Plating Au PCB
At present, Plating Au PCB process, the gold plating process methods of various PCB factories can be divided into four types: chemical nickel gold, electroplated gold fingers, electroplated hard gold, and electroplated soft gold.
1. Plating Au PCB Basic Definition
Principle: The basic principle of electroplating: When metal is dissolved in water, it exists in the form of cation M+n. When a voltage is passed, the metal cation obtains n electrons to reduce to metal and precipitate at the cathode; when no additives are added, Metal crystals will grow in the vertical direction according to the current density.
Therefore, the purpose of adding additives is to make the atomic crystals grow in the horizontal direction, increase their conductivity, improve the current density distribution in the plated object, and improve the glossiness. It relies on other molecules in the additive to fill the gaps in the metal crystals to achieve surface smoothness.
Faraday’s law: The weight of metal precipitation can be calculated from the amount of electricity (time, amperage) and the atomic weight of the metal: W=ItA/ZF
I: Current amperage. Z: metal atomic valence (metal precipitation equivalent) t: energization time F: Faraday constant: 96500
A: Metal atomic weight
Example: Gold-plated 1ASD, deposited thickness in 1min
Weight = (current amperage × energization time × gold atomic weight) / (gold atomic valence × Faraday constant)
Weight × plating efficiency = actual weight
0.122g×45% (acid gold efficiency)=0.05g
Weight = area × thickness × density
0.05=10×10×thickness×16.5 (gold cobalt alloy density)
※Plating efficiency is calculated as 100%
2. Plating Au PCB Basic Process of Electroplating
Gold plating: Gold is an extremely inactive metal element. The main electrochemical reduction mechanisms are Au++e-Au, E=1.7V and Au3++3e-Au, E=1.5V. At present, gold plating usually uses two sources of gold ions such as gold potassium cyanide and gold sulfite, but the most common is gold potassium cyanide. Here, only the gold cyanide potassium reduction mechanism is used to illustrate the gold ion crystallization process: the gold plating process:
Chemical adsorption: Au(CN)2-(AuCN)ad + CN- ad: adsorption
Charge transfer: (AuCN)ad + e-(AuCN)ad-
PCB crystallization (cathode): Au(CN)ad-Au+CN-
Nickel plating: Although the main purpose of nickel plating is to prevent the mutual diffusion of gold and copper ions, the hardness and ductility of nickel also have a good effect on PCBs, so nickel plating has become a necessary process in the gold plating process. Nickel plating reaction mechanism:
Nickel sulfamate: Ni(NH2SO3)2Ni2++2NH2SO3
Nickel chloride: NiCl2Ni2+2Cl-
Anode nickel block: NiNi2+2e-
PCB surface (cathode): Ni2+2e-Ni
3. Plating Au PCB Functions of Main Coatings and Chemical Agents
Gold plating: The main function of the plating gold finger is to increase the wear resistance and hardness of the PCB circuit. Prevent oxidation of the copper layer and increase PCB life. The functions of GT-2080 chemical agents are:
#375 Cobalt gloss agent: The gold-plated layer is usually a matt brown powdery deposit, so when electroplating hard gold and gold fingers, usually a small amount of cobalt is mixed to increase the gloss, hardness, and wear resistance of the plating. #376 Additive: Prevent high current scorching and increase uniformity. #404 conductive salt: increase the specific gravity of the plating solution and increase the plating efficiency. #209Balanced salt: It is a buffer salt to balance the bath. #402 Adjust acid: reduce the PH value of the plating solution.
Potassium gold cyanide: mainly provides gold ions. Commonly known as gold salt, it will dissolve into gold complexions and potassium complexions in water. After being attracted by the ground field of the cathode, the surface of the cathode will have gold atoms attached. Nickel plating: The main purpose is to isolate the copper layer from diffusing to the metal surface to form an oxide layer, which affects the contact resistance of the gold plating layer.
However, the thickness of the nickel layer must be appropriate, because too high a thickness will increase the resistance, and too thin will also cause poor insulation. Nickel sulfamate and nickel chloride: Both are important components of nickel plating tanks, and are the main suppliers of nickel ions. The nickel ions lost in the solution due to the plating layer are dissolved by the nickel block on the anode. , To supplement the nickel ion concentration of these two solutions.
