Method for Electrolysis-Assisted Oxidative Regeneration of Alkaline Copper-Ammonia Chloride Etching Working Solution, and Apparatus Using Same

The present application is a continuation application of PCT application No. PCT/CN2024/071780 filed on Jan. 11, 2024, which claims the benefit of Chinese Patent Application No. 202310061500.5 filed on Jan. 13, 2023. The contents of all of the aforementioned applications are incorporated by reference herein in their entirety.

The present disclosure belongs to the technical field of etching processes for printed circuit boards, and specifically relates to a method for electrolysis-assisted oxidative regeneration of an alkaline copper-ammonia chloride etching working solution, and an apparatus using the same.

The etching processes for printed circuit boards are divided into acidic etching processes and alkaline etching processes. The alkaline etching processes are primarily alkaline copper-ammonia chloride etching processes. In the industry, an etching solution in an etching machine is commonly referred to as an etching working solution. An etching working solution adopted in the alkaline copper-ammonia chloride etching process, namely, an alkaline copper-ammonia chloride etching working solution, mainly includes an ammonia source, ammonium chloride, and a copper-ammonia chloride complex (Cu(NH)Cl), and may also include other ammonium salt additives and/or other additives. The traditional alkaline copper-ammonia chloride etching working solution mainly adopts ammonia water as the ammonia source. To improve the etching performance, a novel alkaline copper-ammonia chloride etching working solution formula, namely, a weakly-alkaline etching solution, has emerged in the industry. In this weakly-alkaline etching solution, ammonium bicarbonate and/or ammonium carbonate are/is adopted as the major ammonia source. This weakly-alkaline etching solution has a lower pH than the traditional alkaline copper-ammonia chloride etching solution, which can reduce the volatilization and pollution of free ammonia during an etching process.

During an etching production process, a desired replenishment solution needs to be continuously fed into an etching machine to balance and stabilize a concentration of each component and a ratio of components in an etching working solution, thereby maintaining the etching performance. Such a replenishment solution is referred to as an etching replenisher. An alkaline copper-ammonia chloride etching replenisher mainly includes an ammonia source and ammonium chloride, and may also include other ammonium salt additives and/or other additives. During the continuous etching production of printed circuit boards, because a large amount of an etching replenisher is fed into an etching machine, an etching working solution overflows from a tank to be outside the etching machine to produce an overflow solution, which is referred to as a spent etching solution.

A copper-etching agent in the alkaline copper-ammonia chloride etching working solution is a divalent copper-ammonia chloride complex Cu(NH)Cl. During an etching process, the divalent copper-ammonia complex reacts with metallic copper and is converted into a monovalent copper-ammonia complex Cu(NH)Cl accordingly (as shown in the following equation), resulting in the loss of copper-etching performance:

To maintain a stable etching rate, a substantial amount of an etching replenisher must be introduced, such that, under the action of an oxidant, a monovalent copper-ammonia complex in an etching working solution can be oxidatively regenerated into a divalent copper-ammonia complex as a copper-etching agent. The corresponding reaction principle is as follows:

For the oxidative regeneration of the alkaline copper-ammonia chloride etching working solution, the following technology is currently adopted: Oxygen in air is adopted as an oxidant. Fresh air is introduced into an etching machine under a negative pressure induced by extraction ventilation, an etching working solution is atomized through spraying to produce liquid droplets, and the liquid droplets are then allowed to contact the fresh air, such that the oxidative regeneration of a copper-etching agent is achieved through oxygen in the fresh air. On the typical alkaline etching machine, a spray device and a tail gas treatment device are arranged, which together constitute a spray-oxygen absorption-exhausting system. The spray device is configured to spray an etching working solution on a printed circuit board to be etched. The tail gas treatment device is configured to neutralize a gas discharged from an etching machine. In the spray-oxygen absorption-exhausting system, a fan is typically adopted as a power source for introducing fresh air into an etching machine, and the fan is commonly arranged at a position of the tail gas treatment device.

However, the above technology presents the following deficiencies:

1. Due to the solution turbulence caused by a spraying operation, an ammonia gas is constantly released from an etching working solution. To meet the oxidative regeneration demand of a copper-etching agent, fresh air needs to be introduced into an etching machine at a high flow rate. Consequently, a large amount of an ammonia gas will be extracted from the etching machine by a high-flow-rate extraction operation of the spray-oxygen absorption-exhausting system, resulting in the waste of an ammonia water raw material and even the potential etching performance failure due to ammonia deficiency in the etching working solution.

