What are the classifications of PCB electroplating processes?
Acid bright copper electroplating
- Process principle and technical characteristics
Acid bright copper electroplating uses a sulfate system electrolyte with copper sulfate as the main salt. By adding brighteners, leveling agents and other additives, copper ions are reduced and deposited at the cathode (PCB copper foil surface). This process has the following technical advantages:
Ultra-high conductivity: the purity of the deposited copper layer is > 99.9%, and the resistivity is as low as 1.72μΩ・cm, which is close to the theoretical value of pure copper
Excellent surface quality: Through the regulation of brightener, a mirror-level flat surface of Ra≤0.2μm can be obtained
Strong bonding: The bonding strength with the substrate copper foil is ≥5N/mm, which meets the mechanical stress requirements of high-density circuit boards
- Application scenarios
Inner/outer layer circuit thickening: Based on the original copper foil of 18-35μm, it can be electroplated to a thickness of 50-100μm to meet the needs of large current transmission
Through-hole interconnection reinforcement: Uniform plating is achieved for micro-holes with a diameter of less than 0.3mm, and the thickness difference of the copper layer in the hole is ≤15%
Precision circuit forming: Supports fine circuit plating with a line width/line spacing of ≤50μm, and the verticality of the plating edge is ≥85°
Nickel / Gold electroplating
- Double-layer structure synergistic effect
This process adopts a composite plating system of "electroplated nickel + electroplated gold":
Bottom nickel plating: thickness 5-15μm, usually using nickel sulfamate system to form a dense corrosion-resistant barrier with a hardness of 200-300HV
Surface gold plating: thickness 0.05-5μm, divided into acidic gold plating (suitable for decorative surfaces) and neutral gold plating (suitable for functional contact points), gold layer purity ≥99.95%
- Application areas
High-frequency signal transmission: The extremely low contact resistance (2μm) can withstand high temperature environments above 300℃, meeting the welding requirements of power devices such as IGBT
What are the PCB electroplating process flows?
Acid dipping → Full-board copper plating → Pattern transfer → Acid degreasing → Secondary countercurrent rinsing → Microetching → Secondary → Acid dipping → Tin plating → Secondary countercurrent rinsing
Countercurrent rinsing → Acid dipping → Pattern copper plating → Secondary countercurrent rinsing → Nickel plating → Secondary water washing → Citric acid dipping → Gold plating → Recycling → 2-3 level pure water washing → Drying
PCB electroplating process flow description
(I) Acid dipping
① Function and purpose:
Remove board surface oxides and activate board surface. The general concentration is 5%, and some are kept at around 10%. The main purpose is to prevent moisture from entering and causing unstable sulfuric acid content in the bath liquid;
② The acid dipping time should not be too long to prevent board surface oxidation; 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 copper plating cylinder and the surface of the board;
③ C, P grade sulfuric acid should be used here:
(II) Full-board copper plating: also called primary copper, panel-plating, Panel-plating
①Function and purpose:
Protect the thin chemical copper just deposited, prevent the chemical copper from being oxidized and corroded by acid, and add it to a certain extent through electroplating
②Related process parameters of full-board electroplating copper:
The main components of the bath are copper sulfate and sulfuric acid. A high-acid and low-copper formula is used to ensure the uniformity of the thickness distribution of the board surface during electroplating and the deep plating ability of deep holes and small holes; the sulfuric acid content is mostly 180 g/L, and the maximum is 240 g/L; the copper sulfate content is generally around 75 g/L, and a trace amount of ammonium ions is added to the bath as an auxiliary gloss agent and copper light agent to jointly exert the gloss effect: the amount of copper light agent added or the amount of opening the cylinder is generally 3-5 ml/L, and the addition of copper light agent is generally supplemented according to the kiloampere-hour method or according to the actual production board effect; the current calculation of the full-board electroplating is generally 2 amperes/square decimeter multiplied by the electroplatable area on the board. For the full-board current, the board length dm×board width dm×2×2A/DM2; The temperature of the copper cylinder is maintained at room temperature, generally not exceeding 32 degrees, and mostly controlled at 22 degrees. Therefore, in summer, due to the high temperature, it is recommended to install a cooling temperature control system for the copper cylinder;
③ Process maintenance:
Replenish the copper light agent in time according to the kiloampere-hour every day, and add it at 100-150ml/KAH; Check whether the filter pump is working properly. Whether there is any air leakage; Use a clean wet rag to clean the cathode conductive rod every 2-3 hours; Regularly analyze the copper sulfate (1 time/week), sulfuric acid (1 time/week), and chloride ion (2 times/week) content of the copper cylinder every week, and adjust the light agent content through the Hall cell test. And replenish relevant raw materials in time; Clean the anode conductive rod and the electrical connectors at both ends of the tank every week. Replenish the anode copper balls in the titanium basket in time, and use low current 0.2-0.5ASD for electrolysis for 6-8 hours; check the titanium basket bag of the anode for damage every month, and replace it in time if it is damaged; check whether there is anode mud accumulated at the bottom of the anode titanium basket, if so, clean it up in time; and use carbon core to filter continuously for 6-8 hours, and remove impurities by low current electrolysis at the same time: every six months or so, decide whether a large treatment (activated carbon powder) is needed according to the pollution status of the tank liquid; replace the filter element of the filter pump every two weeks;
④ Large treatment procedure:
A. Take out the anode, pour out the anode, clean the anode film on the surface of the anode, and then put it in the barrel of the copper anode packaging, use micro-etching agent to roughen the surface of the copper corner until it is uniformly pink, rinse with water and put it into the titanium basket, and then put it into the acid tank for standby.
