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Case Study 3: Opportunities for Acid Recovery and Management

Printed Wiring Board
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    The Design for the Environment (DfE) Printed Wiring Board Project is a voluntary, cooperative effort between the printed wiring board (PWB) industry, the U.S. Environmental Protection Agency (EPA), and other stakeholders dedicated to helping PWB manufacturers reduce risk to their workers and the environment in cost-effective ways. One of the goals of this project is to provide PWB manufacturers with pollution prevention information specific to the PWB industry, so that they are better equipped to incorporate environmental concerns into day-to-day business decisions.

    This case study highlights the pollution prevention efforts of a medium-sized PWB manufacturer whose experience shows that implementing viable pollution prevention alternatives can result in economic as well as environmental benefits. In particular, this case study illustrates:

    • How acid recycling and recovery can reduce process wastes, chemical costs, and occupational exposure, and improve process control.
    • How working together with employees in the facility and with equipment vendors and chemical suppliers can improve the potential for pollution prevention success.
    • How the complexity of pollution prevention efforts can range from simple improvements in operation and maintenance to researching and developing a unique in-process recycling unit.
    In support of EPAs pollution prevention hierarchy, recycling strategies described in this case study should be investigated only after every attempt has been made to implement source reduction options such as changes in materials, processes, practices, or products.

    Company Background

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    Located in Woburn, Massachusetts, Printed Circuit Corporation (PCC) is a manufacturer of double-sided and multilayer printed wiring boards for the electronics industry. PCC employs 300 people and produces 1.8 million surface square feet of board per year in its 100,000 ft2 manufacturing facility.

    PCC, one of the first companies to join EPA's 33/50 Program, has been active in the area of pollution prevention for many years. As part of this program, the company eliminated methylene chloride in 1990 and 1,1,1-trichloroethane in 1993 through chemical substitution. More recently, PCC has begun to regenerate ammoniacal etchant, recycle rinse water, and recover copper using an on-site etchant regeneration system. (For more information on etchant regeneration, refer to PWB Case Study 2.)

    Motivated by the desire to reduce waste, save money, and meet requirements of the Massachusetts Toxics Use Reduction Act (TURA), PCC continues to look for pollution prevention opportunities. TURA requires facilities that use listed chemicals in quantities above certain thresholds to inventory those chemicals and prepare plans on how to reduce or eliminate their use or generation as hazardous byproducts. Sixteen states have similar pollution prevention planning laws. In addition to investigating and implementing chemical substitution and other source reduction options, PCC has promoted environmental protection through acid recovery and improved acid management practices, the focus of this case study.

    Recovery of Methane Sulfonic Acid Tab Strip

    As a surface finish, PCC uses solder-mask-over-bare-copper with hot-air-solder-leveling. This outer layer finish prevents copper oxidation and facilitates solderability during the assembly process. Before panels can then undergo nickel/gold tab plating (also called finger plating, connector plating, or microplating) for electrical conductivity and environmental resistance, the tin/lead solder must be stripped from the panel. In the stripping process, PCC uses methane sulfonic acid (MSA) and applies a reverse electrical current to dissolve tin and lead from the boards.

    In the past, PCC changed the acid every 30,000 ends (one pass of a circuit panel), or approximately every 6 weeks depending on production schedules. MSA is a very expensive acid (~$21/gal.), and accounted for an average of $17,000/year in raw material costs. Spent solution was sent off-site for disposal at a cost of approximately $5,600/year. PCC recognized ann opportunity to conserve acid, prevent hazardous waste generation, and lower employee exposure to corrosive materials using a relatively simple and efficient in-process recycling technology called diffusion dialysis.

    Diffusion Dialysis Cell Pair

    Diffusion dialysis is a technology that uses an anion exchange membrane allowing anions and the hydrogen ions (due to their small size and mobility) to pass through into a water stream that is running counter current to the flow of the spent acid. The acid (e.g., HCl, H2SO4) is reconstituted on the water side of the membrane and directed back to the process tank. The metal-rich, acid-depleted stream can be sent to on-site waste treatment or shipped off-site for treatment. Fresh acid in proportion to the unrecovered amount is added to the bath to maintain the concentration within the correct operating window. In addition to MSA tab strip recovery, diffusion dialysis has potential applications in plating or other wet processing operations such as rack strip, plating bath pre-dip, and microetch that is treated on-site.

    At PCC, the diffusion dialysis recycling unit is hard-piped to the MSA tab stripping bath. The company first evaluated a 5 gallon/day recycling unit in an off-line pilot test. They assessed parameters such as acid recovery and metal rejection rates, as well as the stripping rate of the recovered acid. PCC then proceeded to evaluate the system on-line. After working with the vendor to fine-tune metal rejection and acid recovery rates, PCC was able to maintain a constant solution level in the stripping bath.

