PCB News - Talking about the manufacturing process of PCB board

The PCB manufacturing process starts with a "substrate" made of glass epoxy (Glass Epoxy) or similar materials
Image (forming/wire making)

The first step is to establish the wiring between the parts. We use Subtractive Transfer to express the working film on the metal conductor. This technique is to spread a thin layer of copper foil on the entire surface and eliminate the excess. Additive pattern transfer is another method that less people use. It is a method of laying copper wires only where needed, but we won't talk about it here.
If you are making a double-sided board, then both sides of the PCB substrate will be covered with copper foil. If you are making a multi-layer board, the next step will glue these boards together.
The following flow chart describes how the wires are soldered on the substrate.
Positive photoresist is made of sensitizer, which will dissolve under lighting (negative photoresist will decompose if it is not illuminated). There are many ways to treat the photoresist on the copper surface, but the most common way is to heat it and roll it on the surface containing the photoresist (called dry film photoresist). It can also be sprayed on the head in a liquid way, but the dry film type provides higher resolution and can also produce thinner wires.
The hood is just a template for the PCB layer in manufacturing. Before the photoresist on the PCB board is exposed to UV light, the light shield covering it can prevent some areas of the photoresist from being exposed (assuming a positive photoresist is used). These areas covered by photoresist will become wiring.
After the photoresist is developed, the other bare copper parts to be etched. The etching process can immerse the board in the etching solvent or spray the solvent on the board. Generally used as etching solvents are ferric chloride (Ferric Chloride), alkaline ammonia (Alkaline Ammonia), sulfuric acid plus hydrogen peroxide (Sulfuric Acid + Hydrogen Peroxide), and copper chloride (Cupric Chloride). After the etching, the remaining photoresist is removed. This is called a stripping process.
Drilling and plating
If you are making a multi-layer PCB with buried or blind holes, each layer of the board must be drilled and electroplated before bonding. If you don't go through this step, then there is no way to connect to each other.
After drilling by the machine according to the drilling requirements, the inside of the hole must be electroplated (Plated-Through-Hole technology, PTH). After the inside of Kongbi is metal treated, the inner layers of the circuit can be connected to each other. Before starting electroplating, the debris in the hole must be cleaned. This is because the resin epoxy will produce some chemical changes after heating, and it will cover the internal PCB layer, so it must be removed first. Both the cleaning and electroplating actions are completed in the chemical process.
Multi-layer PCB pressing
Each single layer must be pressed together to make a multilayer board. The pressing action includes adding an insulating layer between the layers and bonding each other firmly. If there are vias through several layers, each layer must be processed repeatedly. The wiring on the outer sides of the multilayer board is usually processed after the multilayer board is laminated.
Process solder mask, screen printing surface and gold finger part plating
Next, cover the solder mask on the outermost wiring, so that the wiring will not touch the plating part. The screen printing surface is printed on it to mark the position of each part. It cannot cover any wiring or gold fingers, otherwise it may reduce the solderability or the stability of the current connection. The gold finger part is usually plated with gold, so that high-quality current connection can be ensured when the expansion slot is inserted.
test
To test whether the PCB has a short circuit or open circuit, you can use optical or electronic testing. The optical method uses scanning to find the defects in each layer, and the electronic test usually uses a flying-probe to check all connections. Electronic testing is more accurate in finding short circuits or open circuits, but optical testing can more easily detect incorrect gaps between conductors.
Parts installation and welding
The last step is to install and weld the parts. Both THT and SMT parts are installed on the PCB using machinery and equipment.
THT parts are usually soldered by a method called Wave Soldering. This allows all parts to be soldered to the PCB at once. First cut the pins close to the board and bend them slightly to allow the parts to be fixed. Then move the PCB to the water wave of the co-solvent, so that the bottom is in contact with the co-solvent, so that the oxide on the bottom metal can be removed. After heating the PCB, this time it is moved to the molten solder, and the soldering is completed after contact with the bottom.
The method of automatically welding SMT parts is called Over Reflow Soldering. The paste solder containing flux and solder is processed once after the parts are mounted on the PCB, and then processed again after the PCB is heated. After the PCB has cooled down, the soldering is completed, and the next step is to prepare for the final test of the PCB.