Printed Wiring Board Industry: Part 2 - 3 Support Operations
A. Waste Treatment
Because of their toxicity, certain cations are of great importance in the treatment and disposal of waste water. Copper, lead, silver, tin, and chromium are toxic in varying degrees to microorganisms used in publicly-owned waste treatment facilities and therefore must be taken into consideration in the disposal of PWB effluents. Some publicly-owned waste treatment facilities have been upset by the introduction of these ions, because microorganisms were killed and their ability to clean water ceased. In sludge digesters, copper is toxic at a concentration of 100 milligrams/liter, chromium and nickel are toxic at concentrations of 500 milligrams/liter, and sodium is also toxic at high concentrations. Other toxic cations include potassium and ammonium at 4000 milligrams/liter. Some toxic anions, including cyanides and chromates, are also present in PWB waste streams. These are found particularly in metal-plating effluents.o
Treatment of waste streams at the manufacturing site requires capital investment, but it is required by governmental regulations. Federal statutes of this type include the Clean Water Act (CWA) and the Resource Conservation and Recovery Act (RCRA). Over the last 30 years, the number of federal laws regulating environmental issues has gone from less than 10 to more than 40.p A proactive policy of waste reduction will help make the industry robust to variations in the regulatory environment.
B. Waste Streams
Solvents are used to strip dry-film laminates and for other cleaning applications. Monoethanolamine is often the chemistry of choice for removing dry-film laminates. Other proprietary formulations exist as well. Methylene chloride was formally used for stripping negative working resist materials; however, it is declining in use, and is unnecessary for fully aqueous films. Alcohol and acetone are often used for simple cleaning operations.
2. Process Baths
Electroless copper lines contribute a large portion of the overall waste from PWB manufacturing. Water use is high due to the critical rinsing required between nearly all of the process steps. Rinse water waste from electroless copper lines is problematic in several ways. Copper is introduced into the waste water stream due to drag-out from the cleaner-conditioner, micro-etch, sulfuric, accelerator, and deposition baths. This copper is usually complexed with EDTA and requires special waste treatment consideration. The volume of waste generation is high, but the concentration of valuable material to recover is low. Since the waste generated does not have very high value as a reclamation material, rewards for regeneration are relatively low. Micro-etch baths are exhausted when 2 to 4 ounces/gallon of copper is dissolved, and this bath life is usually measured in days. While the electroless copper bath is relatively long-lived (usually several weeks or months), a considerable bail-out stream is generated (several gallons of concentrated bath chemistry per day in production shops).
In addition to waste water, the electroless copper plating process also generates significant volumes of formaldehyde, which is one of the most hazardous chemicals used in PWB manufacturing (formaldehyde is very volatile and requires sufficient ventilation to minimize worker exposure). Concentrations of 3 to 10 grams/liter are common in electroless plating baths. Formaldehyde enters the waste stream through drag-out from electroless plating, and it is oxidized in the plating process and must be replenished with new additions. By switching to a process (such as carbon or graphite alternatives) that does not use electroless copper plating to make holes conductive, two important environmental benefits can be obtained: first, formaldehyde is completely eliminated from the process of making holes conductive, and second, the water used (and waste water produced) is greatly reduced when compared to conventional electroless plating.