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A detailed explanation of the production technology and process of PCB with pictures and text. Interested friends should not miss it.
1. Cutting (CUT)
Cutting is the process of cutting raw copper-clad laminates into boards that can be produced on the production line
Firstly, let's understand a few concepts:
(1) UNIT: UNIT refers to the unit graphics designed by PCB design engineers.
(2) SET: SET refers to the process by which engineers assemble multiple UNITs into a cohesive graphic for the purpose of improving production efficiency, facilitating production, and other reasons. That is, what we often refer to as splicing, which includes unit graphics, process edges, and so on.
(3) PANEL: PANEL refers to a board formed by PCB manufacturers by assembling multiple SET together and adding tool board edges to improve efficiency and facilitate production.
2. Inner Dry Film
Inner layer dry film is the process of transferring the inner layer circuit pattern onto the PCB board.
In PCB production, we will mention the concept of graphic transfer because the production of conductive graphics is fundamental to PCB production. So the process of graphic transfer is of great significance for PCB production.
The inner layer dry film includes multiple processes such as inner layer coating, exposure and development, and inner layer etching. The inner layer film is a special photosensitive film that is applied to the surface of a copper plate, which is what we call a dry film. This type of film will solidify when exposed to light, forming a protective film on the board. Exposure development is the process of exposing a film coated board, curing the transparent parts and leaving the opaque parts as dry film. Then, after development, the uncured dry film is removed, and the board with the cured protective film is etched. After further film stripping treatment, the inner layer circuit pattern is transferred to the board. The entire process flow is shown in the following figure.
For designers, our main considerations are the minimum line width, spacing control, and uniformity of wiring. Because the spacing is too small, it can cause film clamping, and the film cannot be completely removed, resulting in a short circuit. The line width is too small, and the adhesion of the film is insufficient, causing an open circuit in the circuit. So the safety spacing during circuit design (including wire to wire, wire to pad, pad to pad, wire to copper surface, etc.) must consider the safety spacing during production.
(1) Pre treatment: Grinding plate
The main function of grinding plate is to solve the problems of surface cleanliness and surface roughness through basic pre-treatment. Remove oxidation and increase the roughness of the copper surface, making it easier for the film to adhere to the copper surface.
(2) Apply film
Paste dry or wet film onto the processed substrate through hot pressing or coated pipes for subsequent exposure production.
(3) Exposure
Align the film with the substrate on which the dry film has been pressed, and transfer the film pattern onto the photosensitive dry film using ultraviolet light irradiation on the exposure machine.
(4) Development
Dissolve and rinse off the unexposed dry/wet film using the weak alkalinity of the developing solution (sodium carbonate), while retaining the exposed portion.
(5) Etching
After the unexposed dry/wet film is removed by the developer, the copper surface will be exposed. Acidic copper chloride is used to dissolve and corrode this exposed copper surface to obtain the desired circuit.
(6) Peel off film
Peel off the exposed dry film protecting the copper surface with sodium hydroxide solution to expose the circuit pattern.
3. Browning
Purpose: To create a microscopic roughness and organic metal layer on the inner copper surface, enhancing the adhesion between layers.
Process principle: By chemical treatment, a uniform and well adhesive organic metal layer structure is generated, which controls the roughening of the copper layer surface before the inner layer is bonded, and is used to enhance the bonding strength between the inner copper layer and the semi cured sheet after pressing.
4. Lamination
Layering is the process of using the adhesive properties of PP sheets to bond the various layers of circuits into a whole. This bonding is achieved through the mutual diffusion and permeation of macromolecules at the interface, resulting in interweaving and pressing of discrete multilayer boards and PP sheets into the required number of layers and thickness of multilayer boards. In actual operation, the copper foil, bonding sheet (semi cured sheet), inner layer board, stainless steel, isolation board, kraft paper, outer steel plate and other data are stacked according to the process requirements.
For designers, the first thing to consider when laminating is symmetry. Because the board is affected by pressure and temperature during the lamination process, there is still stress inside the board after lamination is completed. Therefore, if the laminated board is uneven on both sides, the stress on both sides will be different, causing the board to bend towards one side, greatly affecting the PCB performance.
In addition, even on the same plane, if the distribution of copper is uneven, it will cause different resin flow velocities at each point, resulting in slightly thinner thickness in areas with less copper and slightly thicker thickness in areas with more copper.
In order to avoid these problems, detailed consideration must be given to various factors such as the uniformity of copper distribution, the symmetry of stacking, and the design and layout of blind buried holes during the design process.
5. Drilling hole
Create through holes between PCB layers to achieve the purpose of connecting layers.
6. Copper plating
(1) Sinking copper
Also known as chemical copper, the PCB board after drilling undergoes oxidation-reduction reactions in the copper immersion cylinder, forming a copper layer to metalize the holes, depositing copper on the surface of the originally insulated substrate, achieving interlayer electrical communication.
(2) Plate plating
Thicken the surface and holes of the PCB board that has just been copper plated to 5-8um to prevent the thin copper in the holes from being oxidized and micro corroded before graphic electroplating, which may leak the substrate.
7. Outer dry film
The process is the same as that of the inner dry film.
8. Outer graphic electroplating, SES
Coat the holes and circuit copper layers to a certain thickness (20-25um) to meet the copper thickness requirements of the final PCB board product. And etch away the unused copper on the board to expose useful circuit patterns.
9. 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 achieved through screen printing or coating with solder mask ink, applying a layer of solder mask on the board surface, exposing the disc and hole to be soldered through exposure and development, and covering other areas with solder mask to prevent short circuits during soldering
10. Silk screen characters
Print the required text, trademark, or part symbol onto the board surface using a screen printed pipeline, and then expose the pipeline to ultraviolet radiation on the board surface.
11. Surface treatment
Bare copper itself has good weldability, but it is prone to moisture and oxidation when exposed to air for a long time. It tends to exist in the form of oxides and is unlikely to remain as the original copper for a long time. Therefore, surface treatment of the copper surface is necessary. The most basic purpose of surface treatment is to ensure good weldability or electrical performance.
Common surface treatments: tin spraying, immersion gold, OSP, immersion tin, immersion silver, nickel palladium gold, electroplated gold, electroplated gold fingers, etc.
12. Forming
Cut the PCB into the required external dimensions using a CNC molding machine.
13. Electrical measurement
Simulate the status of the board, conduct electrical performance checks when powered on, and check for any open or short circuits.
14. Final inspection, sampling, packaging
Inspect the appearance, size, aperture, thickness, and markings of the board to meet customer requirements. Pack qualified products into bundles for easy storage and transportation.
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