Nickel chloride can increase the conductivity in the plating solution and improve the efficiency of the plating solution, but when the amount is large, the plating layer is likely to be hard and brittle. Boric acid: Because the pH value of the nickel plating bath has a great influence on the quality of nickel plating, add boric acid to the bath as a pH buffer to avoid changes in the pH value. Ni-505 softener: also known as the primary gloss agent.
The main function is to make the nickel plating layer crystallize finely, make the high and low potential more uniform, and the plating layer is soft. Ni-505 gloss agent: The main function is to provide the gloss and leveling of the nickel plating. The amount of additional required is determined according to the actual needs of the site.
Do not add too much, it is easy to cause the coating to be hard, brittle, and peeling, and the low current is poor. Ni-505 wetting agent: Prevent pinholes caused by cathode hydrogen bubbles. Gold-plating process and its characteristics: electroplating process: sticking anti-plating tape → opening skylight → pressing tape → boarding (conveyor belt splint) → micro erosion → washing → brushing → activation → washing → nickel plating → washing → activation → washing → gold plating → Gold recycling → washing → unloading plate → peeling glue → washing → drying → paste soldering and spray tin tape
Features: Using direct current electroplating, the gold finger position of the PCB is plated with a gold layer containing cobalt metal. Since the conveyor belt clamp position is the other long side of the PCB when the PCB with gold fingers on both sides undergoes the plating gold finger process.
It is necessary to flow up and down through the plating tank twice. If the depth of the plating tank is 10″, the distance between the gold finger and the edge of the PANEL board cannot exceed 9.3″ when designing the PCB layout. Dummy Board: It can be divided into two purposes: removing impurities in the nickel plating bath and preventing the plating area from being uniform:
Removal of impurities in the nickel plating bath: the bath usually contains some plating impurities that are not needed by the PCB. To prevent these impurities from being plated on the PCB, some scrap boards are usually used for electroplating with a lower current before use.
Impurities are plated on the scrap board to ensure the quality of the PCB coating. This type of scrap board is called a dummy board. Prevent uneven plating area: During the golden finger manufacturing process, the PCB travel is carried by the conveyor belt.
In order to prevent the front and back of the PCB from being electroplated and the normal plating area is different, a larger current will flow through the line, causing the plating layer to be burnt or too thick, usually in the front and back of the formal production PCB.
Put a few scrap boards to solve this problem. These scrap boards are also called Dummy Boards. In addition, normal gold fingers are also designed with non-functional gold fingers on the front and rear ends. The effect is to prevent the uneven thickness of the gold finger area.
This gold finger is called Dummy Finger. Electroplating layer thickness control: In the electroplating process, because the current density applied on both sides of the bath can be used, different plating thicknesses can also be produced on both sides of the PCB. (The greater the current density, the faster the plating rate, and the quality also have an impact.
When the current density is too high, it may cause scorching of the plating area or too large crystal particles). Gold plating process specification: gold plating process and plating solution control range:
4.Plating Au PCB Replenishment Management of Plating Solution
Precautions and maintenance of the gold-plated machine: Precautions: For each batch of PCBs with different material numbers, 6-10 pieces should be tested first to determine the required current, voltage, machine speed, and bath temperature.
Check the thickness and tensile test (tested with 3M company’s MN55144 tape), and check whether there is nickel-gold peeling. Check the brush wheel pressure and wheel brush range. Before the operation, inspect the tank liquid level, whether the motor is running normally, and check whether the rubber and PE plastic water-retaining blades in each tank are intact and free of damage.
Check whether the overflow flow of nickel plating and gold tanks is proper and even. Whether the tightness of the cathode conductive copper brush is normal (need to overlap by 1-2mm), the conductive copper brush must not be loose to avoid contamination of the plating solution. Maintenance: Machine and tank maintenance: weekly maintenance, monthly maintenance.