B. Soak the anode titanium basket and anode bag in 10% alkali solution for 6-8 hours, rinse with water, and then soak in 5% dilute sulfuric acid, rinse with water and set aside;
C. Transfer the tank liquid to the spare tank, add 1-3ml/L of 30% hydrogen peroxide, start heating, turn on the air stirring when the temperature reaches about 65 degrees, and keep the air stirring for 2-4 hours;
D. Turn off the air stirring, slowly dissolve the activated carbon powder into the tank liquid at 3-5 grams/liter, and turn on the air stirring after it is completely dissolved, and keep it warm for 2-4 hours;
E. Turn off the air stirring, heat, and let the activated carbon powder slowly settle to the bottom of the tank;
F. When the temperature drops to about 40 degrees, use a 10um PP filter element and filter powder to filter the tank liquid into a clean working tank, turn on the air stirring, put in the anode, and hang it in the electrolytic plate. 5ASD current density low current electrolysis for 6-8 hours.
G. After laboratory analysis, adjust the content of sulphuric acid, copper sulfate and chloride ions in the tank to the normal operating range; add the light agent according to the results of the Hall cell test; H. When the color of the electrolytic plate surface is uniform, stop the electrolysis. Perform electrolytic film treatment at a current density of 5ASD for 1-2 hours, and wait until a uniform, dense and well-adhesive black phosphorus film is formed on the anode; the positive plating test is OK. The anode copper ball contains 0.3-0.6% phosphorus, the main purpose is to reduce the anode dissolution efficiency and reduce the generation of copper powder.
When supplementing drugs, if the amount added is large, such as copper sulfate and sulfuric acid; low current electrolysis should be performed after adding; safety should be paid attention to when adding sulfuric acid, and when the amount added is large (more than 10 liters), it should be added slowly in several times; otherwise, the tank liquid temperature will be too high, the light agent will decompose faster, and the tank liquid will be polluted.
Special attention should be paid to the addition of chloride ions, because the chloride ion content is particularly low (30-90ppm). When adding, it must be accurately weighed with a measuring cylinder or measuring cup before adding; 1ml hydrochloric acid contains about 385ppm of chloride ions. The formula for adding drugs is copper sulfate (unit: kg) = (75-X) × tank volume (liter) / 1000 sulfuric acid (unit: liter) = (10%-X) g/L × tank volume (liter) or (unit: liter) = (180-X) g/L × tank volume (liter) / 1840 hydrochloric acid (unit: ml) = (60-X) ppm × tank volume (liter) / 385
(III) Acid degreasing
① Purpose and function: Remove oxides on the copper surface of the circuit, residual ink film and glue, and ensure the bonding strength between the primary copper and the electroplated copper or nickel of the pattern.
②Remember that an acid degreaser is used here. Why not an alkaline degreaser and why is the degreasing effect of an alkaline degreaser better than that of an acid degreaser? This is mainly because graphic inks are not alkali-resistant and will damage graphic circuits, so only an acid degreaser can be used before graphic electroplating.
③ During production, it is only necessary to control the degreasing agent concentration and time. The degreasing agent concentration is about 10%, and the time is guaranteed to be 6 minutes. A longer time will not have adverse effects; the use and replacement of the tank liquid is also based on 15 square meters/liter of working liquid, and the supplementary addition is based on 0.5-0.8L per 100 square meters;
(IV) Micro-etching
① Purpose and function: Clean and roughen the copper surface of the circuit to ensure the bonding strength between the electroplated copper of the pattern and the primary copper;
② Micro-etching agents mostly use sodium persulfate, the roughening rate is stable and uniform, and the water washing property is good. The sodium persulfate concentration is generally controlled at about 60 grams/liter, the time is controlled at about 20 seconds, and the chemical addition is 3-4 kilograms per 100 square meters; the copper content is controlled below 20 grams/liter; other maintenance and tank replacement are the same as copper micro-etching.
(V) Acid Dipping
① Function and Purpose Remove oxides from the board surface and activate the board surface. The general concentration is 5%, and some are kept at around 10%. The main purpose is to prevent moisture from entering the tank and causing unstable sulfuric acid content;
② The acid dipping 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 is too high, it should be replaced in time to prevent contamination of the electroplating copper tank and the board surface;
③ CP grade sulfuric acid should be used here.
Conclusion
PCB electroplating processes include acid bright copper, nickel/gold electroplating. The flow involves acid dipping, full-board copper plating, pattern copper plating, etc.
South-Electronic excels in PCB electroplating with high-efficiency processes like acid bright copper and nickel/gold plating, suitable for high-frequency and precision circuits.