2. Due to the release of ammonia, a significant amount of an ammonia gas is present in a tail gas discharged during an etching process and requires an environmental treatment, which increases the production cost.

3. When a weakly-alkaline etching process is adopted, due to a low pH of an etching working solution adopted for the weakly-alkaline etching process, the reducibility of a monovalent copper-ammonia chloride complex Cu(NH)Cl in the etching working solution is weakened, resulting in a low oxidation reaction rate of the monovalent copper-ammonia chloride complex with oxygen in air. As a result, the regeneration efficiency of a copper-etching agent by the existing technology for oxidative regeneration of a copper-etching agent is low, and can hardly meet the production requirements.

To address the shortcomings of the existing technology for oxidative regeneration of a copper-etching agent in an alkaline copper-ammonia chloride etching working solution, the essential auxiliary oxidative regeneration process improvement is required, such that an etching rate can meet the production etching rate requirement or the ammonia utilization can be effectively optimized and the ammonia pollution can be reduced.

A first objective of the present disclosure is to provide a method for electrolysis-assisted oxidative regeneration of an alkaline copper-ammonia chloride etching working solution. In this method, a regenerated copper-etching agent is oxidatively regenerated from an etching working solution through electrolysis, which can effectively improve the etching efficiency and/or reduce the ammonia pollution and enhance the ammonia utilization.

A second objective of the present disclosure is to provide an apparatus for electrolysis-assisted oxidative regeneration of an alkaline copper-ammonia chloride etching working solution.

The first objective of the present disclosure is achieved through the following technical solutions:

A method for electrolysis-assisted oxidative regeneration of an alkaline copper-ammonia chloride etching working solution is provided, where the alkaline copper-ammonia chloride etching working solution is used for etching on an etching machine, and the method includes the following steps:

(1) selecting an electrolytic cell provided with an electrolytic cell separator, where the electrolytic cell is divided by the electrolytic cell separator into an anode cell zone and a cathode cell zone; an anode is provided in the anode cell zone and is connected to a positive electrode of an electrolytic power supply; and a cathode is provided in the cathode cell zone and is connected to a negative electrode of the electrolytic power supply;

(2) with the alkaline copper-ammonia chloride etching working solution as an anode electrolyte, conducting electrolysis in the electrolytic cell, where a reaction of oxidatively regenerating a copper-etching agent occurs in the anode cell zone; and

(3) during the electrolysis, controlling an oxidation-reduction potential (ORP) value of the anode electrolyte at 300 mV or less, and feeding an etching replenisher into the alkaline copper-ammonia chloride etching working solution to participate in the reaction of oxidatively regenerating the copper-etching agent, such that a pH value and/or a copper ion concentration of the alkaline copper-ammonia chloride etching working solution on the etching machine are/is controlled within a set process range.

The method of the present disclosure is applicable to the oxidative regeneration of various alkaline copper-ammonia chloride etching working solutions with a copper ion concentration of 60 g/L to 170 g/L and a pH value of 7 to 9. In addition to the traditional alkaline copper-ammonia chloride etching working solutions, the present disclosure also includes other alkaline copper-ammonia chloride etching working solutions each including both an ammonia source and a chloride ion, such as weakly-alkaline etching solutions.

In the step (2), the circulation of the etching working solution between the etching machine and the anode cell zone of the electrolytic cell refers to the circulation of the etching working solution between an etching machine and the anode cell zone of the electrolytic cell.

In the step (3), the etching replenisher can be added to the anode cell zone and/or the etching machine and/or a container connected to either of the anode cell zone and the etching machine and/or a solution mixing junction of the anode cell zone and the etching machine.