5. Plating Au PCB Weekly Maintenance Items
Wipe and clean the machine and the outside of the tank. Check whether the rubber and PE plastic water-proof wiper between each slot is intact and not damaged or deformed, and update it if necessary. Replenish nickel blocks (S-Ni blocks need to be soaked in 5% sulfuric acid first). Check the filter element and filter screen, and replace it once a week. (The specification of the filter element is 1-5μm) Check whether the overflow flow of the nickel plating and gold tanks is appropriate and uniform.
Is the motor running normally? Whether the temperature control system such as heater and condenser is normal. Whether the tightness of the cathode conductive copper brush is uniform, whether the contact conductivity is normal, and whether the copper brush is loose. Check whether the current, voltage, temperature and machine tachometer are normal. Monthly maintenance items:
Includes all matters of weekly maintenance. Testing and maintenance of electromechanical products. If you change the tank, you can clean the inner tank and anode by the way. (The anode block of the nickel tank can be cleaned with 5% sulfuric acid.
The platinum mesh of the gold tank can be cleaned with 5% KOH. The analysis method of the plating solution: nickel tank analysis method: nickel metal: preparation: ammonia water MX indicator 0.05M EDTA (take 18.8 grams dissolved in 1 liter of pure water) Operation: Take 1ml of the sample in a 250ml Erlenmeyer flask, add about 150ml of water. Add 5-10ml of concentrated ammonia. Add 0.1-0.3g of MX indicator. Titrate with 0.05M EDTA until the endpoint is purple. .Calculation: Nickel metal g/L=titration ml×2.9342
Nickel chloride: Preparation: 5% potassium dichromate indicator. 0.1N silver nitrate (take 17.02 g dissolved in 1 liter of pure water). Operation: take 5 ml sample in a 250 ml conical flask. Add 5% potassium dichromate Indicator 1ml. Titration with 0.1N silver nitrate from green to brown as the endpoint. Calculation: Nickel chloride g/L=titration ml×2.38
Nickel sulfate and nickel sulfamate: calculation: nickel sulfate g/L=(nickel-metal g/L-(nickel chloride g/L×0.2463))×4.475
Nickel sulfamate g/L= [nickel metal g/L-(nickel chloride g/L×0.2463)]÷0.1818
Boric acid: preparation: mannitol BCP indicator 0.2N NaOH (take 8.6 g dissolved in 1 liter of water). Operation: take 2 ml of the sample in a 250 ml conical flask, add 100 ml of pure water, add 2 g of mannitol, add BCP indicator 2 -3 drops. Titration with 0.2N NaOH from green to light blue as the endpoint. Calculation: Boric acid g/L=titration ml×6.184
§Hastelloy experiment operating conditions of Plating Au PCB
Acidic fast gold tank analysis method: Gold content: Preparation: concentrated sulfuric acid, concentrated nitric acid, hydrogen peroxide, high-temperature furnace (1000°C), operation: take 10ml sample in an Erlenmeyer flask. Add 5ml concentrated nitric acid and 15ml concentrated sulfuric acid. Heat for 10-15 minutes. Cooldown To room temperature, add 200ml of pure water. Add 40ml of hydrogen peroxide (diluted 10 times). Boil until the solution is double clear, about 5-10 minutes. Cool to room temperature, filter with ash-free filter paper, wash with pure water once after filtration, then wash with acetone Once. Weigh the gangue pot T1. Put the filter paper into the gangue pot and heat it for 2 hours (900℃). Cool to room temperature and weigh T2. Calculation: Gold content g/L=(T1 -T2)×100
※The gold content can also be analyzed by A.An instrument.
6. Plating Au PCB Cobalt Content: Analyze by A.An Instrument
#209 Balanced salt: Preparation: Calcium chloride solution (20% W/V). Potassium permanganate solution N/10. Sulfuric acid. Operation: Take 2ml of the sample into a 1000ml beaker, add 800ml of pure water. Add 10ml of chlorination Calcium solution. Heat to 60-70℃ for 30-60 minutes. Filter the precipitate and wash it several times with pure water. Put the filter paper with the precipitate into a beaker and add 50ml of 30% sulfuric acid. Heat to 70℃ to N/10 potassium permanganate solution is titrated to the endpoint. Calculation: #209g/L=titration ml×4.725