A method for using palladium removal solution

Abstract

This application relates to the field of circuit board manufacturing technology, and discloses a palladium removal solution and its usage method. The palladium removal solution comprises the following raw materials by weight percentage: 0.5-15% oxidant, 0.02-5% chlorine-containing compound, 0.1-3% complexing agent, and the balance being water; the oxidant is one or more of sodium persulfate, ammonium persulfate, potassium persulfate, potassium peroxymonosulfate, sodium chlorate, or ammonium chlorate; the chlorine-containing compound is one or more of hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, or magnesium chloride. The palladium removal solution prepared in this application can efficiently remove palladium metal at high temperatures without corroding the printed circuit board or contaminating the activation tank of the electroless nickel-gold main line. Depending on the final surface treatment, palladium removal can be selected for either positive or negative film processes. It is low-cost and not limited by the process, does not require specific restriction to post-etching use, and does not affect the quality of the substrate, efficiently removing palladium metal in all cases.

Description

Technical Field

[0001] This application relates to the field of circuit board manufacturing technology, and in particular to a palladium removal solution and its application method. Background Technology

[0002] Because of its catalytic activity, palladium is often used as an activation catalyst in the electroless copper plating process of printed circuit board (PCB) manufacturing. It is also an essential material for PCB hole metallization. During PCB hole metallization, palladium is non-selectively adsorbed onto the hole walls to ensure conductivity. However, these hole walls can be categorized as either conductive or non-conductive. The difference lies in the fact that, in design, non-conductive holes, after circuit formation, do not have copper on their walls, meaning they are non-conductive.

[0003] After the board undergoes expansion and desmearing before copper plating, the epoxy resin on the hole walls is oxidized into carbon dioxide and removed. However, the reinforcing material, glass fiber, is not oxidized. Due to the difference in reaction between the two, gaps are created that can penetrate the activation solution, resulting in palladium metal residue on the hole walls. Under normal circumstances, the hole walls of through holes are covered with a layer of copper metal, so the residual palladium metal does not cause any abnormalities. However, if the hole is designed as a non-through hole, the exposed palladium metal will catalyze the deposition of chemical nickel in the chemical nickel bath during the chemical nickel plating process, causing the hole walls to become conductive due to the reduction of nickel metal, resulting in product defects.

[0004] Because palladium metal has extremely strong mechanical properties and corrosion resistance, removing palladium metal is very difficult. Currently, only a few strong oxidizing agents such as aqua regia or strong complexing agents such as sodium cyanide or potassium cyanide are known to achieve this function. However, aqua regia itself is highly dangerous and can also corrode copper surfaces, while sodium cyanide and potassium cyanide are banned due to their high toxicity.

[0005] Therefore, current palladium removal processes typically use sulfur-containing compounds to poison the surface of palladium metal to inhibit its catalytic activity, such as thiourea. However, solutions that use sulfides as palladium poisoning agents cannot be considered palladium removal solutions at all, because in fact, the palladium metal in the non-conductive holes is not removed, but is simply poisoned by sulfides and no longer has a catalytic effect.

[0006] Furthermore, thiourea itself has an unstable structure and easily decomposes into NH3, CO2 and H2S. H2S can easily react with copper ions to form copper sulfide precipitate, causing nozzle blockage. At the same time, thiourea in hydrochloric acid solution can accelerate the dissolution of copper metal to produce divalent copper and monovalent copper. At this time, thiourea will react with monovalent copper to form a white gel-like substance, which will adhere to the copper surface and cause defects in subsequent processes.

[0007] Furthermore, most sulfur-containing compounds currently in use are also aggressive towards copper, causing copper surfaces to oxidize and turn black. Copper oxidation cannot be removed by acid washing or micro-etching in standard electroless nickel-gold pretreatment, ultimately leading to solder pad skipping during electroless nickel-gold plating. Therefore, traditional palladium removal solutions must be used after etching while the copper layer still has a protective layer (electroplated tin or dry film layer) to prevent copper oxidation. Similar sulfur-containing structural substances, such as thioglycolic acid, sodium thiosulfate, and sodium dimethyl dithiocarbamate, all have the effect of poisoning palladium metal. At the same time, if they come into direct contact with the copper surface, they will also cause copper oxidation. Therefore, the use of such sulfur-containing palladium removal solutions is very limited: palladium removal must be performed after etching and before tin stripping or film removal.

[0008] However, according to the normal production process, when the abnormality of gold on non-conductive vias is discovered, the board is already in the electroless nickel-gold process. At this time, because there is no protective layer on the board, it is impossible to use sulfur-containing palladium removal solution to remove palladium.

[0009] Based on this, some companies have used sulfur-containing palladium removal solutions in the pretreatment of electroless nickel-gold plating, and at the same time used strong abrasive brushes to try to solve the problems of copper surface oxidation and residual palladium metal in non-conductive vias. However, these companies would apply tape to the copper-containing areas on the edges of the boards produced in the main process to save gold salts. The chemical residue at the tape application point could not be removed by washing, blowing, or drying. Ultimately, the residual chemical was carried to the activation tank of the electroless nickel-gold plating main line, contaminating the activation tank and causing overall skip plating. In the end, it was still impossible to perfectly solve this problem.

