Four Layered PCB Manufacturing Process (16 Steps)
Printed circuit boards form the core of the design of electronic circuits and devices. These circuit boards help route the electrical signals through the electronic environment of the most commonly used devices. Here, in few simplest steps you will know complete manufacturing process of four layered PCB.
This is how the PCB is manufactured
Using the 4-layer PCB as an example, a complete 4 layer PCB fabrication and assembly process followed at any PCB manufacturer normally involves 16 steps as below.
1. Clean in the Chemical Method
For making a good etching pattern, it is necessary to ensure that corrosion resistant layer is firmly bonded with the surface of the substrate. This requires the surface to be free of oxidation layer, fingerprints, dust, oil and other contaminants. Accordingly, the first thing is to clean up the board surface and make the copper foil surface roughened considerably before coating the corrosion resist layer.
To start making a four-layer PCB, the formation of inner layers (including the second layer and the third layer) should be the first process. The inner plate is a copper plate with the epoxy resin and glass fiber compounded on both surfaces.
2. Laminate Dry Film
In this step, firstly we have to affix the dry film on the inner-layer plate for obtaining the shape we want (dry film also refers to the photoresist, which is made up of the three parts - photoresist film, polyethylene protective film and polyester film). In the step of affixing the dry film, the right order is to peel off the polyethylene film first from it, and then affix it to the copper surface followed by pressing and heating.
3. Image Expose & Develop
In this step, image exposure and image development are accomplished by two chemical reactions. One is the absorption and decomposition of photoinitiator existing on the copper surface with the irradiation of UV light to make the polymer crosslinking reaction happen and also, last for some time with the generation of a polymer which is insoluble in the dilute alkali. Thus, ensure that you do not remove the polyester film immediately once exposed until at least 15 minutes; this is done to ensure the technological stability. Please note that it’s better to remove the film before image developing.
The other chemical reaction happens by the integration of the active group in the photosensitive film and that of the dilute alkali. This results in the generation of soluble substance that forms a pattern after dissolution.
4. Copper Etching
In the process of manufacturing the flexible PCBs or common PCBs, this is an indispensable step for forming the desired circuit pattern. It is carried out by getting rid of the useless part of copper foil (off-circuit part) with the help of chemicals. It is also necessary that the copper below the photoresist should be retained without getting affected by etching process.
5. Strip Resist
This step aims to completely clean up the resistant layer remained in the etching step so as to expose the copper foil. Here, take good care of the waste materials and "Membrane slag", i.e. you must properly filter and handle them for safety and quality. Washing is the priority for cleaning the board after stripping resist, pickling is recommended as the secondary option. In addition, the surface of the board should be totally dry after washing so as to keep it protected from moisture.
6. Lay-up with prepreg
Before going to the laminating press machine, it’s better to get all the needed raw materials of the multi-layer board in place so as to do the layup operation. Once the inner layers are prepared with the oxidation treatment, impregnating the glass fiber with prepreg is also needed. Lay-up operation, as the core of this step, will keep the plates with protective film stacked in order and also between the 2 layers of steel plate.
7. Layup with Copper Foil
In this step, a multi-layered board will be prepared by doing the multi-layer laminating by applying pressure, but before that, you need to cover the both sides of the existing inner plates with a layer of copper foil. Remember to cool the board to the room temperature once the laminating pressure is finished, this is really important.
In this step, the stack board is drilled to make holes at specific points as appearing in the printed circuit. The size of the holes is 100microns in diameter. The drilling is done after finding the target with the help of x-ray locator. A computer-driven machine controls the movement of the drill to a micro level.
Once the drilling is over, the excess copper lining on the edge of the panel is scraped off with the help of a profiling tool.
9. Plating and Copper Deposition
Drilling is followed by plating. In order to enable the through-holes to be conductive among the layers, holes are supposed to be filled with copper. To accomplish this, what should be first done is to plate a thin copper layer in those holes. This is completely accomplished by a chemical deposition process.
10. Outer Layer Imaging
The application of photo resist is repeated in this step. However, only the outer layers of the panel carrying the PCB layout is coated this time. This is done in a completely sterilized environment. The checking for the hardening of the outer layers is done after removing the photo resist applied in the previous stage in inner layers.
11. Copper Plating
It is actually the second copper plating, which is done with the purpose of thickening the copper layer.
12. Tin Plating
This step is followed to prevent the copper conductor from being attacked during the etching process that follows next (protecting all the copper circuits and through-holes). This is followed by strip resist that aims to make the copper on the surface exposed by chemical method.
13. Final Etching
The chemical etching process is carried out to remove the excess copper and the tin works as a shield for the useful copper during this stage. This helps demarcate the conducting areas and connections properly.
14. Solder Mask
Once etching is done, the panel is coated with epoxy solder mask ink. And then it is moved to an oven to secure the solder mask. Doing a good cleaning can facilitate the coming up of surface features nicely.
15. Surface Finish
The challenge is to make the PCB panel solder-able. Thus, its surface is coated with gold or silver; some PCB assembly services add hot air-leveled pads in this step to impart uniformity and brightness. Finally, the PCB board is completed by embossing the related vital information using ink-jet writing on the surface; this step is literally named silkscreen.
16. Electrical Test
Finally, the PCB testing is performed by the technician. The testing involves ensuring the proper functionality. The PCB is also tested for quality and conformation with the original design.
So, now you know how the PCB fabrication and assembly is done. For makers with some experiences of electronics, this tutorial gives you DIY procedure for making PCB at home. As you can see that the process involves technical operations and equipment, so if you want top quality printed circuit boards that suit your varying needs, you must speak to the experts about your requirement. The PCB fabrication service provider like Elecrow is the best place to shop for PCBs where custom and advanced PCBs are made available at affordable prices.