Printed Circuit Board Manufacturing Process Flow
Printed Circuit Boards (PCB) are used almost in every kind of electronic equipment.
The Printed Circuit Board (PCB) manufacturing process can be summarized as the following steps:
Cutting is the process of cutting the dry-cleaned copper-clad panel into small pieces according to the production size, which can be fabricated on the production line. To ensure safe operation and reduce scratch issues, the corners of the board have been rounded.
2. Inner layer dry film lamination
A dry photoresist film is applied onto the board by automatically hot processing. This step occurs in a dust-free room with yellow light because the photoresist film is very sensitive to UV light.
Next, perfectly align the circuit printed film with the board, and then send the board to the printer using UV lamps to harden the photosensitive film according to the circuit printed film. After this step, the circuit is transferred to the dry photoresist film. It is worth noting that the circuit parts are not exposed, but the area without circuits is exposed to UV light, thereby remaining soft.
During the development phase, the alkaline solution is used to wash out left unhardened photoresists. After that, the inner layer image is printed by blue resist, which will resist the chemical solution at the etching stage.
The etching is a key stage of layer imaging, using an acid solution to remove unwanted copper and outline the pattern. After etching, the board is cleaned to wash out the excess chemical solution.
Stripping is to completely peel off the exposed dry photoresist film that protects the copper surface with sodium hydroxide solution to expose the circuit pattern.
7. Inner layer Automatic Optical Inspection (AOI)
This step will accurately confirm a total absence of defects, and ensure that the built circuit board is of high quality without manufacturing faults. The working principle of AOI is to use the high-definition image camera to quickly shoot, and then compare the captured pictures with the original files, which can fundamentally solve the hidden dangers such as short and open circuits.
8. Brown Oxide treatment
The purpose of the brown oxide treatment is to form a microscopic roughness and organic metal layer on the inner layer surface through a chemical treatment to enhance the adhesion between the layers and avoid such problems as delamination.
In actual operation, the discrete multi-layer board and prepreg are pressed together to form a multi-layer board with the required number of layers and thickness. Finally, the copper foil completes the stack-up. The combinations of a copper foil and a prepreg are located on the top and bottom respectively, sandwiching the inner layer to form the stack-up.
The stack-up is processed in the lamination machine, which takes up to 2 hours. After processing under high pressure and temperature, a single laminated board is formed and then moved to the cold press.
In this stage, various factors such as the uniformity of copper distribution, the symmetry of the stack, the design and layout of the blind and buried holes must be considered in detail during the design.
Drilling has 2 main purposes, one is to connect load components, another is to link the copper layers. In this stage, there is no copper in the holes, therefore the current cannot flow through the board.
11. Copper plating
The drilled PCB board undergoes an oxidation-reduction reaction in the sinking copper cylinder to form a copper layer to metalize the holes. The copper is deposited on the surface of the originally insulated substrate to obtain conductive holes, thereby achieving electrical communication between the inner layer and outer layer. The stage takes place in a series of chemical and rinsing baths.
12. Outer layer basic process
The fundamental process of the outer layer is similar to that of the inner, which includes outer layer dry film lamination, exposure, development, etching, stripping off the exposed dry photoresist film, chemical copper deposited, and outer layer Automatic Optical Inspection (AOI).
The outer pattern plating follows the chemical copper process but emphasizes the copper distribution. The copper is not only deposited over the entire outer layer surface but also inside the holes. The whole circuit board, which acts as a cathode for electroplating, passes through several baths with electrolytic copper to produce electrolysis. After this, the copper layer of the outer layer and holes is plated to a certain thickness to meet the requirements of the copper thickness of the final PCB board. Different from the inner layer chemical copper process, the board will also be immersed in the electrolytic tin to protect the copper in the subsequent etching process.
13. Outer layer etching
There are three main steps. Firstly, all residues and the dry film are removed, but the unwanted copper remains. Next, the board passes through the chemical solution to etch away the unwanted copper and tin. Finally, the circuit areas and connections are properly defined.
14. Solder mask
Solder mask is one of the most critical stages in the production of printed circuit boards, mainly by screen printing or coating solder mask ink to coat a layer of solder mask on the surface of the board. Through exposure and development, the pads and holes are exposed, and the solder mask is hardened. Finally, the unprotected and unhardened portions by insolation will be washed out.
This stage prints the required characters or part symbols on the board surface by screen printing, and then exposes it under the UV light.
16. Surface finish
The solderability of bare copper itself is pretty good, but long-term exposure to the air is easy to be damp and oxidized. The bare copper tends to exist in the form of oxide, which is unlikely to remain in its original state for a long time. Therefore, surface treatment of the copper surface is required to ensure good solderability and electrical properties. The most common surface treatments are immersion tin, electroless nickel immersion gold (ENIG), immersion silver, gold plating, etc.
Cut the PCB to the required shape and dimensions.
18. Electrical measurement
Simulate the state of the PCB board and check the electrical performance to see if there is an open or short circuit.
19. Final QC, packaging, and stock
Check the appearance, size, hole diameter, thickness, marking, etc. of the board to meet customer requirements. The qualified products are packed into bundles, which are easy to store and transport.