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The electroplating process of printed circuit boards can be classified into: acidic bright copper plating, nickel/gold plating, and tin plating. This article introduces the technology and process flow of the electroplating process during PCB manufacturing, as well as specific operational methods.
II. Process Flow:
Acid immersion → Full board copper plating → Pattern transfer → Acid degreasing → Secondary countercurrent rinsing → Micro-etching → Secondary countercurrent rinsing → Acid immersion → Tin plating → Secondary countercurrent rinsing → Countercurrent rinsing → Acid immersion → Pattern copper plating → Secondary countercurrent rinsing → Nickel plating → Secondary water rinsing → Acid immersion → Gold plating → Recovery → 2-3 times pure water rinsing → Drying
III. Process Description:
(1) Acid immersion
① Function and purpose: Remove surface oxides, activate the surface, generally at a concentration of 5%, some maintain around 10%, mainly to prevent moisture from causing instability in the sulfuric acid content of the bath; ② The acid immersion time should not be too long to prevent surface oxidation; after a period of use, if the acid solution becomes cloudy or the copper content is too high, it should be replaced in time to prevent contamination of the copper plating tank and the surface of the board; ③ C.P grade sulfuric acid should be used here;
(2) Full board copper plating: also known as panel plating
① Function and purpose: Protect the thin layer of chemical copper just deposited, preventing it from being corroded by acid after oxidation, and enhance it to a certain extent through electroplating.② Related process parameters for full board copper plating: The main components of the bath solution are copper sulfate and sulfuric acid, using a high acid low copper formula to ensure uniform thickness distribution on the board during plating and the ability to deep plate small holes; sulfuric acid content is generally around 180 grams/liter, with a maximum of 240 grams/liter; copper sulfate content is generally around 75 grams/liter, and a small amount of chloride ions is added to the bath solution as a brightener and copper brightener to enhance the brightness effect; the addition amount of the copper brightener is generally 3-5ml/L, and it is supplemented according to the actual production board effect; the current calculation for full board plating is generally based on 2A/dm² multiplied by the area that can be plated on the board, for full board plating, it is the length of the board in dm × width of the board in dm × 2 × 2A/dm²; the temperature of the copper bath is maintained at room temperature, generally not exceeding 32 degrees, mostly controlled at 22 degrees, so in summer, due to high temperatures, it is recommended to install a cooling temperature control system for the copper bath;③ Process maintenance: Daily supplement the copper brightener in time according to the ampere-hour, adding 100-150ml/KAH; check whether the filtration pump is working properly and if there are any air leaks; every 2-3 hours, use a clean wet cloth to wipe the cathode conductive rod; weekly, regularly analyze the copper bath for copper sulfate (once a week), sulfuric acid (once a week), and chloride ions (twice a week), and adjust the brightener content through Hall cell tests, supplementing relevant raw materials in time; weekly clean the anode conductive rod, the electrical joints at both ends of the tank, and timely supplement the anode copper balls in the titanium basket, electrolyzing at low current 0.2-0.5ASD for 6-8 hours; every month, check the titanium basket bag of the anode for any damage, and replace it in time if damaged; check if there is any accumulation of anode mud at the bottom of the titanium basket, and clean it up in time; and filter continuously with carbon core for 6-8 hours while electrolyzing at low current to remove impurities; every six months, depending on the pollution status of the bath solution, determine whether a major treatment (activated carbon powder) is needed; replace the filter element of the filtration pump every two weeks;