The etching machine of the present disclosure is provided with a spray-oxygen absorption-exhausting system. Through multiple repeated experiments, the inventors have discovered that, during the electrolysis-assisted oxidative regeneration of an alkaline copper-ammonia chloride etching working solution in the present disclosure, through different combinations of an electrolytic cell separator and electrolytes and varying electrolyte parameters, oxidants such as oxygen and/or chlorine and/or a hydroxyl radical can be generated on the electrolytic anode. These oxidants promote the oxidative regeneration of a monovalent copper-ammonia complex in an etching working solution into a copper-etching agent, namely, a divalent copper-ammonia chloride complex Cu(NH)Cl. As a result, a concentration of the copper-etching agent in the alkaline copper-ammonia chloride etching working solution can be guaranteed, and the etching efficiency can also meet the production requirements. Consequently, while effectively enhancing the etching efficiency, the present disclosure can significantly mitigate the raw material waste and ammonia pollution issues by reducing a flow rate of fresh air supplied during an etching process. Further, when the etching working solution has a relatively low pH value, a monovalent copper-ammonia chloride complex Cu(NH)Cl exhibits weak reducibility and can hardly be oxidized. In this case, a rate of electrolytic oxidation of the monovalent copper-ammonia chloride complex is higher than a rate of oxygen-exchange oxidation. Thus, the method of the present disclosure can remarkably enhance an etching rate of an alkaline copper-ammonia chloride etching working solution with pH of less than or equal to 8.5, thereby meeting the production requirements.

The alkaline copper-ammonia chloride etching working solution (hereinafter also referred to as an etching working solution) includes a large number of chloride ions. As a result, during the electrolysis-assisted oxidative regeneration of the etching working solution, these chloride ions are easily oxidized at the electrolytic anode to produce chlorine and/or a hypochlorite ion in a solution. Subsequently, depending on the ease of electron losses of reductive substances in the etching working solution during an oxidation reaction, the chlorine and/or the hypochlorite ion will undergo a chemical reaction with these reductive substances sequentially. The chlorine and the hypochlorite ion generated during the electrolysis will preferentially react with the monovalent copper-ammonia complex in the etching working solution.

However, when there are uneven concentrations locally in a reaction solution during the electrolysis, the chlorine and the hypochlorite ion may further react with ammonia and ammonium ions in the etching working solution. Moreover, when the etching working solution further includes at least one of a carbonate, a bicarbonate, and a reducing additive, these substances also participates in the above reaction. The addition of the etching replenisher is intended to compensate those components consumed in the reaction of oxidatively regenerating the copper-etching agent in the etching working solution. However, the components additionally consumed by the chlorine and the hypochlorite ion in the etching working solution cannot be routinely compensated through the etching replenisher due to irregular patterns of consumption and substantial consumption. Correspondingly, the original balance among concentrations of various substances in the etching working solution is disrupted, and a pH value of the etching working solution abnormally decreases, resulting in the imbalance in a chemical reaction for copper etching on the etching machine. Therefore, during the electrolysis, the control of the ORP value of the anode electrolyte at 300 mV or less can ensure the effective regeneration of the copper-etching agent in the etching working solution while effectively preventing the oxidative consumption of reductive substances other than the monovalent copper-ammonia complex in the etching working solution.

When oxygen is generated at the electrolytic anode, the following electrochemical reaction takes place: 4[OH]−4e→2HO+O↑. In this case, the following chemical reactions occur in the anode electrolyte:

When chlorine is generated at the electrolytic anode, the following chemical reactions occur in the anode electrolyte:

A hydroxyl radical —OH is also generated in the anode electrolyte of the present disclosure during the electrolysis, and similarly, the following chemical reaction of oxidatively regenerating the copper-etching agent occurs:

Based on the above reaction mechanisms for copper etching and copper-etching agent regeneration, the etching working solution has a pH value continuously decreasing and a specific gravity value constantly increasing during the etching and regeneration processes. Therefore, at least one parameter of the above two process parameters can be adopted as a control basis. The etching replenisher is fed into the etching working solution based a change of the at least one parameter to control and maintain the continuous etching production.

The electrolytic cell separator in the step (1) is a material capable of effectively blocking entrance of copper ions and ammonium ions from the anode cell zone into the cathode cell zone, and is preferably at least one selected from the group consisting of a bipolar membrane, a reverse osmosis membrane, an anion-exchange membrane, a proton exchange membrane, and an ion selectivity-free membrane. The electrolytic cell separator can be further used in combination with a filter cloth. The reverse osmosis membrane is specifically a reverse osmosis membrane sheet, which generally has a lower price than the bipolar membrane, the ion-exchange membrane, and the proton exchange membrane and thus is economical and practical. More preferably, when the reverse osmosis membrane is adopted as the electrolytic cell separator, a pH value of an electrolyte in the electrolytic cell should be less than or equal to 11 to extend a service life of the reverse osmosis membrane.

