Printed circuit board – the core of electronic components

PCB manufacturing process

Taking a four -layer PCB as an example, let’s take a look at how a PCB is made.

Step 1: chemical cleaning
In order to obtain good-quality etching patterns, it is necessary to ensure that the resist layer is firmly bonded to the surface of the substrate, and the surface of the substrate is required to be free of oxide layer, oil, dust, fingerprints and other dirt. Therefore, before coating the resist layer, it is first necessary to clean the surface of the board and make the surface of the copper foil reach a certain degree of roughness.

Inner layer board: start to make a four-layer PCB, and the inner layer (second and third layers) must be done first. The inner plate is a copper sheet laminated with glass fiber and epoxy resin matrix on the upper and lower surfaces.

Step 2: cutting board , lamination
Apply photoresist: In order to make the shape we need on the inner sheet, we first apply a dry film (photoresist, photoresist) on the inner sheet. The dry film is composed of polyester thin film, photoresist film and polyethylene protective film. When pasting the film, first peel off the polyethylene protective film from the dry film, and then paste the dry film on the copper surface under heat and pressure.

Step 3: exposure and development
Exposure: Under the irradiation of ultraviolet light, the photoinitiator absorbs the light energy and decomposes into free radicals, and the free radicals then initiate the polymerization and crosslinking reaction of the photopolymerizable monomer, and form a polymer structure insoluble in dilute alkali solution after the reaction. The polymerization reaction will continue for a period of time.

In order to ensure the stability of the process, do not tear off the polyester film immediately after exposure, but should stay for more than 15 minutes to allow the polymerization reaction to continue, and tear off the polyester film before developing.

Development: The active groups in the unexposed part of the photosensitive film react with dilute alkali solution to produce soluble substances and dissolve, leaving the photosensitive cross-linked and cured graphic part.

Step 4: etching
In the production process of flexible PCB or PCB, the unnecessary part of the copper foil is removed by chemical reaction to form the required circuit pattern, and the copper under the photoresist is kept from being etched of the influence.

Step 5: film removal, post-etch punching, AOI inspection, oxidation

The purpose of film removal is to remove the resist layer remaining on the PCB surface after etching to expose the underlying copper foil. “Membrane slag” filtration and waste recovery must be properly handled. If the water washing after removing the film can be completely cleaned, it may be considered not to do pickling. After the PCB surface is cleaned, it should be completely dried at the end to avoid moisture residue.

Step 6: laminated board – protective film
Before entering the press machine, the raw materials for each multilayer PCB need to be prepared for the Lay-up operation. In addition to the inner layer that has been oxidized, a protective film (Prepreg)-epoxy resin impregnated glass is still required fiber. The function of lamination is to stack the PCBs covered with protective film in a certain order and place them between the two layers of steel plates.

Step 7: laminated board – copper foil and vacuum lamination
Copper foil – cover the current inner layer with a layer of copper foil on both sides, and then perform multilayer pressing (extrusion that needs to measure temperature and pressure within a fixed time) and cool to room temperature after completion, the rest It’s a multiayer PCB.

Step 8: CNC drilling

Under the precise conditions of the inner layer, CNC drilling drills holes according to the pattern. Drilling requires high precision to ensure that the holes are in the correct position.

Step 9: electroplating – through hole
In order to make the through hole conduct between layers (to metallize the resin and glass fiber bundles on the non-conductive part of the hole wall), the hole must be filled with copper. The first step is to plate a thin layer of copper in the hole, a process that is purely chemical. The final plated copper is 50 millionths of an inch thick.

Step 10: cutting board lamination
Apply photoresist: We once applied photoresist on the outer layer.

Step 11: exposure and development
Outer layer exposure and development.

Step 12: line plating
This time it also be called secondary copper plating, the main purpose is to thicken the line copper and through-hole copper.

Step 13: electroplating tin

Its main purpose is an etch resist, protecting the copper conductors it covers from attack by alkaline copper etching (protecting all copper lines and inside vias).

Step 14: remove the film
We already know the purpose, we just need to chemically expose the copper on the surface.

Step 15: etching

We know the purpose of the etch, and the tinning partially protects the underlying copper foil.

Step 16: pre-hardening exposure development on the solder mask
The solder resist layer is used to expose the pads, which is commonly referred to as the green oil layer. In fact, it is to dig holes in the green oil layer to expose the pads and other places that do not need to be covered by green oil. Proper cleaning will result in the proper surface characteristics.

Step 17: surface treatment
The hot air leveling solder coating process is to dip the printed board with flux first, then dip it in the molten solder, and then pass between two air knives, and use the hot compressed air in the air knife to clean the printed board. The excess solder is blown off, and the excess solder in the metal hole is excluded at the same time, so as to obtain a bright, flat and uniform solder coating.