[0010] Furthermore, due to the high-frequency and high-speed demands brought about by the 5G era, high-speed board substrates have emerged, such as Panasonic M6G / M7 and Shengyi S1000-2M. Board suppliers add certain special materials to the boards, and these materials often affect the deposition of electroless nickel, resulting in nickel or gold splatter during production. Currently, apart from a few electroless nickel plating solutions that can handle this anomaly, the most common countermeasure is to bake the board at 120-150°C before electroless nickel plating. The principle is that baking the copper surface makes the copper layer lattice align regularly and releases internal stress. However, this baking method can cause the sulfur-containing compounds with palladium removal function attached to the palladium metal to desorb or thermally decompose. This desorption or thermal decomposition will cause the active surface of palladium metal in non-conductive vias to be re-exposed, resulting in the loss of palladium removal effect.

[0011] Furthermore, the manufacturing of traditional printed circuit boards is divided into positive and negative processes. Since traditional palladium removal solutions will cause oxidation of the copper surface, palladium must be poisoned when there is a protective layer on top of the copper layer. Therefore, traditional palladium removal solutions are used between alkaline etching and desoldering or between acidic etching and film removal.

[0012] Currently, almost all negative film processes lack palladium removal baths, while almost all positive film processes include them. This is primarily because positive film processes are prone to nickel-gold plating on non-vias. Negative film processes don't use palladium removal solutions because the defect rate is extremely low, so most manufacturers don't use them. Introducing palladium removal solutions for such a low defect rate is not cost-effective. Therefore, standard design practice in the industry for PCB manufacturers that purchase palladium removal solutions is to use them in positive film processes but not in negative film processes.

[0013] However, in reality, both positive and negative film processes may have issues with nickel and gold plating on non-conductive vias. But not all production boards undergo final surface treatments of electroless nickel-gold or electroless nickel-palladium plating. In addition to these two final surface treatments, there are also electroplating nickel-gold, hot air leveling, OSP, electroless silver, and electroless tin. In these final surface treatments, because the reaction principles are different, whether there is residual palladium metal in non-conductive vias will not cause quality abnormalities. That is, regardless of whether it is a positive or negative film process, if the final surface treatment is electroplating nickel-gold, hot air leveling, OSP, electroless silver, or electroless tin, there is no need to use palladium removal solution.

[0014] Therefore, according to the current industry standard process design, it is very unreasonable for the positive film process to pass 100% of the palladium removal solution while the negative film process does not pass the palladium removal solution. The former wastes the palladium removal solution to treat boards that do not need palladium removal, while the latter may miss a few defective boards, which may cause abnormalities in the electroless nickel-gold process.

[0015] Therefore, there is an urgent need to develop a palladium removal solution that is not limited by the process flow or high-speed board usage and does not contaminate the chemical solution. Summary of the Invention

[0016] In order to solve at least one of the above-mentioned technical problems, and to develop a palladium removal solution that is not limited by process and can withstand high temperature and efficiently remove palladium metal, this application provides a palladium removal solution and its usage method.

[0017] On the one hand, the palladium removal solution provided in this application comprises the following raw materials by weight percentage: 0.5-15% oxidant, 0.02-5% chlorine-containing compound, 0.1-3% complexing agent, and the balance being water;

[0018] The oxidant is one or more of sodium persulfate, ammonium persulfate, potassium persulfate, potassium peroxymonosulfate, sodium chlorate, or ammonium chlorate.

[0019] The chlorine-containing compound is one or more of hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, or magnesium chloride.

[0020] By adopting the above technical solution, the palladium removal solution prepared in this application can efficiently remove palladium metal at high temperatures without corroding printed circuit boards, and is not limited to use after etching. This application uses a specific strong oxidant combined with a chlorine-containing compound to generate chlorine gas in the solution, which accumulates on the rough non-conductive hole walls. The accumulated chlorine gas concentration gradually increases to the point that it can oxidize and remove the palladium metal. Furthermore, the chlorine gas dissolves in water to produce hypochlorous acid, which then corrodes the palladium metal. In this way, the palladium metal remaining from the copper plating process is directly removed from the hole walls.

[0021] Optionally, the complexing agent is ethylenediaminetetramethylenephosphonic acid or ethylenediaminetetramethylenephosphonate.

[0022] By adopting the above technical solution, this application selects ethylenediaminetetramethylenephosphonic acid or ethylenediaminetetramethylenephosphonate, which can effectively slow down the generation rate of chlorine gas and avoid the impact on the environment and personnel when the ventilation in the production workshop is insufficient.

