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PCB circuit board copper plating process

date:2021-04-21 15:46:11

The electroplated copper layer has the advantages of good electrical conductivity, thermal conductivity and mechanical ductility. It is one of the indispensable key electroplating technologies in the manufacture of printed circuit boards (PCB).

Copper electroplating on printed circuit boards includes full-board copper plating, graphic circuit copper plating and micro-hole production copper plating. Commonly used plating solutions include sulfate plating solution, pyrophosphoric acid direct plating solution and cyanide plating solution. It is an acid sulfate bath.

Let's take this as an example to introduce the process technology of PCB electroplating copper.

1. Pickling

The purpose of acid pickling is to remove oxides on the surface of the board and activate the surface of the board. The general concentration is 5%, and some are kept at about 10%, mainly to prevent the instability of the sulfuric acid content of the bath liquid caused by moisture.

In operation, attention should be paid to controlling the pickling time, not too long to prevent oxidation of the board surface.

For the acid solution, after a period of use, if the acid solution becomes turbid or the copper content is too high, it should be replaced in time to prevent contamination of the surface of the electroplating copper cylinder and the plate. The sulfuric acid used for pickling should generally be CP grade sulfuric acid.

2. Preparation of plating solution

Acidic copper sulfate plating solution has the advantages of good dispersion ability and deep plating ability, high current efficiency, low cost, etc., which makes it very widely used in the production of printed boards.

Acidic sulfate copper plating solution is generally composed of copper sulfate (CuSO4), sulfuric acid (H2SO4), hydrochloric acid (the main function is chloride ion Cl-) and organic additives.

Copper sulfate is the main salt and the main source of Cu2+ ions in the solution. Pay attention to the concentration of copper sulfate when preparing. If the concentration is too low, the deposition rate will be slow, and if the concentration is too high, the deposition rate will be too fast, the crystal particles will be coarse, and the deep plating ability of the plating solution will be affected, making the thickness difference between the plate surface and the hole too large.

In general use, the content of CuSO4•5H2O is controlled at 60g/L~100g/L. The main function of sulfuric acid in the plating solution is to increase the conductivity of the plating solution and prevent the hydrolysis of Cu2+. Pay attention to controlling the concentration when using it.

If the concentration is too high, the dispersibility of the plating solution will be poor, but too low will increase the brittleness of the coating and decrease the toughness. Especially in the plated through-hole plating operation of the printed board, it is necessary to maintain a proper and stable ratio of ρ(H2SO4)/ρ(Cu2+) in order to achieve a better deep plating effect.

According to practice, the content of H2SO4 should be controlled within 180g/L~220g/L. Chloride ion (hydrochloric acid) can increase the activity of the anode, promote the normal dissolution of the anode, and prevent the anode passivation; it can also reduce the copper powder generated by the incomplete anode dissolution, improve the brightness and leveling ability of the coating, and improve the quality of the coating.

The content of chloride ions in the plating solution is generally low, and can be controlled within 30mg/L~80mg/L. Additives generally include carrier, brightener, leveling agent, etc., which play a very important role in acid copper sulfate electroplating. They can change the surface adsorption status of the electrode, and then change the structure of the coating.

However, additives usually require several synergistic effects to achieve the desired effect. Therefore, it is difficult to accurately grasp the amount of additives in the actual preparation and electroplating process. This is also the problem of high-thickness-to-diameter ratio microporous electroplating in high-density printed boards. There have been studies abroad to develop technologies that do not use additives by changing the conditions of pulse electroplating.

3. Full board copper plating

Also called one-time copper plating. Its role is to protect the thin chemical copper that has just been deposited. Full-board electroplating is to use the entire printed board as the cathode after the hole is metallized, and the copper layer is thickened to a certain extent by electroplating, and then the circuit pattern is formed by etching to prevent the electroless copper layer from being too thin by subsequent processes Etched away and cause the product to be scrapped. The control of the process parameters related to copper electroplating on the whole board is:

(1) The main components of the bath are copper sulfate and sulfuric acid. The high-acid and low-copper formula is adopted to ensure the uniformity of the plate surface thickness distribution and the deep plating ability for deep holes and small holes during electroplating; the sulfuric acid content is generally 180g/L, more It can reach 240g/L; copper sulfate content is generally controlled at about 75g/L.

