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Printed circuit board (PCB) manufacturing technology is a highly complex and comprehensive processing technology. It can be divided into dry processing (design and wiring, stencil making, drilling, lamination, exposure, and surface finishing, etc.) and wet processing (inner layer black film oxidation, resin degreasing of hole walls, copper plating, electroplating, development, etching, film removal, screen printing, hot air leveling, etc.). Especially in the wet processing, a large amount of water is required, resulting in the discharge of various heavy metal and organic wastewater with complex compositions and significant treatment challenges. Assuming a copper foil utilization rate of 30%–40% for PCBs, the copper content in the wastewater is considerable. For 10,000 square meters of double-sided boards (each with a copper foil thickness of 35 micrometers), the wastewater would contain approximately 4,500 kilograms of copper, along with other heavy and precious metals. Discharging these metals in the wastewater without treatment not only wastes resources but also pollutes the environment. Therefore, wastewater treatment and the recovery of metals such as copper in the printed circuit board (PCB) production process are significant and indispensable parts of PCB manufacturing.
As is well known, the wastewater generated during PCB production contains a large amount of copper, with trace amounts of lead, tin, gold, silver, fluorine, ammonia, organic matter, and organic complexes.
The processes that generate copper wastewater mainly include: copper plating, full-board copper plating, patterned copper plating, etching, and various PCB pretreatment processes (chemical pretreatment, brushing pretreatment, volcanic ash grinding pretreatment, etc.).
The copper-containing wastewater generated from these processes can be broadly classified into complex wastewater and non-complex wastewater based on its composition. To ensure that wastewater treatment meets national emission standards, the maximum allowable discharge concentration of copper and its compounds is 1 mg/L (calculated as copper). Different wastewater treatment methods must be adopted for different types of copper-containing wastewater.
Wastewater Sources and Composition
Chemical Copper Deposition Process:
The wastewater mainly contains complexing agents such as EDTA, sodium tartrate, or others, and Cu2+. Cu2+ forms highly stable complexes with these complexing agents, making conventional neutralization and precipitation methods ineffective.
Alkaline Etching Process:
The wastewater mainly contains Cu2+ and NH3·H2O. When the NH4+ content is high and under alkaline conditions, Cu2+ and NH4+ can form copper-ammonia complexes, which cannot be treated by neutralization and precipitation.
Micro-etching (Ammonium Persulfate-Sulfuric Acid) Process:
The wastewater mainly contains Cu2+ and NH4+. Under acidic conditions, Cu2+ and NH4+ in the wastewater cannot form complexes, but under alkaline conditions, they can.
Other Processes:
For processes such as acidic degreasing, alkaline degreasing, degumming, descaling, and expansion, the wastewater may contain complexing agents depending on the chemicals used. Therefore, general neutralization and precipitation methods are not suitable.
Main Methods for Treating Complexed Wastewater Domestically and Internationally
Ion Exchange Method
Using ion exchange to treat complexed heavy metals has many advantages: less land required, no need for wastewater classification, and relatively low cost. However, this method has many disadvantages: high investment, high resin requirements, and difficulty in control and management. The treatment process is as follows:
Complex Decomposition Method
This method mainly uses strong oxidation to destroy the structure of the complexing agent, causing it to form non-complexes. After complex decomposition treatment, the wastewater containing complexes can be treated using general neutralization and precipitation methods. The treatment process is as follows:
Displacement Treatment Method
Utilizing the instability of heavy metal complexes under acidic conditions, resulting in dissociation, Cu2+ is displaced by adding Ca2+ and Fe2+, and then the pH value is increased to precipitate Cu2+.
Chemical Precipitation Method
Using chemicals that can form more stable precipitates with heavy metals than their complexes, such as Na2S, CaS, and H2S, the purpose of removing heavy metals is achieved.
Heavy Metal Capturing Agent Precipitation Method
Using high-molecular-weight heavy metal capturing agents, which can strongly chelate with heavy metal ions and are unaffected by the concentration of heavy metal ions, they can form precipitates to achieve the purpose of removing heavy metals.
Wastewater Sources
The main sources are rinsing water from full-plate electroplating, pattern electroplating, acid etching, and other processes.
Main Methods for Treating Non-Complex Wastewater
The main method is chemical precipitation. When the wastewater is alkaline, it forms insoluble hydroxide, carbonate, or sulfide precipitates. Typically, lime (calcium oxide) is added to acidic wastewater to make it alkaline and form hydroxide precipitates.