[0023] Optionally, the ethylenediaminetetramethylenephosphonate is sodium ethylenediaminetetramethylenephosphonate or potassium ethylenediaminetetramethylenephosphonate.

[0024] Optionally, the palladium removal solution further includes sulfuric acid, wherein the sulfuric acid content is 1 to 6 wt% of the palladium removal solution content.

[0025] By adopting the above technical solution, this application also adds sulfuric acid, and the prepared palladium removal solution can not only remove palladium metal but also has a micro-etching effect.

[0026] On the other hand, a method of using palladium removal solution includes use in positive film processes, negative film processes, any chemical bath in electroless nickel-gold pretreatment, or in a micro-etching bath in the electroless nickel-gold mainline.

[0027] Optionally, when used in the positive film process, the palladium removal solution is placed between the alkaline etching tank and the desoldering tank.

[0028] Optionally, when used in the negative film process, the palladium removal solution is placed between the acid etching tank and the film stripping solution tank.

[0029] By adopting the above technical solution, the palladium removal solution prepared in this application is not limited by the process and does not require specific restrictions on its use after etching, thus not affecting the quality of the substrate. It can be used not only in the traditional application areas of palladium removal solutions—that is, placed between the alkaline etching tank and the desoldering tank in the positive wafer process, and between the acidic etching tank and the stripping solution tank in the negative wafer process—but also in any chemical bath in the electroless nickel-gold pretreatment process or in the micro-etching tank in the electroless nickel-gold main line, all of which can efficiently remove palladium metal.

[0030] Optionally, palladium removal can be performed by spraying or immersion.

[0031] By adopting the above technical solution, the palladium removal solution prepared in this application can be directly prepared and then used for palladium removal by immersion or spraying, with excellent palladium removal effect.

[0032] Optionally, the spraying pressure is 0.5-2.5 MPa; the working temperature of the palladium removal solution is 10-50℃.

[0033] Optionally, during the soaking process, the working temperature of the palladium solution is 20-40℃, and the soaking time is 4-5 minutes.

[0034] In summary, the present invention has at least one of the following beneficial technical effects:

[0035] 1. The palladium removal solution prepared in this application can efficiently remove palladium metal at high temperature without corroding the printed circuit board or contaminating the activation tank of the electroless nickel-gold main line. Depending on the final surface treatment, palladium removal can be selected for the positive or negative process, avoiding the waste of palladium removal solution and increased cost caused by removing palladium from printed circuit boards that do not need it.

[0036] 2. This application uses a combination of a strong oxidant and a chlorine-containing compound to generate chlorine gas in the chemical solution. The chlorine gas, combined with hypochlorous acid dissolved in water, corrodes palladium metal, thereby directly removing the palladium metal remaining from the hole wall in the previous copper plating process. 3. The palladium removal solution prepared in this application is not limited by the process and does not require specific restrictions on its use after etching. It does not affect the quality of the substrate and can be used not only in the traditional palladium removal solution application areas, i.e., placed between the alkaline etching tank and the tin stripping tank in the positive film process, and placed between the acidic etching tank and the film stripping solution tank in the negative film process; it can also be used in any chemical tank in the electroless nickel-gold pretreatment or in the micro-etching tank in the electroless nickel-gold main line, and can efficiently remove palladium metal. Attached Figure Description

[0037] Figure 1 Metallographic structure of the printed circuit board after palladium removal treatment in Example 10;

[0038] Figure 2 The appearance of the copper surface of the printed circuit board after palladium removal treatment in Example 10;

[0039] Figure 3 The metallographic structure of the printed circuit board after palladium removal treatment is shown in Comparative Example 2.

[0040] Figure 4 The copper surface appearance of the printed circuit board after palladium removal treatment is shown in Comparative Example 2. Detailed Implementation

[0041] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0042] This application describes a palladium removal solution comprising the following raw materials by weight percentage: 0.5-15% oxidant, 0.02-5% chlorine-containing compound, 0.1-3% complexing agent, and the balance being water;

[0043] The oxidant is one or more of sodium persulfate, ammonium persulfate, potassium persulfate, potassium peroxymonosulfate, sodium chlorate, or ammonium chlorate.

[0044] The chlorine-containing compound is one or more of hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, or magnesium chloride.

[0045] A method of using palladium removal solution includes use in positive film processing, negative film processing, any chemical bath in electroless nickel-gold pretreatment, or in a micro-etching bath in electroless nickel-gold mainline.

