What is the Industrial PCB Production Process?
1. Board cut
Board cutting is the process of cutting the original copper-clad laminate into boards that can be made on the production line.
First, let’s understand a few concepts:
- UNIT: Refers to the unit graphics designed by PCB design engineers.
- SET: Refers to multiple units that engineers put together to improve production efficiency and facilitate production. This is what we often call the puzzle, which includes unit graphics, process edges,etc.
- PANEL: Refers to a board composed of multiple sets. It is added with tool board edges to improve efficiency and facilitate production when PCB manufacturers producethe product.
2. Innerdry 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 dry film application includes many processes such as inner layer filming, exposure, development, and etching. The inner film is to paste a special photosensitive film on the copper plate’s surface, which we call a dry film. This film will cure when exposed to light, forming a protective film on the board. Exposure and development expose the board with the film, the transparent part is cured, and the non-transparent part is still the dry film. 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. The whole process flow is shown in the picture below.
For designers, our main consideration is the minimum line width, spacing control, and wiring uniformity. If the distance is too small, it will cause film clamping. If this occurs, the film cannot be removed entirely and will cause a short circuit. If the line width is too small, the film’s adhesion is insufficient, resulting in an open circuit. Therefore, the safety spacing during circuit design (including line to line, line to pad, pad to pad, line to the copper surface, etc.) must be considered during production.
- Grinding board
The primary function of the grinding plate: The basic pretreatment is mainly to solve the problems of surface cleanliness and roughness. Remove oxidation and increase the copper surface’s roughness, which will increase the film’s adhesion effect to the copper surface.
Paste the processed substrate with dry film or wet film by hot pressing or coating to facilitate subsequent exposure production.
Align the negative film with the substrate on which the dry film is pressed, and use ultraviolet light on the exposure machine to transfer the negative film pattern to the photosensitive dry film.
Use the weak alkalinity of sodium carbonate to dissolve and rinse the unexposed dry film/wet film, leaving the exposed part.
After the unexposed dry film and wet film are removed, the copper surface will be exposed. Use acid copper chloride to dissolve and corrode the exposed copper surface to obtain the required circuit.
- Film stripping
Use sodium hydroxide solution to peel off the exposed dry film that protects the copper surface to expose the circuit pattern.
3. Brown oxidation
Purpose: To form a microscopic roughness and organic metal layer on the inner copper surface to enhance the adhesion between the layers.
Process principle: Through chemical treatment, a uniform and good adhesion characteristic of the organic metal layer structure is produced. The copper layer’s surface, which is before the inner layer, is roughened in a controlled matter. This is to strengthen the inner copper layer and the prepreg after pressing the plate strength.
Stack up is the process of bonding each layer into a whole utilizing the PP sheet’s adhesiveness. This bonding is achieved by mutual diffusion, penetration between macromolecules on the interface, and interweaving. The multi-layer boards and PP sheets are pressed together to form a multi-layer board with the required number of layers and thickness. In actual operation, the copper foil, bonding sheet (prepreg), inner layer, stainless steel, isolation board, kraft paper, outer layer, and other materials are laminated according to the process requirements.
For designers, the first consideration for stack up is symmetry. Because the PCB will be affected by pressure and temperature during the lamination process, stress will remain on 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 PCB’s performance.
In addition, even in the same layer, if the copper distribution is uneven, the resin flow rate at each point will be different. As a result, the area with less copper will be slightly thinner, and the area with more copper will be slightly thicker.
In order to avoid these problems, various factors such as the uniformity of the copper distribution, the symmetry of the stack, the design and layout of the blind and buried holes, etc., must be considered in detail during the design.
Drilling is the process of creating through-holes between the layers to connect the layers.
6.Plated through holes(PTH) and plating
- Plated through holes(PTH)
Plated through-holes are also called chemical copper. After drilling, the PCB board undergoes redox reactions in the sinking copper cylinder to form a copper layer to metalize the holes. Copper is deposited on the surface of the original insulating substrate to achieve an interlayer electrical connection.
Plating creates the PCB board surface that has just been copper-immersed. The copper in the hole is thickened to 5-8um to prevent the thin copper in the hole from being oxidized or micro-etched and leaking the substrate before pattern plating.
(1) Outer dry film
The process is the same as the inner dry film.
8.Outer layer pattern plating, SES
The copper layer of the hole and circuit is plated to a specific thickness (20-25um) to meet the copper thickness requirements of the final PCB board. The useless copper on the board surface is etched away, exposing useful circuit patterns.
9. Solder mask
Solder mask, also called and green oil, is one of the most critical PCB fabrication processes. Solder mask is achieved mainly through screen printing or coating solder mask ink, coating a layer of solder mask on the board surface, and developing through exposure. The processes also expose the disk and hole to be soldered and cover other areas with solder mask to prevent short circuit during soldering.
The required text, trademark, or part symbol is printed on the board surface by screen printing. Afterward, it is exposed on the board surface by ultraviolet radiation.
11. Surface treatment
The solderability of bare copper itself is very good, but long-term exposure to the air can cause oxidation and saturation. It tends to exist in the form of oxides and is unlikely to remain as the original copper for an extended period. Therefore, surface treatment of the copper surface is required. The primary purpose of surface treatment is to ensure good solderability or electrical properties.
Regular surface treatments: HASL, immersion gold(ENIG), OSP, immersion tin, immersion silver, electric hard gold, electric gold fingers, etc.
The PCB is cut into the required dimensions with a CNC molding machine.
13. Electrical test(E-test)
Simulate the status of the PCB and check the electrical performance after power on to see if there is an open or short circuit.
14. Final inspection, sampling test, packaging
Check the appearance, size, hole diameter, thickness, and marking of the PCB to meet customer requirements. The qualified products are packed into bundles, which are easy to store and transport.