The cathode electrolyte in the cathode cell zone may be any ammonia and/or ammonium-containing alkaline solution. An etching working solution, an etching replenisher, and a spent etching solution that include ammonia and/or ammonium adopted in an alkaline etching process all can be adopted as the ammonia and/or ammonium-containing alkaline solution. At the electrolytic cathode of the present disclosure, the following electrolytic reaction of water to produce hydrogen mainly occurs: 4H+4e→2H↑, and/or an electrochemical reaction of reducing a high-valence ion into a low-valence ion or a metal occurs. That is, when the cathode electrolyte includes a copper ion, a copper metal may be generated at the electrolytic cathode during the electrolysis.

Preferably, the cathode electrolyte in the cathode cell zone of the electrolytic cell is one or a mixed solution of two or more selected from the group consisting of a spent alkaline copper-ammonia chloride etching solution, an alkaline copper-ammonia chloride etching replenisher, an alkaline copper-ammonia chloride etching working solution, a copper-ammonia complex solution, ammonia water, an ammonium bicarbonate solution, and an ammonium carbonate solution.

Oxygen does not react with ammonia or ammonium ions, and exhibits a significantly lower reaction rate with carbonates, bicarbonates, and most reducing additives than chlorine. Consequently, any of the following preferred embodiments is adopted. Through a specific combination of an electrolytic cell separator and a cathode electrolyte, an electrochemical reaction environment favorable for oxygen evolution is created, such that an electrochemical reaction of electrolyzing water primarily occurs at the electrolytic anode of the electrolytic cell to produce oxygen. As a result, chloride ions can be effectively prevented from being oxidized through an electrochemical reaction at the anode to produce chlorine, thereby minimizing the generation of chlorine and hypochlorite ions and further protecting reductive substances other than the monovalent copper-ammonia complex in the etching working solution from oxidative consumption:

(1) The electrolytic cell separator is at least one selected from the group consisting of a reverse osmosis membrane, a bipolar membrane, a proton exchange membrane, and an ion selectivity-free membrane, and a cathode electrolyte is an ammonia and/or ammonium-containing alkaline solution.

(2) The electrolytic cell separator is an anion-exchange membrane, and the cathode electrolyte is ammonia water.

In the embodiment (1), when the bipolar membrane is adopted as the electrolytic cell separator, water molecules in the bipolar membrane are ionized to produce hydroxide ions and hydrogen ions during the electrolysis. The hydroxide ions migrate from an inside of the bipolar membrane into the anode electrolyte under an attraction action of an electric field, and thus are enriched, which promotes the electrochemical reaction of converting hydroxide ions into oxygen for evolution at the electrolytic anode. When the reverse osmosis membrane is adopted as the electrolytic cell separator, only water, hydrogen ions, and hydroxide ions are allowed to pass through due to special material properties. Thus, during the electrolysis, hydroxide ions in the cathode electrolyte pass through the electrolytic cell separator and enter the anode electrolyte under an attraction action of an electric field, and thus are enriched, which promotes the electrochemical reaction of converting hydroxide ions into oxygen for evolution at the electrolytic anode. When the proton exchange membrane is adopted as the electrolytic cell separator, hydrogen ions and hydronium ions are primarily allowed to pass through due to special material characteristics, which promotes the water electrolysis and oxygen evolution at the electrolytic anode during the electrolysis. When the ion selectivity-free membrane is adopted as the electrolytic cell separator, a pore size of the ion selectivity-free membrane blocks large ions while permitting small ions or molecules such as hydrogen ions and hydroxide ions to pass through, which promotes the occurrence of water electrolysis and oxygen evolution primarily at the electrolytic anode during the electrolysis. The ion selectivity-free membrane is a porous membrane or a porous film.

In the embodiment (2), when the anion-exchange membrane is adopted as the electrolytic cell separator and the ammonia water is adopted as the cathode electrolyte, during the electrolysis, hydroxide ions in the cathode electrolyte pass through the electrolytic cell separator and enter the anode electrolyte under an attraction action of an electric field, and thus are enriched, which promotes the electrochemical reaction of converting hydroxide ions into oxygen for evolution at the electrolytic anode.