Introduction to a Comprehensive Treatment Technology for Printed Circuit Board Wastewater
The author has over ten years of experience in printed circuit board materials, production, processes, and quality management. Having worked in foreign printed circuit board manufacturing companies, domestic joint ventures, and currently in a state-owned printed circuit board enterprise, the author has accumulated considerable experience. The author presents some insights into printed circuit board wastewater treatment as follows:
Overall Approach to Printed Circuit Board Wastewater Treatment
The effectiveness of printed circuit board wastewater treatment is directly related to whether it is treated in a classified manner. The author believes that a comprehensive consideration is needed based on the wastewater source, copper content, wastewater volume per unit time, and whether treatment is worthwhile.
Based on this idea, the author believes that printed circuit board wastewater can be classified into the following types and treated using different methods for each.
(1) Wastewater containing copper complexes;
(2) Wastewater containing copper non-complexes;
(3) General copper-containing rinsing water;
(4) Acidic and alkaline wastewater containing other heavy metals;
(5) General acidic and alkaline wastewater without copper;
(6) Alkaline etching wastewater with very high copper content.
Treatment methods for the above-mentioned wastewater
(1) Treatment method for wastewater containing copper complexes:
(2) Treatment method for wastewater containing copper non-complexes:
(3) Treatment method for general copper-containing rinsing water:
(4) Treatment method for acidic and alkaline wastewater containing other heavy metals:
(5) Treatment method for general acidic and alkaline wastewater without copper:
(6) Treatment method for alkaline etching wastewater with very high copper content:
For etching wastewater with very high copper content (generally around 130~150g/l), we cooperate with relevant manufacturers to transport it for external treatment in various ways, without needing to treat it ourselves. What is wastewater to us is raw material needed for the production of other manufacturers. This approach saves on wastewater treatment costs and generates economic benefits.
Several Measures to Ensure Wastewater Treatment Meets Standards
(1) Ensure all copper-containing wastewater is treated at a wastewater treatment plant:
Major production processes, such as chemical copper plating, full-plate electroplating, and pattern electroplating, should be equipped with water collection trays at the production site. This allows for the collection of solutions that may leak during normal production or production line maintenance, effectively preventing spills and leaks of electroplating chemicals and ensuring wastewater treatment meets standards.
(2) Ensure different types of wastewater flow to the wastewater treatment plant via their respective pipelines: Strictly distinguish between different types of wastewater, and discharge pipelines according to the equipment layout of the production site, marking them accordingly. This way, even if the production process changes, the wastewater can be redirected to the appropriate pipelines based on the composition of each chemical, ensuring wastewater treatment meets standards.
(3) Strictly adhere to process controls and prohibit the indiscriminate dumping of copper-containing wastewater: Printed circuit board manufacturers have varying production conditions and processing procedures. Due to constraints imposed by printed circuit board (PCB) processing equipment or the production environment, temporary measures are sometimes necessary. For example, when PCBs are transferred between processes, to prevent copper oxidation, they must be placed in a plastic tank containing dilute acid; and the phosphor bronze balls used in copper plating require separate maintenance and cleaning.
In these situations, strict process operation discipline must be maintained, and the aforementioned copper-containing wastewater must be poured into the appropriate treatment tank. Indiscriminate dumping is prohibited to ensure wastewater treatment meets standards.
(4) Continuously improve equipment to reduce unnecessary copper-containing wastewater treatment:
For example, the PCB brushing machine used in the pretreatment of each PCB process generates a large volume of water and contains a high amount of copper powder. Including it in the treatment process is highly uneconomical. Equipment upgrades should be implemented, and a copper powder recovery machine should be used in conjunction with the process to recover and reuse this wastewater, thereby reducing the burden on wastewater treatment and ensuring that wastewater treatment meets standards. (5) Continuously improve processes to reduce the amount of copper-containing complex wastewater to be treated: Under economic and supply conditions, implement process reforms. For example, replace ammonium persulfate used in the chemical pretreatment of printed circuit boards with sodium persulfate. Although sodium persulfate is currently more expensive than ammonium persulfate, its use significantly reduces the amount of copper-containing complex wastewater to be treated, saving wastewater treatment costs, making it worthwhile. More importantly, it further ensures that wastewater treatment meets standards.
In addition, replacing the currently widely used chemical copper plating process with direct electroplating is also a good way to reduce the amount of copper-containing complex wastewater to be treated and alleviate the pressure on wastewater treatment.