④ Major treatment procedure: A. Remove the anode, pour out the anode, clean the surface of the anode membrane, then place it in a barrel for packaged copper anodes, use a micro-etching agent to roughen the copper corner surface to an even pink color, rinse with water, and then place it in the titanium basket for use in the acid tank; B. Soak the anode titanium basket and anode bag in 10% alkaline solution for 6-8 hours, rinse with water, and then soak in 5% dilute sulfuric acid, rinse with water for later use; C. Transfer the bath solution to a standby tank, add 1-3ml/L of 30% hydrogen peroxide, start heating, and when the temperature reaches around 65 degrees, turn on air stirring, maintaining air stirring for 2-4 hours; D. Turn off air stirring, slowly dissolve activated carbon powder into the bath solution at 3-5 grams/liter, and after complete dissolution, turn on air stirring, maintaining for 2-4 hours; E. Turn off air stirring, heat, and allow the activated carbon powder to slowly settle to the bottom of the tank; F. When the temperature drops to around 40 degrees, filter the bath solution to the clean working tank using a 10um PP filter element with auxiliary filter powder, turn on air stirring, place the anode, hang the electrolytic board, and electrolyze at low current density of 0.2-0.5ASD for 6-8 hours; G. After testing and analysis, adjust the sulfuric acid, copper sulfate, and chloride ion content in the tank to the normal operating range; supplement the brightener according to the results of the Hall cell test; H. When the color of the electrolytic board surface is uniform, stop electrolysis, and then perform electrolysis for film formation treatment at a current density of 1-1.5ASD for 1-2 hours until a uniform, dense, and well-adhered black phosphorous film is formed on the anode; I. Test plating OK, and it can be done;⑤ The anode copper balls contain 0.3-0.6% phosphorus, mainly to reduce the anode dissolution efficiency and decrease the production of copper powder;⑥ When supplementing chemicals, if the addition amount is large, such as copper sulfate or sulfuric acid; after adding, a low current electrolysis should be performed; when adding sulfuric acid in large amounts (more than 10 liters), it should be added slowly in several times; otherwise, it may cause the bath solution temperature to be too high, accelerate the decomposition of the brightener, and pollute the bath solution;⑦ The addition of chloride ions should be particularly noted, as the chloride ion content is particularly low (30-90ppm), and when adding, it must be accurately measured with a measuring cylinder or cup before adding; 1ml of hydrochloric acid contains about 385ppm of chloride ions;⑧ Chemical addition calculation formulas:Copper sulfate (unit: kg) = (75-X) × tank volume (liters) / 1000Sulfuric acid (unit: liters) = (10%-X) g/L × tank volume (liters) or (unit: liters) = (180-X) g/L × tank volume (liters) / 1840Hydrochloric acid (unit: ml) = (60-X) ppm × tank volume (liters) / 385
(3) Acid degreasing
① Purpose and function: Remove oxides, ink residues, and residual glue on the copper surface of the circuit, ensuring the bonding strength between copper and pattern electroplated copper or nickel.② Remember to use acidic degreasing agents here; why not use alkaline degreasing agents, even though alkaline degreasing agents are more effective? Mainly because the pattern ink is not resistant to alkalinity and will damage the pattern circuit, so only acidic degreasing agents can be used before pattern electroplating.③ During production, just control the concentration and time of the degreasing agent; the concentration of the degreasing agent is around 10%, and the time should be ensured at 6 minutes; slightly longer time will not have adverse effects; the bath solution is replaced according to 15 square meters/liter of working solution, and supplemented according to 100 square meters 0.5-0.8L;
(4) Micro-etching:
① Purpose and function: Clean and roughen the copper surface of the circuit, ensuring the bonding strength between pattern electroplated copper and copper.② Micro-etching agents mostly use sodium persulfate, with stable and uniform roughening rates, and good washability; the concentration of sodium persulfate is generally controlled at around 60 grams/liter, with a time control of about 20 seconds, and the chemical addition is 3-4 kg per 100 square meters; copper content is controlled below 20 grams/liter; other maintenance and tank replacement are the same as for copper micro-etching.