[0046] The substrate of a printed circuit board (PCB) is an insulator. To enable circuit conduction and interconnection between the two sides, the walls of the insulating vias must be metallized. The PCB manufacturing process includes drilling, chemical copper plating and electroplating, pattern transfer, resist removal, etching, tin stripping, solder mask application, text addition, and final surface treatment. Chemical nickel plating is the key process for the final surface treatment. The substrate is composed of resin and glass fiber to enhance its strength. During the copper plating process, a large amount of strong oxidants such as potassium permanganate or sodium permanganate are used for desizing. These strong oxidants attack the resin inside the holes but cannot treat the glass fiber, which is mainly composed of silicon dioxide. As a result, during the subsequent palladium activation reaction, some of the activation solution remains in the tiny voids between the glass fiber and the epoxy resin. After the entire copper plating process, this activation solution is reduced or desizing into palladium metal and remains on the hole wall. The palladium metal remaining in the non-conductive holes will catalyze the reduction of nickel metal onto the palladium metal, causing abnormal nickel on the non-conductive holes. Slight nickel on the non-conductive holes affects the appearance, while severe nickel on the conductive holes will make the non-conductive holes conductive, that is, short-circuited, causing product defects.

[0047] However, palladium metal has extremely strong mechanical properties and corrosion resistance, making it very difficult to remove.

[0048] Currently, only a few strong oxidizing agents, such as aqua regia, or strong complexing agents, such as sodium cyanide or potassium cyanide, are known to achieve this function. However, aqua regia itself is highly dangerous and can also damage copper surfaces, while sodium cyanide and potassium cyanide are banned due to their extreme toxicity.

[0049] Therefore, the current traditional palladium removal process mainly uses sulfur-containing compounds to poison the surface of palladium metal to inhibit its catalytic activity, such as thiourea and its derivative formulations.

[0050] However, thiourea itself has an unstable structure and is easily decomposed into NH3, CO2 and H2S. H2S can easily react with copper ions to form copper sulfide precipitate, causing nozzle blockage. At the same time, thiourea in hydrochloric acid solution can accelerate the dissolution of copper metal to produce divalent copper and monovalent copper. At this time, thiourea will react with monovalent copper to form a white gel-like substance. This substance will adhere to the copper surface and cause defects in subsequent processes.

[0051] Furthermore, most sulfur-containing compounds are also aggressive towards copper, causing the copper surface to oxidize and turn black, which will lead to skipping of the solder pads during electroless nickel-gold plating. Therefore, the use of such sulfur-containing palladium removal solutions is very limited. They must be used after etching and when the copper layer still has a protective layer to avoid copper surface oxidation. That is, palladium removal must be performed after etching and before tin stripping or film removal.

[0052] Based on this, some companies have used sulfur-containing palladium removal solutions directly in the pretreatment of electroless nickel-gold plating, and at the same time used strong abrasive brushes to try to solve the problems of copper surface oxidation and residual palladium metal in non-through holes. However, the companies would apply tape to the copper-containing areas on the edges of the boards produced in the positive film process in order to save gold salts. The chemical residue at the tape application point could not be removed by washing, blowing, and drying. Ultimately, the residual chemical was carried to the activation tank of the electroless nickel-gold main line, contaminating the activation tank and causing overall skip plating.

[0053] Furthermore, with the emergence of high-speed board substrates, board suppliers add certain special materials to the boards. These materials often affect the deposition of electroless nickel, resulting in nickel or gold splatter during production. The most common countermeasure is to bake the high-speed board substrate at 120-150°C before electroless nickel plating. However, this baking method can cause the sulfur-containing compounds with palladium removal function attached to the palladium metal to desorb or thermally decompose, which in turn causes the active surface of palladium metal in non-through holes to be re-exposed, resulting in the loss of palladium removal effect.

[0054] Currently, traditional printed circuit board (PCB) manufacturing is divided into positive and negative processes. Almost all negative processes lack a palladium removal bath, while almost all positive processes include one. However, in reality, both positive and negative processes can encounter issues with nickel or gold plating on non-vias, and whether a PCB undergoes palladium removal depends on the type of final surface treatment chosen.

[0055] Therefore, according to the current industry standard process design, it is very unreasonable for the positive film process to pass 100% of the palladium removal solution while the negative film process does not pass the palladium removal solution. The former wastes the palladium removal solution to treat boards that do not need palladium removal, while the latter may miss a few defective boards, which may cause abnormalities in the electroless nickel-gold process.

[0056] The etching principles in the negative and positive film processes are as follows:

[0057] Acid etching:

[0058] Cu + CuCl₂ → 2CuCl Etching process

[0059] Regeneration process: CuCl + NaClO3 + 6HCl → NaCl + 3CuCl2 + 3H2O

[0060] Alkaline etching:

[0061] Etching process: Cu + Cu(NH3)4Cl2 → 2Cu(NH3)2Cl

[0062] 4Cu(NH3)2Cl + 4NH3 + 4NH4Cl + O2 → Cu(NH3)4Cl2 + 2H2O (Regeneration process)

[0063] Whether acidic or alkaline, the principle behind etching copper is the disproportionation reaction of copper: divalent copper etches copper metal and produces twice the amount of monovalent copper. The regeneration process involves oxidizing monovalent copper back to divalent copper. In acidic etching solutions, sodium chlorate and hydrochloric acid are used as oxidants to regenerate monovalent copper back to divalent copper; in alkaline etching solutions, oxygen is used to complete the regeneration process. Therefore, acidic etching solutions contain the oxidants sodium chlorate and hydrochloric acid, while alkaline etching solutions do not.

