Pcb process flow

Cutting is the process of cutting the original copper clad laminate into plates that can be manufactured on the production line.

First of all, let's understand a few concepts:

(1) unit: unit refers to the unit figure designed by PCB design engineer.

(2) Assembly: Assembly refers to a whole drawing that engineers put together several units in order to improve production efficiency and facilitate production. That's what we often call a puzzle, including unit graphics, process edges and so on.

(3) Panel: Panel refers to a panel composed of multiple groups and tool edges in order to improve efficiency and facilitate production.

2. Internal dry film

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

We will mention the concept of pattern transfer in PCB production, because the production of conductive patterns is the basis of PCB production. Therefore, the graphic transfer process is of great significance to PCB production.

The inner dry film includes many processes such as inner film sticking, exposure and development, and inner etching. The inner film is to stick a special photosensitive film on the surface of copper plate, which is what we call dry film. This film will cure when exposed to light, forming a protective film on the board. Exposure development is to expose the board with a good film, and the light-transmitting part is cured, and the light-transmitting part is still a dry film. Then, after development, the uncured dry film is removed, and the plate with the cured protective film is etched. After the film is uncovered, the circuit pattern of the inner layer is transferred to the board. The whole process flow is as follows.

For designers, we mainly consider the minimum line width, spacing control and wiring uniformity. Because the distance is too small, the film will be caught, and the film will not fade, resulting in short circuit. The line width is too small, and the film adhesion is insufficient, which leads to open circuit. Therefore, the safe spacing in circuit design (including wire to wire, wire to pad, pad to pad, wire to copper surface, etc.). ) must be considered in production.

(1) Pretreatment: plate grinding.

The main function of the grinding disc: the basic pretreatment is mainly to solve the problems of surface cleanliness and surface roughness. Remove oxidation, increase the roughness of copper surface, and promote the adhesion of thin film to copper surface.

(2) Sticking film

The dry film or wet film is attached to the treated substrate by hot pressing or coating, which is convenient for subsequent exposure production.

(3) exposure

When the dry film is under pressure, the negative film is aligned with the substrate, and the negative film pattern is transferred to the photosensitive dry film by ultraviolet irradiation on the exposure machine.

Negative physical picture

(4) development

Using the weak alkalinity of developing solution (sodium carbonate), the unexposed dry film/wet film is dissolved and washed away, leaving the exposed part.

(5) etching

After the unexposed dry film/wet film is removed by developer, the copper surface will be exposed, and the exposed copper surface will be dissolved and corroded by acidic copper chloride to obtain the required circuit.

(6) demoulding

Stripping the exposed dry film on the protective copper surface with sodium hydroxide solution to expose the circuit pattern.

Step 3 wear brown

Objective: To form microscopic roughness and organic metal layer on the inner surface of copper, and enhance the adhesion between layers.

Process principle:

Through chemical treatment, the organic metal layer structure with uniform and good adhesion characteristics is produced, which coarsens the surface of the copper layer before the inner layer is bonded, and is used to enhance the bonding strength between the inner layer copper layer and the prepreg after pressing.

Step 4: Sheet

Lamination is a process of bonding all layers of circuits into a whole through the bonding of pp sheets. This adhesion is achieved by mutual diffusion and infiltration of macromolecules at the interface, and then interweaving, so that discrete multilayer boards and pp sheets are pressed together to form multilayer boards with the required number of layers and thickness. In practice, copper foil, adhesive sheet (prepreg), inner plate, stainless steel, isolation plate, kraft paper, outer steel plate and other materials are laminated according to the process requirements.

For designers, the first thing to consider in lamination is symmetry. Because the plate will be affected by pressure and temperature during the lamination process, there is still stress in the plate after the lamination is completed. Therefore, if the two sides of the laminate are uneven and the two sides bear different forces, the board will bend to one side, which will greatly affect the performance of PCB.

In addition, even on the same plane, if the distribution of copper is uneven, the resin flow rate 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 slightly thicker.

In order to avoid these problems, the uniformity of copper distribution, the symmetry of laminated plates, the design and layout of blind buried holes and other factors must be considered in detail in the design.

5, drilling

Through holes are generated between the layers of the circuit board to achieve the purpose of connecting the layers.

Bit cutter

6. Copper plating

(1) copper deposit

Also known as chemical copper, after the PCB board is drilled, the redox reaction takes place in the copper sink to form a copper layer, which metallizes the hole and deposits copper on the surface of the original insulating substrate to realize interlayer electrical communication.

(2) Electroplating

Immediately after copper deposition, the copper in the surface and holes of PCB is thickened to 5-8 μm to prevent the thin copper in the holes from being oxidized and etched before pattern electroplating and leaking the substrate.

7. External dry film

The process is the same as the internal dry film.

8. Outer pattern electroplating and SES

The copper layers of holes and wires are electroplated to a certain thickness (20-25 μm) to meet the copper thickness requirements of the final PCB. The useless copper on the surface of the circuit board is etched away, revealing useful circuit patterns.

9. Welding resistance

Resistance welding, also known as resistance welding and green oil, is one of the most critical processes in PCB production. Mainly by screen printing or painting solder resist ink, a solder resist layer is coated on the board surface, and the board and holes to be welded are exposed through exposure and development, and other places are covered with solder resist to prevent short circuit during welding.

10, screen-printed characters

The required characters, trademarks or part symbols are printed on the board by screen printing, and then exposed on the board by ultraviolet radiation.

1 1, surface treatment

Bare copper itself has good solderability, but it is easy to be oxidized by moisture when exposed to air for a long time, and tends to exist in the form of oxide, which is unlikely to remain as original copper for a long time, so it is necessary to carry out surface treatment on the copper surface. The most basic purpose of surface treatment is to ensure good solderability or electrical performance.

Common surface treatments: tin spraying, gold plating, OSP, tin plating, silver plating, nickel palladium gold plating, hard gold plating, gold finger plating, etc.

12, molding

PCB is cut into required size by CNC forming machine.

13, electrical measurement

Simulate the board status, power on and check the electrical performance, and see if there is an open circuit or short circuit.

14, final inspection, sampling and packaging

Check the appearance, size, aperture, thickness and marking of the board to meet the requirements of customers. Qualified products are packed in bundles, which is convenient for storage and transportation.