Preferably, in the preferred embodiment (1), the electrolytic cell separator is the reverse osmosis membrane and/or the bipolar membrane and/or the ion selectivity-free membrane, and the cathode electrolyte of the electrolytic cell is a spent etching solution from the same etching solution system as the anode electrolyte. For example, a spent etching solution from the same etching machine as the anode electrolyte can be adopted, which can prevent the hydrogen evolution in the cathode cell zone during the electrolysis to avoid the generation of a new hazard source.

In order to effectively avoid the excessive rapid consumption of reductive substances other than a monovalent copper-ammonia complex in the alkaline copper-ammonia chloride etching working solution, such as ammonia, ammonium ions, carbonates, bicarbonates, and reducing additives, the present disclosure may take at least one of the following improvement measures when a consumption rate of a reductive substance or some reductive substances other than the monovalent copper-ammonia complex exceeds a preset value during an etching production process:

Measure 1: During the electrolysis, the ORP value of the anode electrolyte is reduced. Preferably, the ORP value of the anode electrolyte is controlled at less than or equal to 280 mV.

Measure 2: During the electrolysis, an effective electrolytic area of the anode in the electrolytic cell is increased. When etching requirements are met, the smaller the current density per unit area of the anode, the better. Similarly, at a same electrolysis power, the larger the surface area of the anode, the better. A method for increasing the effective electrolytic area of the anode includes, but is not limited to, increasing a volume of the anode, using an anode with a hollow or network structure, providing a protrusion at the anode, and adding an electrical conductor that is in contact with the anode and is not easily soluble in the anode electrolyte. The electrical conductor that is not easily soluble in the anode electrolyte is preferably at least one selected from the group consisting of gold, platinum, graphite, and a titanium-based coated insoluble anode.

Measure 3: A solution circulation flow rate between the anode cell zone of the electrolytic cell and the etching machine is increased to allow solution exchange and mixing, such that Cu(NH)Cl in the etching working solution is more likely to approach the anode and be oxidatively regenerated into a copper-etching agent Cu(NH)Cl.

Measure 4: A solution mixing-exchange tank is provided on a connecting pipeline between the anode cell zone of the electrolytic cell and the etching machine, and the solution mixing-exchange tank is connected to the anode cell zone of the electrolytic cell and the etching machine. A solution circulation flow rate between the anode cell zone of the electrolytic cell and the solution mixing-exchange tank is increased or a solution circulation flow rate between the anode cell zone of the electrolytic cell and the etching machine and the solution circulation flow rate between the anode cell zone of the electrolytic cell and the solution mixing-exchange tank are increased to allow solution exchange and mixing. As a result, Cu(NH)Cl in the etching working solution is more likely to approach the anode and be oxidatively regenerated into a copper-etching agent Cu(NH)Cl.

The above measures all can maintain a high concentration of the reductive substance Cu(NH)Cl in the anode electrolyte surrounding the anode, which leads to the additional advantage that the oxidant nitrogen trichloride can hardly be generated during the electrolysis. In the measure 4, the solution mixing-exchange tank can temporarily store a substantial amount of a solution in which a copper-etching agent has been oxidatively regenerated, such that an electrolysis-assisted oxidative regeneration system can rapidly respond to a change in an etching production load.

The present disclosure can be improved as follows: At least two parameters selected from the group consisting of an ORP value, the pH value, and a specific gravity value of the alkaline copper-ammonia chloride etching working solution on the etching machine are detected and monitored, and operations of the electrolytic cell and a feeding device for the etching replenisher are controlled based on detected parameter results. In this way, the electrolysis-assisted oxidation is added to the traditional oxidative regeneration process based on spray-oxygen absorption. Further, a control mode of combining an etching reaction of the etching machine and an electrochemical reaction of an electrolysis apparatus is adopted to stabilize the balance of an etching working solution, thereby enabling the continuous etching production of the etching working solution. Specifically, an output working current or start/stop of the electrolytic power supply of the electrolytic cell is controlled according to a measured ORP of the etching working solution on the etching machine, and/or a flow rate of an etching working solution produced from the electrolysis in the anode cell zone of the electrolytic cell to enter the etching machine is controlled, and the feeding of an etching replenisher and/or an etching solution raw material and/or water is controlled based on a measured pH and/or specific gravity value of the etching working solution on the etching machine.