In the epoch of the electronics boom, chips and integrated circuits serve as a foundation stone. All the electronic devices launched in the electronic revolution are not produced as available. Like any other product or device launching, it goes through its own process, just like cooking a dish. In essence, for electronic innovation to occur, we need some basic utensils. That role is intricately played by a Printed Circuit Board, ensuring the resonating purpose of performance and reliability. PCBs serve as a key electronic component. In this article, we will talk about the detailed steps of PCB board fabrication.
Day by day electronics is lynching towards integration technologies scale integration, large scale integration, very large-scale integration or most commonly known as VLSI and ultra large scale integration. While this technology has miniaturised our daily use devices, such as mobile phones, laptops or PCs and even our televisions, it is breathtaking to look at the behind-the-scenes of this dramatic revolution. With these integration technologies, our devices are getting smaller and smaller but simultaneously we need to accommodate our chips on small circuit boards as we know that the chips are mounted on PCBs or printed circuit boards. Initially, the circuit boards were made using physical wires, and each and every conducting path was represented by an actual physical wire of varying diameters depending on the use.
Nowadays our circuit board boards have become minuscule in size, it wouldn’t be a great fit for the minute chips to be physically connected via jumper wires or copper wires and additionally, it might have become almost impossible to look after each and every line of the circuit with accuracy and precision. Now, for the same reason, we use some software to design our circuit board layouts.
The Main Steps in PCB Board Fabrication:
To get a clear vision of the manufacturing process of PCBs, let’s dive into a step-by-step guide on how PCBs are developed from an idea on paper to a final product. While delving into the process of how PCBs are manufactured, it is noteworthy to remember that PCBs have types and different levels of complexity. The number of steps we need to accomplish is directly proportional to the complexity of the PCB board in question and are here as follows:
1. PCB Layout Design:
Before manufacturing any PCB board, a layout or a design is required. The layout of a PCB board is nothing but circuit diagrams in which each and every node has a clear path. The layouts are not drawn by pen and paper but with the assistance of some software like Altium Designer, OrCAD, Pads, KiCad, Eagle CAD and EasyEDA. Using software not only saves time but makes it easier to make any last minute changes to the layout. After the design is finalized, it is converted into a appropriate format for the use of the PCB board manufacturer. The most commonly used extension for PCB layout diagrams is Extended Gerber files. Each Gerber file marks one design of a PCB board. Gerber files go through a DFM check (Design for Manufacturing) at the hands of manufacturers to check whether all industry standards are matched or not. Upon receiving clearance, the layout is printed. A special type of printer called as Plotting Printer is used for printing PCB layouts. A plotting printer is not a regular Inkjet printer but a special type of laser printer. PCB layouts can not be printed on a regular A4 paper but on some special films.
While joining two or more traces on a printed circuit board design, making a 90-degree angle is avoided as it makes traces more prone to corrosion on the board and it gets scabbed easily. 90-degree angles are more susceptible to radio frequency effects as well and thus decrease stability. In comparison to that, it has been found and tested that 45-degree angle ensures that we get the maximum possible tracks in the least area on the PCB board and is not much prone to corrosion. In addition to that, we might have seen on PCB boards parallel lines running parallel to each other but of different lengths. It is called the Length Matching of traces. It is the same as the name suggests in that it matches the lengths of paths in order to make two or more signals reach at a certain node simultaneously and accurately so that the sometimes we need signals to reach to a specific point at a certain time.
2. Base Substrate Cutting:
In this step, we will cut big sheets of PCB base materials into small pieces. For example, the base material dimension is around 36×48 inches, which is very difficult in PCB production to cut into smaller pieces with 18×24 inches. Its core is comprised of copper sheets and a dielectric substrate. Epoxy resin (FR-4) is mostly used for multi-layer PCB board fabrication and glass fibre is used for making boards strong and sturdy and shock-resistant. Laminate serves as the perfect host for the copper reception on both sides, it is also called copper-clad laminate(CCL).
3. Pattern Printing:
PCB layout drawings come out of plotting printers on photonegative films with different inked areas marked on them. These films are also layer specific i.e., for inner layers of PCB board, black ink represents conductive path and the other areas represent non-conductive paths. However, for the outer layers of PCB boards, the color of the ink completely changes its meaning. In the outer layers, black represents non-conducting paths and other regions mark the conducting paths.
4. Photoresist Coating:
Once cleaning is finished, a coating of photoresist film is painted over the boards. Photoresist film comprises of photoreactive chemicals which harden upon the reception of UV light. Photoresist coating is done in order to trace down the conducting paths from the negative film to the laminate board with the help of UV rays. Registration holes are punched through all of them using drilling machines to achieve the perfect alignment of all the layers. After that electroless copper is made to pass through the board so that the conductivity of the PCB remains intact throughout the board and both the surfaces the top and the bottom remain connected.