(5) Acid immersion
① Function and purpose: Remove surface oxides, activate the surface, generally at a concentration of 5%, some maintain around 10%, mainly to prevent moisture from causing instability in the sulfuric acid content of the bath; ② The acid immersion time should not be too long to prevent surface oxidation; after a period of use, if the acid solution becomes cloudy or the copper content is too high, it should be replaced in time to prevent contamination of the copper plating tank and the surface of the board; ③ C.P grade sulfuric acid should be used here;
(6) Pattern copper plating: also known as secondary copper, circuit copper plating
① Purpose and function: To meet the rated current load of each circuit, the copper thickness of each circuit and hole needs to reach a certain thickness; the purpose of circuit copper plating is to timely thicken the hole copper and circuit copper to a certain thickness;② Other items are the same as full board copper plating
(7) Tin plating
① Purpose and function: The main purpose of pattern electroplating pure tin is to use pure tin as a metal corrosion-resistant layer to protect the circuit from etching;② The bath solution mainly consists of stannous sulfate, sulfuric acid, and additives; the stannous sulfate content is controlled at around 35 grams/liter, and sulfuric acid is controlled at around 10%; the addition of tin plating additives is generally supplemented according to the ampere-hour method or based on the actual production board effect; the current calculation for tin plating is generally based on 1.5A/dm² multiplied by the area that can be plated on the board; the tin bath temperature is maintained at room temperature, generally not exceeding 30 degrees, mostly controlled at 22 degrees, so in summer, due to high temperatures, it is recommended to install a cooling temperature control system for the tin bath;③ Process maintenance: Daily supplement the tin plating additives in time according to the ampere-hour; check whether the filtration pump is working properly and if there are any air leaks; every 2-3 hours, use a clean wet cloth to wipe the cathode conductive rod; weekly, regularly analyze the tin bath for stannous sulfate (once a week), sulfuric acid (once a week), and adjust the tin plating additive content through Hall cell tests, supplementing relevant raw materials in time; weekly clean the anode conductive rod and the electrical joints at both ends of the tank; electrolyze at low current 0.2-0.5ASD for 6-8 hours; every month, check the anode bag for any damage, and replace it in time if damaged; check if there is any accumulation of anode mud at the bottom of the anode bag, and clean it up in time; filter continuously with carbon core for 6-8 hours while electrolyzing at low current to remove impurities; every six months, depending on the pollution status of the bath solution, determine whether a major treatment (activated carbon powder) is needed; replace the filter element of the filtration pump every two weeks;
④ Major treatment procedure: A. Remove the anode, take off the anode bag, clean the surface of the anode with a copper brush, rinse with water, and then place it in the anode bag, put it in the acid tank for later use; B. Soak the anode bag in 10% alkaline solution for 6-8 hours, rinse with water, and then soak in 5% dilute sulfuric acid, rinse with water for later use; C. Transfer the bath solution to a standby tank, slowly dissolve activated carbon powder into the bath solution at 3-5 grams/liter, and after complete dissolution, adsorb for 4-6 hours, then filter the bath solution to the clean working tank using a 10um PP filter element with auxiliary filter powder, place the anode, hang the electrolytic board, and electrolyze at low current density of 0.2-0.5ASD for 6-8 hours; D. After testing and analysis, adjust the sulfuric acid and stannous sulfate content in the tank to the normal operating range; supplement the tin plating additives according to the results of the Hall cell test; E. When the color of the electrolytic board surface is uniform, stop electrolysis; F. Test plating OK, and it can be done;
⑤ When supplementing chemicals, if the addition amount is large, such as stannous sulfate or sulfuric acid; after adding, a low current electrolysis should be performed; when adding sulfuric acid in large amounts (more than 10 liters), it should be added slowly in several times; otherwise, it may cause the bath solution temperature to be too high, accelerate the oxidation of stannous, and speed up the aging of the bath solution;⑥ Chemical addition calculation formulas:Stannous sulfate (unit: kg) = (40-X) × tank volume (liters) / 1000Sulfuric acid (unit: liters) = (10%-X) g/L × tank volume (liters) or (unit: liters) = (180-X) g/L × tank volume (liters) / 1840
(8) Nickel plating
① Purpose and function: The nickel layer mainly serves as a barrier layer between the copper layer and the gold layer, preventing mutual diffusion of gold and copper, affecting the solderability and service life of the board; at the same time, the nickel layer also greatly increases the mechanical strength of the gold layer;② Related process parameters for full board copper plating: The addition of nickel plating additives is generally supplemented according to the ampere-hour method or based on the actual production board effect, with an addition amount of about 200ml/KAH; the current calculation for pattern nickel plating is generally based on 2A/dm² multiplied by the area that can be plated on the board; the nickel bath temperature is maintained at 40-55 degrees, generally around 50 degrees, so the nickel bath should be equipped with heating and temperature control systems;③ Process maintenance: Daily supplement the nickel plating additives in time according to the ampere-hour; check whether the filtration