistry, properties, and problems are given in this section, along with suggestions for selection and control.
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Alkaline Ammonia Alkaline etching with ammonium hydroxide complexing is increasingly used because of its continuous operation, compatibility with most metallic and organic resists, high capacity for dissolved copper, and fast etch rates. Continuous (open-loop) spray machine chemical control systems are universally used. This operation provides constant etch rates, high work output, ease of control and replenishment, and improved pollution control. However, rinsing after
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etching is critical, and the ammonium ion introduced to the rinses presents a waste-treatment problem. On-site closed-loop regeneration with chemical recycling is commercially available but not routinely practiced because of facility requirements, capital cost, fluctuating economics depending on copper commodity pricing, and worker requirements. The general economical and environmentally appropriate operating strategy is to recycle the by-product etchant products under contract to a supplier who reclaims or reconstitutes the copper contained and regenerates the ammoniacal constituents into a reformulated replenisher solution for return to fabricators. Chemistry. The main chemical constituents function as follows:
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1. Ammonium hydroxide (NH4OH) acts as a complexing agent and holds copper in solution. 2. Ammonium chloride (NH4Cl) increases etch rate, copper-holding capacity, and solution stability. 3. Copper ion (Cu2+) is an oxidizing agent that reacts with and dissolves metallic copper. 4. Ammonium bicarbonate (NH4HCO3) is a buffer and as such retains clean solder holes and surface. 5. Ammonium phosphate [(NH4)3PO4] retains clean solder and plated through-holes. 6. Ammonium nitrate (NH4NO3) increases etch rate and retains clean solder. 7. Additional additives are included in most formulations to enhance speed and/or sidewall protection. Thiourea or its derivatives are often used, although a newer thiourea-free formulation with improved undercut protection is available. 8. Continuous operations consist of single-solution makeup buffered to a pH of 7.5 to 9.5. Alkaline etching solutions dissolve exposed field copper on printed boards by a chemical process of oxidation, solubilizing, and complexing. Ammonium hydroxide and ammonium salts combine with copper ions to form cupric ammonium complex ions [Cu(NH3)42+], which hold the etched and dissolved copper in solution at 18 to 30 oz/gal. Typical oxidation reactions for closed-loop systems are shown by the reaction of cupric ion on copper, and air (O2) oxidation of the cuprous complex ion: Cu + Cu(NH3)42+ 2Cu(NH3)2+ 2Cu(NH3)2+ + 2NH 4+ + 2NH3 + 1 2O2 2Cu(NH3) 42+ + H2O (34.1) (34.2)
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There is strong evidence that the etching rate is dependent on the diffusion of Cu(NH3)2+ from the copper surface (Eq. 34.1) into the bulk of the active solution, where oxidation per Eq. 34.2 occurs.3 Etching can be continued with the formation of Cu(NH3) 42+ oxidizer from air during spray etching and as long as the copper-holding capacity supported by Cl ions is not exceeded. Properties and Control. Early versions of alkaline etchants were batch operated. They had a low copper capacity, and the etch rates dropped off rapidly as copper content increased.4,5,6 It was found to be necessary to add controlled amounts of dissolved oxidizing agents to speed up the rate and increase copper capacity at a constant temperature. Batch operation is no longer supported by commercial suppliers. Etching solutions are operated at 120 to 130 F and are well suited to spray etching. Efficient exhaust systems are required because ammonia fumes are released during operation.7 Etching machines must have a slight negative pressure and moderate exhausting to retain the ammonia necessary for holding dissolved copper in solution. Care must be taken that sufficient fresh air to supply needed O2 is introduced to balance the extraction. Currently available solutions offer constant etching of 1 oz (35 mm) copper in 1 min or less, with a dissolved copper content of 18 to 24 oz/gal.
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