5. Tracing Exposure:
In PCB construction, the most significant process is to trace the design from negative films to copper clads. As previously mentioned, films have different regions traced onto them. When a copper-clad board and its corresponding layout is stacked up on it, some visible regions are present. These regions mark conductive traces. When UV light is passed, it makes its way through the visible regions onto the UV sensitive photoresist film and it melts right away and gets hardened on the copper clad.Only that portion of photochemical hardening, which could be seen from the conducting traces of the film, rest of it is blocked by black ink and remains in semi-liquid form. With all the paths traced successfully, the board gets ready to wash off the unwanted photoresist. For that purpose, it is made to pass through an alkaline solution, which cleans the whole of the wet photoresist material on the PCB board with the hardened resist covering up the conducting paths.
6. Out Layer Copper Etching:
With the PCB board almost ready, now the only thing unwanted remaining on it is the copper in the non-conducting areas. This process is called as Etching. To easily clean the extra copper, a solution of Ferrous chloride is prepared in the ratio of 1:1 and the board is allowed to sink in it for a couple of hours and the solution removes the copper from the board and we are left with copper with the photoresist covering only. To wash off the hardened photoresist, acetone is used on the board followed by water.
7. Alignment Check:
To make sure registration holes have stayed intact, a final inspection is done before combining all the layers with each other. All the layers are placed inside of a machine which uses the original Gerber file as its model to accurately check the positions and alignment of registration holes through all the layers. If no inaccuracy is found, the layers move for the final stages of the PCB board manufacturing process; if found otherwise, inconsistencies are displayed on a screen for the manufacturer to mend.
Generally speaking, in the process, we will put inner copper layers and out copper layers together, and put dielectrics between the copper layers, and then hot press them to adhere to each other. For example, when we procure a four-layer printed circuit board, we will produce the core board first (copper layers 2 and 3) and then put thin layers of dielectric (here we call them prepreg) and sheets on the top and bottom core board, and then press them together by lamination machine.
Now that a full-fledged PCB board is ready with all the layers intact, it is ready for the drilling purpose. Drilling is mainly used to connect all layers. Drilling is done for elements to be soldered on PCB board. Nowadays, mostly surface mount devices (SMDs) are used but still, this option is kept open. Holes are also made to keep the use of PCBs versatile in nature.
10. Copper Plating:
Copper plating has two functions: firstly, it is used to plate copper into the drill hole wall to connect all the copper layers; secondly, it will add thicker copper on the copper trace on top and bottom layers.
11. Outer Layer Imaging and Etching:
In this process, the tracing of the outer layers of the PCB board is done in a similar manner as in the case of inner layers. Once the tracing is done, unwanted copper and other materials are also etched in a similar fashion. A protective layer is applied to newly formed boards to protect them against further etching processes wherein the desired conducting paths can suffer damage and might make the board faulty. After all the etching processes are complete, it is later removed chemically.
12. Solder Mask:
It is usually a green-coloured mask that we usually see on printed circuit boards. Solder mask is coated over the PCB surface to keep them safe from the damage of oxidation and corrosion and provide them with isolation from other circuits during PCB assembly process.
13. Silk Screen:
Finally if there is any label or any icon that needs to be printed on the PCB board which was present in the layout design that is to be that is done via a silk screen (Inkjet writings). The silkscreen is used for information graphics on the PCB board.
14. Surface Finish:
It is a layer of tin solder or immersion silver, gold plating or other metal that is applied to the PCB boards. It is done to give the boards extra protection against dust, moisture and oxidation, and also good for PCB assembly.
15. Electrical Testing:
Before sending out the boards in the market, all are electrically tested first and some manufacturers sometimes offer some special tools to do so. If found fit, boards are sent out to markets to be used by innovators, engineers and all those in need.
16. Final Inspection:
After testing, the PCB board fabrication is almost finished. But before shipping out, we still need to check minor visual defects such as scraches by inspection machine or manually.
This was all about a PCB board fabrication. But we must take into account that this is just a standard process for multilayer circuit boards. There are some major differences between the processes of double-layer (or single-layer) PCBs and multilayer PCBs. In dual-layer PCB boards, we have copper cladding or copper pathways present on both sides but in multilayer we have multiple copper layers stacked one upon each other, so no lamination is needed for double-layer PCBs. In multilayer PCB boards, blind and buried vias may be needed in complex design.