pump is working properly and if there are any air leaks; every 2-3 hours, use a clean wet cloth to wipe the cathode conductive rod; weekly, regularly analyze the copper bath for nickel sulfate (ammonium sulfate nickel) (once a week), nickel chloride (once a week), and boric acid (once a week), and adjust the nickel plating additive content through Hall cell tests, supplementing relevant raw materials in time; weekly clean the anode conductive rod and the electrical joints at both ends of the tank, timely supplement the anode nickel corners in the titanium basket, electrolyzing at low current 0.2-0.5ASD for 6-8 hours; every month, check the titanium basket bag of the anode for any damage, and replace it in time if damaged; check if there is any accumulation of anode mud at the bottom of the titanium basket, and clean it up in time; filter continuously with carbon core for 6-8 hours while electrolyzing at low current to remove impurities; every six months, depending on the pollution status of the bath solution, determine whether a major treatment (activated carbon powder) is needed; replace the filter element of the filtration pump every two weeks;
④ Major treatment procedure: A. Remove the anode, pour out the anode, clean the anode, and then place it in a barrel for packaged nickel corners, use a micro-etching agent to roughen the nickel corner surface to an even pink color, rinse with water, and then place it in the titanium basket for use in the acid tank; B. Soak the anode titanium basket and anode bag in 10% alkaline solution for 6-8 hours, rinse with water, and then soak in 5% dilute sulfuric acid, rinse with water for later use; C. Transfer the bath solution to a standby tank, add 1-3ml/L of 30% hydrogen peroxide, start heating, and when the temperature reaches around 65 degrees, turn on air stirring, maintaining air stirring for 2-4 hours; D. Turn off air stirring, slowly dissolve activated carbon powder into the bath solution at 3-5 grams/liter, and after complete dissolution, turn on air stirring, maintaining for 2-4 hours; E. Turn off air stirring, heat, and allow the activated carbon powder to slowly settle to the bottom of the tank; F. When the temperature drops to around 40 degrees, filter the bath solution to the clean working tank using a 10um PP filter element with auxiliary filter powder, turn on air stirring, place the anode, hang the electrolytic board, and electrolyze at low current density of 0.2-0.5ASD for 6-8 hours; G. After testing and analysis, adjust the nickel sulfate or ammonium sulfate nickel, nickel chloride, and boric acid content in the tank to the normal operating range; supplement the nickel plating additives according to the results of the Hall cell test; H. When the color of the electrolytic board surface is uniform, stop electrolysis, and then perform electrolysis for activation treatment at a current density of 1-1.5ASD for 10-20 minutes; I. Test plating OK, and it can be done;
⑤ When supplementing chemicals, if the addition amount is large, such as nickel sulfate or ammonium sulfate nickel, nickel chloride; after adding, a low current electrolysis should be performed; when adding boric acid, the added amount of boric acid should be placed in a clean anode bag and hung in the nickel bath, and should not be directly added to the tank;⑥ After nickel plating, it is recommended to add a recovery rinse using pure water to open the tank, which can be used to supplement the nickel bath due to evaporation; after recovery rinsing, connect to secondary countercurrent rinsing;⑦ Chemical addition calculation formulas:Nickel sulfate (unit: kg) = (280-X) × tank volume (liters) / 1000Nickel chloride (unit: kg) = (45-X) × tank volume (liters) / 1000Boric acid (unit: kg) = (45-X) × tank volume (liters) / 1000
(9) Gold plating:
Divided into hard gold plating (gold alloy) and soft gold (pure gold) processes, the hard gold bath and soft gold bath solutions are basically the same, except that the hard gold bath contains some trace metals such as nickel, cobalt, or iron;① Purpose and function: Gold, as a precious metal, has good solderability, oxidation resistance, corrosion resistance, low contact resistance, and good wear resistance, among other excellent characteristics;② Currently, the gold plating on printed circuit boards mainly uses citric acid gold baths, which are widely used due to their simple maintenance and easy operation;③ The gold content in the soft gold bath is controlled at around 1 gram/liter, with a pH of about 4.5, a temperature of 35 degrees, and a specific gravity of about 14 Baumé, with a current density of around 1ASD;④ The main additives include acidic adjustment salts and alkaline adjustment salts to adjust pH, conductive salts to adjust specific gravity, and gold salts and other gold plating additives;⑤ To protect the gold bath, a citric acid immersion tank should be added before the gold bath to effectively reduce pollution to the gold bath and maintain stability;⑥ After gold plating, a pure water rinse should be used as a recovery rinse, which can also be used to supplement the liquid level of the gold bath due to evaporation; after recovery rinsing, connect to secondary countercurrent pure water rinsing, and after rinsing the gold board, place it in 10 grams/liter of alkaline solution to prevent oxidation of the gold board;⑦ The gold bath should use a platinum-titanium mesh as the anode, as stainless steel 316 is prone to dissolve, leading to contamination of the gold bath with metals such as nickel, iron, and chromium, causing defects such as whitening, exposure, and blackening in gold plating;⑧ For organic pollution in the gold bath, continuous filtration with carbon core should be applied, and an appropriate amount of gold plating additives should be supplemented.
END
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