[0064] Therefore, the inventors deduced that sodium chlorate, hydrochloric acid, or a combination of sodium chlorate and hydrochloric acid can oxidize and remove palladium metal, making it almost unnecessary to use palladium removal solution in traditional acidic etching solutions, while alkaline etching solutions, due to the absence of strong oxidants, almost always require the addition of a palladium removal process in the positive film process.

[0065] Based on this, the inventors of this application discovered that by combining a strong oxidant with a chlorine-containing compound, chlorine gas is generated in the chemical solution. The chlorine gas, combined with hypochlorous acid dissolved in water, has a corrosive effect on palladium metal, thereby removing the palladium metal remaining from the previous copper plating process. This method is not limited by the process and does not require specific restrictions on use after etching, nor does it affect the quality of the substrate. It can be used not only in the traditional palladium removal solution application areas, i.e., placed between the alkaline etching tank and the desoldering tank in the positive film process, and placed between the acid etching tank and the stripping solution tank in the negative film process; it can also be used in any chemical solution tank in the pretreatment of electroless nickel-gold or in the micro-etching tank in the electroless nickel-gold main line, all of which can efficiently remove palladium metal. Furthermore, it can efficiently remove palladium metal even under high-temperature baking without corroding the printed circuit board or contaminating the activation tank of the electroless nickel-gold main line. Depending on the final surface treatment, palladium removal can be selected for either the positive or negative film process. It is low-cost and avoids wasting palladium removal solution on boards that do not need palladium removal, as well as missing a few defective boards that could lead to abnormalities in the electroless nickel-gold process.

[0066] The raw material specifications for this application are as follows. Unless otherwise specified, all raw materials used in this application are commercially available:

[0067] Sodium persulfate: analytical grade;

[0068] Ammonium persulfate: 99.99% purity; Potassium persulfate: ≥99% purity;

[0069] Potassium persulfate: 98% purity;

[0070] Sodium chlorate: purity ≥ 99%;

[0071] Ammonium chlorate: purity ≥ 95%;

[0072] Hydrochloric acid: purity 31%;

[0073] Ammonium chloride: purity ≥ 99.5%;

[0074] Sodium chloride: purity ≥ 99.5%;

[0075] Potassium chloride: purity ≥ 99%;

[0076] Calcium chloride: purity ≥ 97%;

[0077] Magnesium chloride: purity ≥ 98%;

[0078] Ethylenediaminetetramethylenephosphonic acid: 98% purity;

[0079] Potassium ethylenediaminetetramethylenephosphonate: 99% purity;

[0080] Sodium ethylenediaminetetramethylenephosphonate: 99% purity;

[0081] Sulfuric acid: 98% purity.

[0082] Testing items and methods:

[0083] By grinding and polishing to make sections, the presence of gold on non-copper-plated holes was observed under a metallographic microscope, and the difference in palladium removal effect was measured by the coverage of adsorbed metal inside the holes.

[0084] The changes in the appearance of the nickel-gold surface were observed using scanning electron microscopy (SEM) to assess the impact of the palladium removal solution on the appearance of the copper surface. The morphology of the final product was also examined using SEM to check for any abnormalities.

[0085] Among them, the metallurgical microscope is Shenzhen Aonuo Optical Technology CX-40M;

[0086] Scanning electron microscope: Hitachi S-3000N. Specific Implementation

[0088] Examples 1-6

[0089] A palladium removal solution comprises the following raw materials in parts by weight: oxidant, chlorine-containing compound, complexing agent, and water; the specific parts by weight of the raw materials for the palladium removal solution are shown in Table 1.

[0090] Table 1 Specific raw material proportions except palladium solution

[0091]

[0092]

[0093] The process involves preparing 1L of palladium-removing solution, with the remainder being water.

[0094] Example 1

[0095] The oxidizing agents are sodium persulfate, potassium persulfate, sodium chlorate, and ammonium chlorate; the chlorine-containing compound is hydrochloric acid; and the complexing agent is ethylenediaminetetramethylenephosphonic acid. The palladium is removed by immersion in a solution at a working temperature of 20°C for 4 minutes.

[0096] The weight ratio of sodium persulfate, potassium persulfate, sodium chlorate, and ammonium chlorate is 0.1:0.3:0.4:0.2.

[0097] Example 2

[0098] The oxidizing agents are sodium persulfate and ammonium persulfate, the chlorine-containing compounds are hydrochloric acid, ammonium chloride and sodium chloride, and the complexing agent is sodium ethylenediaminetetramethylenephosphonate; palladium is removed by immersion, the working temperature of the palladium removal solution is 30℃, and the immersion time is 5min;

[0099] The weight ratio of sodium persulfate to ammonium persulfate is 1:1, and the weight ratio of hydrochloric acid, ammonium chloride, and sodium chloride is 0.2:0.5:0.3.