    Based on the projects costs and savings, as outlined below, the payback on the investment was approximately 6 to 7 months.


    Annual Savings:

      Process chemicals $14,500
      Waste disposal $5,600
    Other Benefits:
    • Reduces long-term liability associated with hazardous waste shipments.
    • Reduces employee exposure associated with bath dumping.
    Capital Costs:
      Pure Cycle AJ-10 $10,800
      Acid Recycling System
    Concerns/Disadvantages:
    • Metals stripped from the board must still be either sent off-site in a low acid matrix (same volume), or treated in the on-site wastewater treatment facility. Based on the total volume of strip generated and the metal content of the spent material, the background lead concentration in the influent to the treatment facility rose <1 ppm; however, PCCs treatment facility is able to treat this increase such that the discharge still meets regulatory standards.
    • Labor costs will actually increase slightly because the solution must be analyzed and additions made, if necessary.
    Payback:
      Approximately 6 - 7 months

    Microetchant Regeneration

    Microetching is a ubiquitous process found as a preclean step for many of the stages of PWB manufacturing. Microetching removes anywhere from 10 - 70 microinches of copper to rid the panels of oxidation prior to the subsequent process, such as pattern plate, soldermask application, or hot-air-solder-leveling. PCC generally uses a sulfuric-acid/hydrogen-peroxide solution as the microetchant.

    PCC had been decanting 138 galloons of spent microetch solution per week from the electroless copper line and 35 gallons/week from the black oxide line. This spent solution was being sent off-site for recycling. In order to conserve sulfuric acid and prolong bath life, PCC and a chemical vendor worked together to install an electrolytic plate-out cell to plate out copper from the microetch. Electrolytic recovery has been used to recover valuable metals from bath rinses, but in this case, it is the regenerated microetch and the associateed savings that motivated PCC to explore this recycling technique.

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    This setup uses dimensionally stable anodes and cheap scrap laminate as the cathode onto which the copper is plated. The pumps are hard-piped for batch transfer from the microetch process bath to the electrolytic plate-out cell. PCC chose only the electroless copper and the black oxide lines for microetchant regeneration because other preclean processes do not have high copper concentrations, due to a high rate of copper dragout.

    The continuous-batch plate-out system allows for better process control because the copper concentration remains more stable, which in turn provides for a more stable etching rate. In addition, the copper ion concentration in the microetch is lowered to 25 - 45 g/l, from an average of 45 - 80 g/l by the old decant method. The reduced copper concentration has the effect of decreasing the average amount of copper dragged into the subsequent rinse and then into waste treatment by about 50%. More importantly, the spent microetch is no longer decanted from these processes each week and sent off-site. Not only are disposal and materials handling costs cut, but employee exposure is reduced.

    Better Process Control Through Microetch Regeneration

    Annual Savings:
      Chemical purchases $1,750
      Off-site transportation and recycling $15,625
      Scrap copper sales $3,950
    Additional Benefits:
    • Reduced wastewater treatment and sludge disposal costs due to decreased copper dragout into rinsewater.
    • Reduced employee exposure associated with bath dumping.
    • Reduced materials handlinglabor time associated with bath dumping.
    • Reduced liability associated with shipping spent microetch off-site.
    Capital Costs:
      200 gallon tank $1,000
      500 amp rectifier $2,500
      2 double diaphragm pumps $1,500
      4 dimensionally stable anodes $4,000
    Annual Operating Costs:
      Energy costs $600
      Labor (1 hr. every other day) $2,500
    Payback:
      5 months, with annual operating costs of ~$3,100

    Full Panel Solder Strip Recycling

    Recycling spent solution is not always as easy as hooking up a unit and adding fresh solution periodically. It may require extensive experimentation and teamwork. Understanding the chemistries involved in the process is the key to regenerating bath solutions successfully.

    After outer layer etching, PCC strips the tin/lead etch-resist with a nitric acid and ferric nitrate solution. The nitric acid is used to strip the tin/lead layer, and the ferric nitrate component is necessary to remove the intermetallic layer that forms when the tin and copper diffuse into each other. These solutions also contain wetting agents, copper etching inhibitors, and anti-tarnishing agents.

    PCC currently has a $15,000 grant from Massachusetts Toxics Use Reduction Institute to study the feasibility of recycling the nitric acid stripping solution using diffusion dialysis. The project involves the use of diffusion technology to separate the stripped metals from the stripping solution, rendering it reusable. This would be a continuous, on-line recycling system similar to that used for MSA recovery.

    The major roadblock to this process is the presence of an iron component in the proprietary stripping solution. This component is necessary in order to dissolve the intermetallic layer that forms when the solder (tin/lead) is plated onto the copper surface of the PWBs. Recall from the discussion of MSA recycling, however, that the diffusion dialysis process will reject from the spent solution all metals, including the iron, which is essential to the stripping process.

