PCB process flow
1.Cutting (CUT)
Cutting is the process of cutting the original copper clad board into boards that can be made on the production line
First, let us understand a few concepts:
(1) UNIT: UNIT refers to the unit pattern designed by the PCB design engineer.
(2) SET: SET refers to the pattern that engineers put together multiple UNITs to improve production efficiency and facilitate production. It is also what we often call a panel, which includes unit patterns, process edges, etc.
(3) PANEL: PANEL refers to a board composed of multiple SETs put together and added with tool board edges when the PCB manufacturer produces in order to improve efficiency and facilitate production.
2.Inner dry film
Inner dry film is the process of transferring the inner line pattern to the PCB board.
In PCB production, we will mention the concept of pattern transfer, because the production of conductive patterns is the foundation of PCB production. Therefore, the pattern transfer process is of great significance to PCB production.
Inner dry film includes multiple processes such as inner film lamination, exposure and development, and inner etching. The inner layer film is to stick a special photosensitive film on the surface of the copper plate, which is what we call dry film.
This film will solidify when exposed to light, forming a protective film on the board. Exposure and development is to expose the board with the film, and the light-transmitting part will be solidified, while the non-light-transmitting part will remain as dry film. Then after development, the uncured dry film will be removed, and the board with the cured protective film will be etched. After the film is removed, the inner layer circuit pattern will be transferred to the board. The whole process flow is shown in the figure below.
For designers, our main considerations are the minimum line width, spacing control and wiring uniformity of the wiring.
Because too small spacing will cause film sandwiching, the film cannot be completely removed and cause short circuit. If the line width is too small, the adhesion of the film is insufficient, causing the line to open. Therefore, the safety spacing during circuit design (including line and line, line and pad, pad and pad, line and copper surface, etc.) must be considered during production.
(1) Pretreatment: grinding plate
The main function of grinding plate: basic pretreatment is mainly to solve the problems of surface cleanliness and surface roughness. Remove oxidation and increase the roughness of the copper surface to facilitate film adhesion on the copper surface.
(2) Film pasting
The processed substrate is pasted with a dry film or wet film by hot pressing or coating to facilitate subsequent exposure production.
(3) Exposure
Align the film with the substrate with the pressed dry film, and use ultraviolet light on the exposure machine to transfer the film pattern to the photosensitive dry film.
Actual picture of the film
(4) Development
Use the weak alkalinity of the developer (sodium carbonate) to dissolve and wash away the unexposed dry film/wet film, and retain the exposed part.
(5) Etching
After the unexposed dry film/wet film is removed by the developer, the copper surface will be exposed. Use acidic copper chloride to dissolve and etch this exposed copper surface to obtain the required circuit.
(6) Film stripping
The exposed dry film protecting the copper surface is peeled off with sodium hydroxide solution to expose the circuit pattern.
3.Browning
Purpose: To form microscopic roughness and organic metal layer on the inner copper surface to enhance the bonding strength between layers.
Process principle:
Through chemical treatment, a uniform organic metal layer structure with good bonding properties is produced, so that the surface of the copper layer before the inner layer is bonded is controlled to be coarsened, which is used to enhance the bonding strength between the inner copper layer and the semi-cured sheet after pressing.
4.Lamination
Lamination is the process of bonding each layer of circuits into a whole by means of the adhesion of the PP sheet. This bonding is achieved through the mutual diffusion and penetration between macromolecules on the interface, and then the mutual interweaving is achieved, and the discrete multi-layer board is pressed together with the PP sheet into a multi-layer board with the required number of layers and thickness. In actual operation, copper foil, bonding sheet (semi-cured sheet), inner layer board, stainless steel, isolation board, kraft paper, outer steel plate and other materials are overlapped according to process requirements.
For designers, the first thing to consider in lamination is symmetry.
Because the board will be affected by pressure and temperature during the lamination process, there is still stress in the board after the lamination is completed. Therefore, if the two sides of the laminated board are not uniform, the stress on the two sides will be different, causing the board to bend to one side, greatly affecting the performance of the PCB.
In addition, even on the same plane, if the copper distribution is uneven, the resin flow rate at each point will be different, so that the thickness of the area with less copper will be slightly thinner, and the thickness of the area with more copper will be slightly thicker.
In order to avoid these problems, various factors such as the uniformity of copper distribution, the symmetry of the stacking, the design and layout of blind and buried holes, etc. must be considered in detail during the design.
5.Drilling
Create through holes between the layers of the circuit board to achieve the purpose of connecting the layers.
The legendary drill
6.Copper plating
(1). Copper plating
Also called chemical copper, the PCB board after drilling undergoes an oxidation-reduction reaction in the copper plating tank to form a copper layer to metallize the holes, so that copper is deposited on the surface of the original insulating substrate to achieve electrical connectivity between the layers.
(2) Board plating
The copper on the board and inside the hole of the newly copper-plated PCB is thickened to 5-8um to prevent the thin copper in the hole from being oxidized and micro-etched before the pattern electroplating, which may leak the substrate.
- Outer dry film
The process is the same as that of the inner dry film. - Outer pattern electroplating, SES
The hole and line copper layer are plated to a certain thickness (20-25um) to meet the copper thickness requirements of the final PCB board. And the unused copper on the board is etched away to expose the useful line pattern. - Solder mask
Solder mask, also known as solder mask or green oil, is one of the most critical processes in the production of printed circuit boards. It is mainly applied by screen printing or coating solder mask ink to apply a layer of solder mask on the board surface. Through exposure and development, the pads and holes to be soldered are exposed, and the solder mask is covered in other places to prevent short circuits during welding. - Silk screen characters
The required text, trademark or part symbol is printed on the board surface by screen printing, and then exposed to ultraviolet light. - Surface treatment
Bare copper itself has good solderability, but it is easily oxidized by moisture when exposed to air for a long time. It tends to exist in the form of oxides and is unlikely to remain as original copper for a long time. Therefore, the copper surface needs to be surface treated. The most basic purpose of surface treatment is to ensure good solderability or electrical properties.
Common surface treatments: tin spraying, immersion gold, OSP, immersion tin, immersion silver, nickel-palladium gold, electric hard gold, electric gold fingers, etc.
- Molding
The PCB is cut into the required dimensions by a CNC molding machine. - Electrical testing
The state of the analog board is checked for electrical performance after power is turned on to see if there is an open or short circuit. - Final inspection, sampling, and packaging
The appearance, size, aperture, board thickness, marking, etc. of the board are checked to meet customer requirements. Qualified products are packaged into bundles for easy storage and transportation.