[0100] Example 3

[0101] The oxidizing agents are sodium persulfate, ammonium persulfate, potassium persulfate, and sodium chlorate; the chlorine-containing compounds are hydrochloric acid, ammonium chloride, and sodium chloride; and the complexing agent is sodium ethylenediaminetetramethylenephosphonate. The palladium is removed by immersion at a working temperature of 40°C for 4 minutes.

[0102] The weight ratio of sodium persulfate, ammonium persulfate, potassium persulfate and sodium chlorate is 0.2:0.2:0.3:0.3, and the weight ratio of hydrochloric acid, ammonium chloride and sodium chloride is 0.4:0.3:0.3.

[0103] Example 4

[0104] The oxidant is sodium persulfate, the chlorine-containing compound is hydrochloric acid, and the complexing agent is ethylenediaminetetramethylenephosphonic acid; palladium is removed by immersion, the working temperature of the palladium removal solution is 35℃, and the immersion time is 4min.

[0105] Example 5

[0106] The oxidizing agents are ammonium persulfate, sodium chlorate, and ammonium chlorate; the chlorine-containing compounds are ammonium chloride, potassium chloride, calcium chloride, and magnesium chloride; and the complexing agent is potassium ethylenediaminetetramethylenephosphonate. Palladium is removed by immersion in a solution at a working temperature of 25°C for 5 minutes.

[0107] The weight ratio of ammonium persulfate, sodium chlorate, and ammonium chlorate is 0.4:0.3:0.3, and the weight ratio of ammonium chloride, potassium chloride, calcium chloride, and magnesium chloride is 0.2:0.2:0.3:0.3.

[0108] Example 6

[0109] The oxidizing agents are sodium persulfate, ammonium persulfate, potassium persulfate, potassium perhydrosulfate, sodium chlorate, and ammonium chlorate; the chlorine-containing compounds are hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, and magnesium chloride; and the complexing agent is potassium ethylenediaminetetramethylenephosphonate. Palladium is removed by immersion at a working temperature of 28°C for 4 minutes.

[0110] The weight ratio of sodium persulfate, ammonium persulfate, potassium persulfate, potassium peroxymonosulfate, sodium chlorate, and ammonium chlorate is 0.2:0.2:0.2:0.1:0.1:0.2, and the weight ratio of hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, and magnesium chloride is 0.2:0.2:0.1:0.1:0.1:0.3.

[0111] Examples 7-9

[0112] In addition to palladium solution, sulfuric acid is also added, and horizontal spraying is used.

[0113] Example 7

[0114] Based on Example 1, except that sulfuric acid was added to the palladium solution, the other components and preparation methods were the same as in Example 1. The content of sulfuric acid was 3 wt% of the content of the palladium solution, the spraying pressure was 0.5 MPa, and the working temperature of the palladium solution was 10°C.

[0115] Example 8

[0116] Based on Example 1, except that sulfuric acid was added to the palladium solution, the other components and preparation methods were the same as in Example 1. The content of sulfuric acid was 6 wt% of the content of the palladium solution, the spraying pressure was 2.5 MPa, and the working temperature of the palladium solution was 35°C.

[0117] Example 9

[0118] Based on Example 1, except that sulfuric acid was added to the palladium solution, the other components and preparation methods were the same as in Example 1. The content of sulfuric acid was 1 wt% of the content of the palladium solution, the spraying pressure was 1.2 MPa, and the working temperature of the palladium solution was 50°C.

[0119] Examples 10-13

[0120] Example 10

[0121] Based on Example 1, except that after removing palladium from the printed circuit board, it is baked at 150°C for 30 minutes before electroless nickel-gold plating, the other components and preparation methods are the same as in Example 1.

[0122] Example 11

[0123] Based on Example 1, except that after removing palladium from the printed circuit board, it is baked at 200°C for 30 minutes before electroless nickel-gold plating, the other components and preparation methods are the same as in Example 1.

[0124] Example 12

[0125] Based on Example 7, except that after removing palladium from the printed circuit board, it is baked at 150°C for 30 minutes before electroless nickel-gold coating, the other components and preparation methods are the same as in Example 7.

[0126] Example 13

[0127] Based on Example 7, except that after removing palladium from the printed circuit board, it is baked at 200°C for 30 minutes before electroless nickel-gold coating, the other components and preparation methods are the same as in Example 7.

[0128] Comparative Examples 1-5

[0129] Comparative Example 1

[0130] Based on Example 1, except that an equal amount of thiourea is used to replace the oxidant and no complexing agent is contained, the other components and preparation methods are the same as in Example 1.

[0131] Comparative Example 2

[0132] Based on Example 10, except that an equal amount of thiourea is used to replace the oxidant and no complexing agent is contained, the other components and preparation methods are the same as in Example 10.

