Copper Electroplating Nickel/Gold Electroplating Tin Process Flow

Copper Electroplating Nickel/Gold Electroplating Tin Process Flow
Process flow: acid dip → panel plating copper → acid degreasing → micro-etching → acid dip → tin plating → acid dip → pattern plating copper → nickel plating → citric acid dip → gold plating.
Process description.
(1) Acid dip.
① Function and purpose: to remove oxides from the board surface and activate the surface, typically at a concentration of about 5% to 10%, mainly to prevent moisture from being carried into the tank, which could cause unstable sulfuric acid concentration.
② Use C.P. grade sulfuric acid; the acid dip time should not be too long to prevent oxidation of the board surface; after a period of use, if the acid solution becomes turbid or the copper content becomes too high, it should be replaced promptly to avoid contaminating the copper plating tank and the board surface.
(2) Panel plating copper.
① Function and purpose: to protect the freshly deposited thin electroless copper from being etched away by acid, and to build up the thickness to a certain level through electroplating.
② Relevant process parameters for panel plating copper: the bath mainly contains copper sulfate and sulfuric acid, using a high-acid low-copper formulation to ensure uniform thickness distribution on the board surface and deep plating capability for deep holes; sulfuric acid content is typically 180 to 240 g/L; copper sulfate content is generally around 75 g/L; the bath may contain trace chloride ions, which work together with auxiliary brighteners and copper brighteners to achieve a brightening effect; the copper brightener addition rate is 3 to 5 ml/L, and it is usually replenished based on ampere-hours or according to the actual production board effect; the current for panel plating is generally calculated as 2 A/dm² multiplied by the plateable area of the board; the copper tank temperature is generally controlled at 22 to 32°C.
③ Process maintenance: replenish copper brightener daily based on ampere-hours; check whether the filter pump is operating normally; every 2-3 hours, wipe the cathode conductor rod clean with a clean wet cloth; weekly, perform periodic analysis and adjust the brightener content through Hull cell tests, and replenish as needed; weekly, clean the anode conductor rod and the electrical contacts at both ends of the tank, and promptly replenish the copper balls in the titanium baskets; monthly, check the anode titanium basket bags for damage and replace them promptly, and check whether anode sludge has accumulated at the bottom of the titanium baskets; approximately every six months, decide whether a major treatment is needed based on the bath contamination level; replace the filter pump cartridge every two weeks.
④ The anode copper balls contain a small amount of phosphorus, which is intended to reduce the anode dissolution efficiency and minimize the generation of copper powder.
⑤ When replenishing chemicals, if large amounts of copper sulfate or sulfuric acid are to be added, they should be added slowly in several batches; otherwise, the bath temperature may become too high, accelerating the decomposition of brighteners and contaminating the bath solution.
(3) Acidic degreasing.
① Purpose and function: To remove oxides from the circuit copper surface, ensuring the adhesion between the primary copper and the pattern-plated copper or nickel.
② Use an acidic degreaser; during production, only the degreaser concentration and treatment time need to be controlled.
(4) Micro-etching.
① Purpose and function: To clean and roughen the circuit copper surface, ensuring the adhesion between the pattern-plated copper and the primary copper.
② Sodium persulfate is used as the micro-etching agent.
(5) Acid dip.
① Purpose and function: To remove oxides from the board surface and prevent moisture from being carried into the bath, which could cause instability in the sulfuric acid concentration.
② Use C.P. grade sulfuric acid for pickling, but do not immerse for too long to prevent oxidation.
(6) Pattern electroplating copper, also known as secondary copper.
Purpose and function: To meet the rated current load of each circuit, the circuit and hole copper must reach a certain thickness. Pattern plating copper thickens the hole copper and circuit copper to the required thickness.
(7) Tin electroplating.
① Purpose and function: The purpose of pattern electroplating pure tin is to use pure tin solely as a metallic etch resist to protect the circuit during etching.
② The bath solution mainly consists of stannous sulfate, sulfuric acid, and additives; the stannous sulfate content is controlled at approximately 35 g/L, and sulfuric acid at about 10%; tin plating additives are typically replenished based on kilo-ampere-hours or according to the actual production board effect; the current for tin plating is generally calculated as 1.5 A/dm² multiplied by the platable area on the board; the tin bath temperature is maintained at room temperature, generally controlled between 22–30°C. Therefore, in summer, when temperatures are too high, a cooling temperature control system may be installed.
③ Process maintenance: Replenish tin plating additives daily based on kilo-ampere-hours; check whether the filter pump is operating normally; every 2–3 hours, wipe the cathode conductor rod clean with a clean damp cloth; weekly, perform regular analysis and adjust the tin plating additive content through Hull cell tests; weekly, clean the anode conductor rod and the electrical contacts at both ends of the tank; monthly, check the anode bags for damage and replace them promptly, and inspect the anode sludge at the bottom of the anode bags; every two weeks, replace the filter pump cartridge.
④ The process for replenishing chemicals is the same as above and will not be detailed again.
(8) Nickel plating.
① Purpose and function: The nickel plating layer primarily serves as a barrier layer between the copper layer and the gold layer, preventing mutual diffusion of gold and copper, which could affect the solderability and service life of the board; additionally, the nickel layer underneath greatly increases the mechanical strength of the gold layer.
② Process parameters for full-panel copper electroplating: Nickel plating additives are generally replenished based on ampere-hours or according to actual production board results, with an addition rate of approximately 200 ml/KAH. The current for pattern plating nickel is typically calculated as 2 amps per square decimeter multiplied by the plate's electroplatable area. The nickel tank temperature should be maintained between 40 and 55°C.
③ Process maintenance: Replenish nickel plating additives daily based on ampere-hours. Check whether the filter pump is operating properly. Every 2 to 3 hours, wipe the cathode conductor rods clean with a clean damp cloth. Weekly, analyze and adjust the nickel plating additive content through Hull cell tests, and replenish as needed. Also weekly, clean the anode conductor rods and the electrical contacts at both ends of the tank, replenish the nickel anode angles in the titanium baskets, and perform low-current electrolysis for 6 to 8 hours. Monthly, inspect the anode titanium basket bags for damage and replace them promptly, and check whether anode sludge has accumulated at the bottom of the titanium baskets. Replace the filter pump cartridges every two weeks.
⑤ The process for adding chemicals is the same as described above and will not be repeated in detail.
(9) Gold electroplating: This is divided into hard gold and soft gold (water gold) processes. The bath compositions are essentially the same, except that the hard gold bath contains trace amounts of metals such as nickel, cobalt, and iron.
① Purpose and function: Gold is a precious metal with properties such as solderability, oxidation resistance, corrosion resistance, low contact resistance, and good alloy wear resistance.
② Currently, gold electroplating for PCBs primarily uses a citric acid-based gold bath, which is easy to maintain and simple to operate.
③ For soft gold (water gold), the gold content is controlled at approximately 1 g/L, the pH value at around 4.5, and the temperature at approximately 35°C.
④ The main chemicals added include acid adjustment salts, alkaline adjustment salts, conductive salts, gold plating replenishment additives, and gold salts.
⑤ After gold plating, the boards should be rinsed with pure water as a recovery rinse, which can also be used to replenish the tank level to compensate for evaporation losses.
⑥ The gold tank should use a platinum-plated titanium mesh as the anode.
⑦ For organic contamination in the gold plating tank, apply continuous filtration with a carbon core and replenish an appropriate amount of gold plating additives.