PCB Tech - PCB printed circuit board detailed process introduction

PCB detailed process introduction:

1. Cutting (CUT)

Cutting is the process of cutting the original copper clad laminate into a board that can be made on the production line

First, let's understand a few concepts:

(1) UNIT: UNIT refers to the unit graphics designed by PCB design engineers.

(2) SET: SET refers to a graphic that engineers put together a number of UNITs for reasons such as improving production efficiency and facilitating production. This is what we often call the puzzle, which includes unit graphics, process edges, and so on.

(3) PANEL: PANEL refers to a board formed by putting together multiple SETs and adding tool board edges during the production of PCB manufacturers in order to improve efficiency and facilitate production.

2. INNER DRY FILM

The inner layer dry film is the process of transferring the inner layer circuit pattern to the PCB board.

In PCB production, we will mention the concept of graphics transfer, because the production of conductive graphics is the basis of PCB production. Therefore, the graphics transfer process is of great significance to PCB production.

The inner layer dry film includes multiple processes such as inner layer filming, exposure and development, and inner layer etching. The inner film is to paste a special photosensitive film on the surface of the copper plate, which is what we call a 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, the light-transmitting part is cured, and the non-light-transmitting part is still the dry film. Then, after developing, the uncured dry film is removed, and the board with the cured protective film is etched. After removing the film, the inner circuit pattern is transferred to the board.

For PCB designers, our main consideration is the minimum line width of the wiring, the control of the spacing and the uniformity of the wiring. Because the distance is too small, it will cause the film to be sandwiched, and the film cannot be completely removed and cause a short circuit. If the line width is too small, the adhesion of the film is insufficient, resulting in an open circuit. 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) Pre-treatment: grinding plate

The main function of the grinding plate: the basic pre-treatment is mainly to solve the problems of surface cleanliness and surface roughness. Remove oxidation, increase the roughness of the copper surface, and facilitate the film to adhere to the copper surface.

(2) Film

The processed substrate is pasted with dry film or wet film by hot pressing or coating, which is convenient for subsequent exposure production.

(3) Exposure

Align the negative film with the dry film substrate, and use the ultraviolet light on the exposure machine to transfer the negative film pattern to the photosensitive dry film.

(4) Development

Use the weak alkalinity of the developing solution (sodium carbonate) to dissolve and rinse the unexposed dry film/wet film, leaving the exposed part.

(5) Etching

After the unexposed dry film/wet film is removed by the developer, the copper surface will be exposed. Use acid copper chloride to dissolve and corrode the exposed copper surface to obtain the required circuit.

(6) Removal of the film

Peel off the exposed dry film that protects the copper surface with sodium hydroxide solution to expose the circuit pattern.

3. Browning

Purpose: to form a microscopic roughness and organic metal layer on the inner copper surface to enhance the adhesion between the layers.

Principle of the process: Through chemical treatment, a uniform and good adhesion characteristic of the organic metal layer structure is produced, so that the surface of the copper layer before the inner layer is bonded is controlled to be roughened, which is used to enhance the adhesion between the inner copper layer and the prepreg after pressing the plate.合 Strength.

4. Laminate

Lamination is the process of bonding each layer of circuits into a whole by means of the adhesiveness of the pp sheet. This bonding is achieved by mutual diffusion and penetration between macromolecules at the interface, and then interweaving. The discrete multilayer board and pp sheet are pressed together to form a multilayer board with the required number of layers and thickness. In actual operation, the copper foil, bonding sheet (prepreg), inner layer board, stainless steel, isolation board, kraft paper, outer layer steel plate and other materials are laminated according to the process requirements.

For PCB designers, the first consideration for lamination is symmetry. Because the board will be affected by pressure and temperature during the lamination process, there will still be 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, which greatly affects the performance of the PCB.

In addition, even in the same plane, if the distribution of copper is uneven, the resin flow speed at each point will be different, so that the thickness of the place with less copper will be slightly thinner, and the thickness of the place with more copper will be thicker. Some.

In order to avoid these problems, various factors such as the uniformity of the copper distribution, the symmetry of the laminate, the design and layout of the blind and buried holes, etc. must be considered in detail during the design.

5. Drilling

Make through holes between PCB layers to achieve the purpose of connecting the layers.

6. Immersion copper plate plating

(1). Shen copper

Also called chemical copper, the drilled PCB board undergoes redox reaction in the sinking copper cylinder to form a copper layer to metalize the holes, so that copper is deposited on the surface of the originally insulated substrate to achieve inter-layer electrical communication.

(2). Plate plating

Thicken the surface of the PCB board that has just been copper-immersed, and the copper in the hole to 5-8um, to prevent the thin copper in the hole from being oxidized and micro-etched away before pattern plating and leaking the base material.

7. Outer dry film

The process is the same as the inner dry film.

9. Outer layer pattern plating, SES

The copper layer of the hole and circuit is plated to a certain thickness (20-25um) to meet the copper thickness requirements of the final PCB board. And etch away the useless copper on the board surface, exposing useful circuit patterns.

10.PCB solder mask

Solder mask, also called solder mask and green oil, is one of the most critical processes in the production of printed boards. It is mainly through screen printing or coating solder mask ink, coating a layer of solder mask on the surface of the board, and developing through exposure., Expose the disk and hole to be soldered, cover other places with solder mask to prevent short circuit during soldering

11. Silkscreen characters

The required text, trademark or part symbol is printed on the board surface by screen printing, and then exposed on the board surface by ultraviolet radiation.

12.PCB surface treatment

The solderability of bare copper itself is very good, but long-term exposure to the air is easy to be damp and oxidized. It tends to exist in the form of oxide and is unlikely to remain as the original copper for a long time. Therefore, surface treatment of the copper surface is required. The most basic purpose of surface treatment is to ensure good solderability or electrical properties.