[0133] Comparative Example 3

[0134] Based on Example 1, except that an equal amount of thiourea is used to replace the oxidant, an equal amount of sulfuric acid is used to replace the chlorine-containing compound, and no complexing agent is contained, the other components and preparation methods are the same as in Example 1.

[0135] Comparative Example 4

[0136] Based on Example 1, except that thiourea is used instead of oxidant, an equal amount of sulfuric acid is used instead of chlorine-containing compounds, and no complexing agent is contained, the content of thiourea is 0.01 wt% of the palladium solution content, and the other components and preparation methods are the same as in Example 1.

[0137] Comparative Example 5

[0138] Based on Example 1, except that thiourea is used instead of oxidant, an equal amount of sulfuric acid is used instead of chlorine-containing compounds, and no complexing agent is contained, the content of thiourea is 7 wt% of the palladium solution content, and the other components and preparation methods are the same as in Example 1.

[0139] Preparation of the circuit board to be tested:

[0140] 1) Take several etched circuit boards without non-conductive vias, cut them into 10cm*10cm sizes, and drill 5 holes on each board using a mechanical drill to obtain 5 non-conductive vias. Then perform chemical copper plating process, which includes expansion, desmearing, neutralization, hole shaping, pre-immersion, activation, and acceleration. Water washing is performed between each tank. To verify the palladium removal effect of the palladium removal solution of the present invention, the immersion time of the circuit board in the activation tank in the chemical copper plating process is extended from the conventional 5min to 60min to ensure that the palladium in the activation solution enters the gaps in the glass fiber and remains. At the same time, after acceleration, it is directly washed and dried with water because the palladium metal has been exposed after the palladium colloid has been desmeared by acceleration, and there is no need to perform copper plating. At this time, a circuit board with 5 non-conductive vias is obtained for testing.

[0141] 2) The circuit boards to be treated were subjected to palladium removal treatment in the prepared palladium removal solution. After the palladium removal treatment was completed, the treated circuit boards were tested according to the chemical nickel-gold process. After the experiment was completed, the circuit boards were washed and dried, and the tests were compared. The experimental conditions of the chemical nickel-gold process are shown in Table 2, and the test results are shown in Table 3.

[0142] Examples 1-2, 10-11, and Comparative Examples 1-5 are used in the positive film process, that is, between the alkaline etching bath and the desoldering bath, to perform palladium removal operation;

[0143] Example 3 is used in the negative film process, that is, between the acid etching tank and the stripping solution tank, to perform palladium removal operation;

[0144] Example 4 describes the palladium removal operation performed in the degreasing tank during the pretreatment of electroless nickel-gold alloys.

[0145] Example 5 describes the palladium removal operation performed in the activation tank during the electroless nickel-gold pretreatment process;

[0146] Examples 7-8 and 12-13 describe palladium removal operations performed in micro-etching tanks during the pretreatment of electroless nickel-gold; Example 9 describes palladium removal operations performed in micro-etching tanks during the main electroless nickel-gold process.

[0147] Table 2. Electroless Nickel-Gold Process Parameters (Water washing between tanks is omitted here)

[0148]

[0149] Table 3 Test Results

[0150]

[0151]

[0152] If the nickel and gold are not applied inside the via, it indicates that there is no palladium removal effect.

[0153] As can be seen from Examples 1-6, 10-11 and Table 3, the palladium removal solution prepared in this application has excellent palladium removal effect regardless of whether it has been baked at high temperature. Moreover, the appearance of the copper surface and the finished product are normal after palladium removal, indicating that the palladium removal solution of this application can withstand high temperature and can maintain the removal effect on palladium metal while preventing oxidation and corrosion of the copper surface and the resulting skipping and color difference, thus ensuring the quality of the circuit board. Furthermore, it is not limited by the process and can be used in any chemical bath in the positive film process, the negative film process, the pretreatment of electroless nickel gold, or the micro-etching bath in the electroless nickel gold main line, with excellent palladium removal effect.

[0154] As shown in Examples 7-9, 12-13 and Table 3, the palladium removal solution prepared in this application also contains sulfuric acid, which has both palladium removal and micro-etching effects, and is not affected by high-temperature baking. As shown in Example 1, Comparative Example 1 and Table 3, although the non-conductive holes were not nickel-gold coated with the palladium removal solution prepared by replacing the oxidant with thiourea, the palladium removal solution of Comparative Example 1 had a certain oxidizing effect on the copper surface, causing oxidation and color difference on the copper surface. The oxidized copper surface could not be fully replaced by palladium ions in the activation tank to activate the copper surface, resulting in the appearance of the plate after electroless nickel-gold plating showing skipped plating and color difference on the gold surface.

[0155] As can be seen from Example 1, Comparative Examples 1-2 and Table 3, after baking at 150℃ for 30 minutes on the basis of Comparative Example 1, the palladium removal effect of Comparative Example 2 is lower than that of Comparative Example 1, resulting in slight nickel and gold plating on non-conductive holes. For the same reason, the sulfur-containing palladium removal solution has a certain oxidizing effect on the copper surface, causing oxidation and color difference on the copper surface. The oxidized copper surface cannot be fully replaced by palladium ions in the activation tank to activate the copper surface, resulting in the appearance of the plate after electroless nickel plating showing skipped plating and color difference on the gold surface.