(2) A small amount of chloride ions should be added to the bath solution to act as an auxiliary gloss agent and copper gloss agent to jointly exert the gloss effect.

(3) The addition of the copper light agent should be supplemented according to the actual production board effect or according to the method of thousand ampere hours. For example, the copper light agent should be supplemented in time according to the thousand ampere hours every day, that is, 100ml/KAH~150ml/KAH. The amount of copper light agent added or the amount of cylinder opening is generally 3ml/L~5ml/L, the current calculation of the whole plate electroplating is generally based on 2A/dm2×the plateable area of the plate, for the whole plate electroplating, it is based on the plate length ( dm)×board width (dm)×2×2A/dm2.

(4) The temperature of the copper cylinder is maintained at room temperature, and is generally controlled at 22°C and not more than 32°C. If the temperature is too high in summer, the copper cylinder should be equipped with a cooling temperature control system.

(5) Pay attention to check whether the filter pump is working properly and whether there is any air leakage; wipe the cathode conductive rod with a clean wet cloth every 2h~3h.

(6) Analyze the content of copper sulfate (1 time/week), sulfuric acid (1 time/week), and chloride ion (2 times/week) in the copper cylinder every week, and adjust the content of the light agent through the Hall cell test. Supplement related raw materials.

Pay attention to safety when supplementing sulfuric acid. When the supplement is large (above 10L), it should be added slowly in several times; otherwise, the temperature of the bath will be too high, the decomposition of the light agent will accelerate, and the bath will be contaminated; when adding chloride ions, Pay special attention to the fact that because the content of chloride ions is particularly low, it must be accurately weighed with a graduated cylinder or cup before adding; 1ml of hydrochloric acid contains about 385×10-6 chloride ions.

In addition, clean the anode conductive rod and the electrical joints at both ends of the tank every week, timely replenish the anode copper balls in the titanium basket, and electrolyze with low current 0.2ASD~0.5ASD for 6h~8h; check the anode titanium basket bag every month If there is no damage, the damaged ones should be replaced in time; and check whether there is anode mud accumulated at the bottom of the anode titanium basket, if there is any anode mud, it should be cleaned up in time; and the carbon core is used to continuously filter for 6h~8h, and the low current electricity is used to remove the impurities; every six months or so according to the tank The liquid pollution status determines whether a major treatment (active carbon powder) is required; the filter element of the filter pump should be replaced every two weeks.

The large processing program operation is mainly carried out according to the following methods:

(1) Take out the anode, pour the anode out, clean the anode film on the surface of the anode, and then put it in the barrel where the copper anode is packaged. Use a microetching agent to roughen the surface of the copper corner to a uniform pink color. After washing and drying, put it in Inside the titanium basket, put it into the acid tank for later use;

(2) Put the anode titanium basket and anode bag into 10% lye and soak for 6h~8h, wash with water and dry, then soak with% dilute sulfuric acid, wash and dry for later use;

(3) Transfer the tank liquid to the spare tank, add 1ml/L~3ml/L of 30% hydrogen peroxide, start heating, wait until the temperature is increased to about 65°, turn on the air stirring, and heat the air for 2h~4h;

(4) Turn off the air stirring, and slowly dissolve the activated carbon powder into the bath at 3g/L~5g/L. After the dissolution is complete, turn on the air stirring and keep it warm for 2h~4h;

(5) Turn off the air and stir, heat up, and let the activated carbon powder slowly settle to the bottom of the tank;

(6) When the temperature drops to about 40°, use a 10μm prepreg filter element and add filter powder filter tank solution to the clean working tank, turn on the air and stir, put in the anode, hang into the electrolytic plate, press 0.2ASD~0.5ASD Current density low current electrolysis 6h~8h;

(7) After laboratory analysis, adjust the sulfuric acid, copper sulfate, and chloride ion content in the tank to within the normal operating range; supplement the brightener according to the results of the Hall tank test;

(8) After the color of the surface of the electrolysis plate is uniform, stop the electrolysis, and then perform the electrolysis-forming film treatment at a current density of 1ASD~1.5ASD for 1h~2h, and wait until a uniform and dense black phosphorous film with good adhesion is formed on the anode. That's it. The anode copper ball contains 0.3%~0.6% phosphorus, its purpose is to reduce the anode dissolution efficiency and reduce the production of copper powder;

(9) The trial plating can meet the requirements.

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