PCB manufacturing process can be roughly divided into the following four steps:
The first step of PCB production is film plate making.
1. Draw the base map.
Most of the basemaps are drawn by designers, and PCB manufacturers need to check and modify these basemaps to ensure the quality of PCB processing. If it does not meet the requirements, it needs to be redrawn.
2. Photographic plate making
Shoot the plate with the drawn base map, and the layout size should be consistent with the PCB size.
The process of making PCB by photography is basically the same as that of ordinary photography, which can be divided into: film cutting-exposure-development-fixing-washing-drying-modification. Check the correctness of the basemap before taking pictures, especially the basemap that has been placed for a long time.
Adjust the focal length before exposure, and keep the two focal lengths of the double-panel photo negatives consistent before and after photography; Photo negatives need to be repaired after drying.
The second step of PCB manufacturing is graphic transfer.
Transferring the printed circuit pattern of PCB on the phase board to the copper clad laminate is called PCB pattern transfer. There are many methods to transfer PCB graphics, such as screen printing and photochemical method.
1. Missing screen printing.
Screen-missing printing is similar to mimeograph, that is, a layer of paint film or glue film is pasted on the screen, and then the printed circuit diagram is made into a hollow figure according to the technical requirements. Screen printing is an ancient printing process with simple operation and low cost. It can be realized by manual, semi-automatic or automatic screen printing machine. The steps of manual screen printing are as follows:
1) Put the copper clad laminate on the bottom plate, and put the printing materials into the frame of the fixed screen.
2) Scraping the embossed material with a rubber plate to make the screen directly contact with the copper clad laminate, and then forming a pattern on the copper clad laminate.
3) then drying and correcting.
The third optical method of PCB manufacturing
(1) direct photosensitive method
The process flow is as follows: surface treatment of copper clad laminate, coating with photosensitive adhesive, exposure, development, fixing film and revision. Modification is a necessary work before etching. Burrs, broken wires and sand holes can be repaired. (PCB resource network
(2) photosensitive dry film method
This process is the same as the direct photosensitive method, except that the film is used as the photosensitive material instead of photosensitive glue. This film consists of three layers of materials: polyester film, photosensitive adhesive film and polyethylene film, with the photosensitive adhesive film sandwiched between them. When in use, the protective film on the outer layer is peeled off, and the photosensitive adhesive film is pasted on the copper clad laminate with a film pasting machine.
(3) chemical etching
It uses chemical methods to remove unnecessary copper foil on the circuit board, leaving pads, printed lines and symbols that form patterns. Commonly used etching solutions include acidic copper chloride, alkaline copper chloride and ferric chloride.
The fourth step of PCB manufacturing is the processing of vias and copper foil.
1. Metallized hole
Metallized hole is to deposit copper on the hole wall that runs through the wires or pads on both sides, so as to metallize the original non-metallic hole wall, also known as copper deposition. This is an essential process in double-sided and multilayer PCB.
In actual production, it can be completed through a series of technological processes, such as drilling, deoiling, coarsening, soaking in cleaning solution, hole wall activation, electroless copper plating, electroplating and thickening.
The quality of metallized holes is very important for double-sided PCB, so it must be checked, the metal layer should be uniform and complete, and the connection with copper foil should be reliable. In surface mount high-density board, blind hole method (copper deposition fills the whole hole) is adopted for this metallized hole, which reduces the area occupied by the via hole and improves the density.
2. Metal plating
In order to improve the conductivity, solderability, wear resistance and decoration of PCB printed circuit, prolong the service life of PCB and improve electrical reliability, metal coating is often carried out on the copper foil of PCB. Commonly used coating materials are gold, silver and lead-tin alloy.
The fifth step of PCB manufacturing: welding aid and welding resistance treatment
After the PCB surface is plated with metal, it can be soldered or soldered according to different needs. Coating flux can improve solderability; On the high-density lead-tin alloy board, in order to protect the board surface and ensure the welding accuracy, a solder mask can be added on the board surface to expose the bonding pad, and other parts are under the solder mask. Solder-resistant coatings are divided into two types: thermosetting and photocuring, and the color is dark green or light green.