    PCC believes it may be possible to determine the rate of loss of iron from the diffusion dialysis process and replace the iron with a concentrated replenisher. The difficulties here include adjusting for the losses of the other components, since rejection of organics and non-metal inorganic materials varies, depending on the charge and size.

    In order to make these determinations, PCC contacted its solder strip chemical vendor. PCC arranged a meeting with the company's process engineers, representatives from its chemical vendors, and the diffusion dialysis equipment vendor. Together they designed an off-line pilot system to test the acid reclaim efficiencies and metal rejection rates at various ratios of virgin to spent solder strip.

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    Currently, PCC is awaiting further test results from its chemical vendor on parameters such as solder stripping rates, intermetallic removal, copper etching inhibition, and anti-tarnish capability. Based on the findings, the chemical vendor will be able to determine the additive package of chemical constituents that would replace the components lost from the diffusion dialysis process.

    The next step will be to determine if the project is technically and economically feasible. PCC will base its decision on the cost of the chemical vendors additive package, the cost savings from the recycling process, and other factors such as reduced risk of exposure. If the project is deemed feasible, the recycling unit would be attached directly to the solder strip sump. The reclaimed solution from the unit would be sent back to the sump. The reject would be plumbed to waste treatment. PCC would then analyze the solution regularly, making additions of the package prepared by the vendor as necessary. Stripping rate and copper etch rate would also be monitored, along with nitric acid reclaim efficiency and metals rejection.

    Pollution Prevention Through Process Control

    In focusing on new technologies for recovering spent materials or other source reduction opportunities, a company may overlook simple changes in operations and maintenance that can yield great benefits. Despite its success with pollution prevention over the past several years, PCC learned an important lesson about the need to understand its processes to avoid unnecessary waste. PCCs story, which is described beld below, illustrates how a thorough process evaluation helped the company solve problems in its preclean step for primary image dry-film photoresist lamination.

    In the preclean step, PCC uses a 10% sulfuric acid spray and aluminum oxide scrubbing in a conveyorized spray process. This process accounted for 50% of the sulfuric acid used at the plant. A PCC environmental engineer initiated an investigation of the preclean process after the production manager first asked the chemistry lab personnel to dump and replenish the sulfuric acid solution twice a day, then three times a day, instead of once. Next, the environmental engineer assembled a team consisting of the plant engineer, the area process engineer, and the production supervisor to discuss the rolee of sulfuric acid in the preclean step and to identify the conditions that precipitated the requests for more frequent bath changes.

    The primary reason for requesting more frequent bath dumps was staining on the panels. The process engineers were concerrrned that the staining would interfere with adhesion to the dry-film. They decided that every time the staining occurred, they would investigate the process in an attempt to identify the root cause of the problem. To their surprise, the cause of the problem was not the chemistry; instead, the problem was caused by plugged nozzles and rinses that either did not operate properly or were not turned on.

    By correcting the mechanical failures, increasing the maintenance schedule, and improving training of the line operators, PCC was actually able to reduce the number of bath dumps required to once a week. Not only was the bath dumped less frequently, but through these simple improvements, PCC has also cut its sulfuric acid usage by more than 85% for the process. This translates to a savings of over 26 tons of sulfuric acid per year and over $12,000 in chemical costs. Thus, by working together and understanding the chemical and mechanical components involved in any process, the chances for pollution prevention success improve dramatically.

    The Design for the Environment
    (DfE) Approach

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    This case study describes how teamwork and thorough process evaluations helped one company reduce waste, save money, and improve process control through acid recovery and improved acid management. The EPA's Design for the Environment Program encourages you to evaluate systematically the technologies, practices, and procedures in your facility that may affect the environment. Our goal in working with the PWB manufacturers is to help you to make informed choices, now and in the future, by promoting the search for and evaluation of cleaner alternatives.

    Acknowledgements

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    EPA's Design for the Environment Program would like to thank Printed Circuit Corporation for participating in this case study and PWB DfE Project participants from the following organizations who provided advice and guidance: Circuit Center Inc., EPA - New England, Institute for Interconnections and Packaging Electronic Circuits, Morton International, and National Security Agency.

    Mention of product trade names does not constitute endorsement or recommendation of use.

    The DfE Program wants your feedback. If you have implemented any of the ideas in this case study series, please tell us about it by calling the DfE Program at 202-260-1678 or via email at oppt.dfe@epamail.epa.gov

    For additional PWB case studies, other publications specific to pollution prevention in the PWB industry, and more information on EPAs Design for the Environment Program or the DfE Printed Wiring Board Project, contact:

    Pollution Prevention Information Clearinghouse (PPIC)
    U.S. EPA
    1200 Pennsylvania Ave. NW (7407-T)
    Washington, DC 20460-0001
    Phone: 202-566-0799
    FAX: 202-566-0794
    e-mail: PPIC@epamail.epa.gov

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