[0156] As can be seen from Comparative Examples 3 and 1 and Table 3, although Comparative Example 3 used sulfuric acid instead of hydrochloric acid in the Comparative Example, it also used thiourea to prepare the palladium removal solution. Like Comparative Example 1, this resulted in the appearance of the plate after electroless nickel plating showing a color difference between the plate and the gold surface.

[0157] As can be seen from Comparative Examples 4 and 3 and Table 3, reducing the amount of thiourea significantly improves the oxidation of the copper surface. However, the complexation oxidation effect of thiourea on copper does not disappear. It can significantly improve skip plating, but relatively speaking, it produces more watermarks. Chemical nickel gold is a final surface treatment with very strict requirements for appearance. Watermarks originate from the copper surface and affect the gold surface, which is unacceptable.

[0158] As can be seen from Comparative Example 5 and Table 3, Comparative Example 5 is a new copper plating activation tank installed by a certain company, but no palladium removal tank was designed. Therefore, there are a lot of abnormalities in its non-through holes. In the absence of a palladium removal tank, the only way to improve the situation is to reduce the activity of the nickel tank, but it cannot eliminate the problem. However, the effective area of ​​this company's board is only 5% on average, and the activity can not be reduced any further (further reduction will easily cause skip plating). At the same time, reducing the activity of the nickel tank can only reduce the defect rate, but cannot eliminate the abnormality of nickel plating on non-through holes.

[0159] In summary, the palladium removal solution of this application is a genuine palladium removal solution, not a traditional palladium poisoning agent, thus avoiding many problems associated with sulfur-containing palladium removal solutions. This application can be used not only in areas where traditional palladium removal solutions are used, i.e., placed between the alkaline etching tank and the desoldering tank in the positive film process, and placed between the acidic etching tank and the stripping solution tank in the negative film process; it can also be used in the micro-etching tank or any chemical tank in the pretreatment of electroless nickel-gold (electroless nickel-palladium-gold), and can also be used in the micro-etching tank in the main electroless nickel-gold (electroless nickel-palladium-gold) process; even palladium can be removed by immersion after the chemical solution is prepared directly. Since the palladium removal process can be placed in the pretreatment of electroless nickel-gold, it truly achieves targeted treatment, avoiding the waste caused by performing palladium removal on boards that do not require palladium removal, and avoiding abnormal board loss that may be caused by not using palladium removal solution in the negative film process; it is also suitable for various high-speed boards that require baking, truly achieving the effects of production cost optimization and process yield improvement.

[0160] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the principles of this application should be covered within the scope of protection of this application.

Claims

1. A method for using a palladium removal solution, characterized in that, The palladium removal solution comprises the following raw materials by weight percentage: 0.5-15% oxidant, 0.02-5% chlorine-containing compound, 0.1-3% complexing agent, and the balance being water; The oxidant is one or more of sodium persulfate, ammonium persulfate, potassium persulfate, potassium peroxymonosulfate, sodium chlorate, or ammonium chlorate. The chlorine-containing compound is one or more of hydrochloric acid, ammonium chloride, sodium chloride, potassium chloride, calcium chloride, or magnesium chloride. The complexing agent is ethylenediaminetetramethylenephosphonic acid or ethylenediaminetetramethylenephosphonate; The palladium removal solution is used in the positive film process, the negative film process, any chemical bath in the electroless nickel-gold pretreatment process, or in the micro-etching bath in the electroless nickel-gold main line. The palladium removal solution also includes sulfuric acid, and the sulfuric acid content is 1 to 6 wt% of the palladium removal solution content.

2. The method of using the palladium removal solution according to claim 1, characterized in that, The ethylenediaminetetramethylene phosphonate is sodium ethylenediaminetetramethylene phosphonate or potassium ethylenediaminetetramethylene phosphonate.

3. The method of using the palladium removal solution according to claim 1, characterized in that, When used in the positive film process, the palladium removal solution is placed between the alkaline etching tank and the desoldering tank.

4. The method of using the palladium removal solution according to claim 1, characterized in that, When used in the negative film process, the palladium removal solution is placed between the acid etching tank and the film stripping solution tank.

5. The method of using the palladium removal solution according to claim 1, characterized in that, Palladium removal is performed by spraying or immersion.

6. The method of using the palladium removal solution according to claim 5, characterized in that, During the spraying process, the pressure is 0.5-2.5 MPa; the working temperature of the palladium removal solution is 10-50℃.

7. The method of using the palladium removal solution according to claim 5, characterized in that, During the soaking process, the working temperature of the palladium solution is 20-40℃, and the soaking time is 4-5 minutes.

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