The manufacturing process of PCB begins with a "substrate" made of glass epoxy or similar materials.
Image (forming/thread making)
The first step in manufacturing is to establish the connection between parts. We use negative transfer to show the working film on the metal conductor. This technology is to spread a thin layer of copper foil on the whole surface to eliminate the redundant parts. Additional pattern transfer is another rarely used method. It is a method of laying copper wire only where necessary, so I won't talk about it here.
If it is a double panel, both sides of the PCB substrate should be covered with copper foil. If multi-layer boards are made, these boards will be bonded together in the next step.
The next flow chart introduces how the wires are soldered on the substrate.
Image (forming/thread making), continued
Positive photoresist is made of photosensitizer and will dissolve under light (negative photoresist is not only decomposed). There are many methods to treat photoresist on copper surface, but the most common method is to roll it on the surface containing photoresist after heating (called dry film photoresist). It can also be sprayed on it in liquid form, but the dry film type provides higher resolution and can also make thinner wires.
The hood is only a template for PCB layer in manufacturing. Before the photoresist on PCB is exposed to ultraviolet light, the photomask covering it can prevent the photoresist in some areas from being exposed (assuming that positive photoresist is used). These areas covered with photoresist will become wiring.
Other bare copper parts to be etched after photoresist development. During the etching process, the plate can be immersed in the etching solvent or sprayed with the solvent. Commonly used as etching solvents are ferric chloride, basic ammonia, sulfuric acid plus hydrogen peroxide and copper chloride. After etching, the remaining photoresist is removed. This is called the stripping procedure.
You can see how the copper wire is wired from the picture below.
This step can be used to route wires on both sides at the same time.
Drilling and electroplating
If a multi-layer PCB is made and it contains buried holes or blind holes, each layer of PCB must be drilled and electroplated before bonding. If you don't go through this step, then there is no way to contact each other.
After the machines and equipment are drilled according to the drilling requirements, the inner side of the hole wall must be electroplated (PTH). After metal treatment in the hole wall, the inner circuits can be connected with each other. Impurities in the holes must be removed before electroplating. This is because the epoxy resin will produce some chemical changes after heating, which will cover the internal PCB layer, so it should be removed first. Cleaning and electroplating actions will be completed in the chemical process.
Multilayer printed circuit board pressing
Each layer must be pressed into a multilayer board. The pressing action includes adding insulating layers between layers and sticking them together. If there are several layers of through holes, each layer must be treated repeatedly. The wiring on the outer two sides of the multilayer board is usually processed after the multilayer board is pressed.
Treatment of solder resist, screen printing surface and gold finger electroplating
Next, cover the outermost wiring with solder resist paint so that the wiring will not contact the electroplated part. Screen printing is printed on it, indicating the position of each part. It cannot cover any wiring or gold fingers, otherwise it may reduce solderability or the stability of current connection. Gold fingers are generally gold-plated to ensure high-quality current connection when inserted into the expansion slot.
test
Test PCB for short circuit or open circuit, which can be tested by optical or electronic means. Optical scanning is used to find out the defects of each layer, while electronic testing usually uses flying needles to check all connections. Electronic testing is more accurate in finding short circuits or open circuits, but optical testing can more easily detect the problem of incorrect gaps between conductors.
Parts installation and welding
The last step is to install and weld the parts. THT and SMT parts are installed and placed on PCB by machines and equipment.
THT parts are usually welded by a method called wave soldering. This allows all devices to be soldered to the PCB at the same time. First, cut the pins near the circuit board and bend them slightly so that the parts can be fixed. Then, the PCB is moved into the water wave of the cosolvent, so that the bottom touches the cosolvent, so that the oxide on the bottom metal can be removed. After heating the PCB, it is moved to the molten solder this time, and the welding is completed after touching the bottom.
Automatic soldering of SMT parts is called reflow soldering. Solder paste containing cosolvent and solder is treated once after the parts are mounted on PCB, and then treated again after the PCB is heated. After the PCB is cooled, the welding is completed, and then the PCB is ready for final testing.