With the development of multi-functional electronic appliances, higher standards have been established for manufacturing flexible PCBs. Since they control the functionality of the entire electrical assembly, it is imperative to maintain high-quality during the manufacturing process. Any design or manufacturing flaw in a flexible PCB will seriously affect the efficiency, working performance, and durability of the end product. Its manufacturing process is time-consuming and laborious and involves several steps that ought to be carried out with extreme care and accuracy.
This article aims to provide a step-by-step guide to help our readers understand the basics of the flexible PCB manufacturing process.
So, lets find out!
Selection of Material for Flexible PCBs
We know that flexible PCBs are designed to provide resistance to high temperature and pressure, and that is why it is necessary to spend due time and energy on the selection of the substrate material that will determine the performance of a flexible PCB. The substrate material primarily contains a flexible adhesive film and a flexible dielectric film. The latter may include:
- Polyester: used in simple or low-end products
- Polyimide: most commonly used type
- fluoropolymer (PTFE): used in high-tech and sophisticated appliances like aerospace, medical, and military products.
Flexible PCB Manufacturing Technology
Generally speaking, flexible PCBs are designed by orderly stacking flexible selective layers through drilled holes, which are responsible for electrical connection between the layers. Various types of standard flexible PCBs are shown in the figure below.
Steps in the Manufacturing of Flexible PCBs
A typical Flexible PCB is developed in the following order:
1.Cleaning the Laminate/Base Material
The first step involved in the assembly of flexible PCBs is cleaning the substrate material. Usually, an anti-tarnish coating is applied to the substrate that contains a copper layer for conductivity by vendors to protect against oxidation. This coating may create hindrance in the electrical conductivity of the PCB in the future and so, it needs to be removed. Generally, developers use the following technique to remove it:
- First of all, they either submerge copper coil in an acidic solution or expose it to acidic spray.
- Then, it is treated with sodium persulfate for micro-etching – to remove the thin, deformed coating by dissolving in the solution.
- Lastly, the copper coil is coated by appropriate oxidation agents to prevent oxidation and adhesion.
2. Developing Circuit Pattern
After properly cleaning the laminate, the developers proceed to generate a circuit pattern a very crucial step in the manufacturing process. It helps to identify various components, parts, marks, logos, etc. on the printed circuit. At present, it is done using two major methods, which are:
Photo Imaging: Albeit older, but photo Imaging is still the most commonly used method to generate circuit traces onto the surface of the copper substrate. In this method:
- Firstly, a dry photoresist film containing the required circuit pattern is put close to the substrate.
- Then, this whole assembly both the substrate and the photoresist film is then exposed to Ultraviolet radiation that deposits the pattern from the photoresist film on the laminate.
- Finally, chemical methods are used to remove the film with the required circuit pattern left on the substrate.
Screen Printing: Using this method, developers print the circuit pattern directly onto the surface of the laminate.
3. Etching the Circuit Pattern: Once the circuit pattern is developed, developers, then, etch the substrate material. Now, you can either immerse the copper laminate in the Etch Bath – to develop a thin etched surface or spray the assembly with an etched solution.
You may etch both sides of the lamination simultaneously if required.
4. Drilling Holes in the Substrate: After etching the circuit pattern, you have to drill holes and pads in the circuit according to the requirement. High-speed, small hole capable, and high precision drilling tools are available in the market to drill precision holes. Drillers often use the Laser-based drilling system for making ultra-small holes in the laminate. Normally, CO2 and Excimer YAG lasers are used by flexible manufacturers for this purpose.
5. Through-Hole Plating: This step requires extreme care and precision as it involves chemical plating of copper along the drilled holes. This process provides a layer to layer electrical connections.
After drilling the required number of holes, electrolytic copper you can use fully automated copper if required – is plated through the holes to provide a conductive path from one end of the circuit to the other. This process is also called Electroless Plating.
6. Apply Cover lay or Cover coat: As the name suggests, cover lay or cover coat is used to encapsulate and protect the top and bottom side of the flexible PCB. This layer protects the PCB against tough weather conditions, and wear and tear during its working. In most cases, a polyimide film with adhesive properties is used as a cover layer. This protective layer is deposited on the surface using screen printing (explained earlier) and then exposed to UV radiation for curing.
Cover coat, on the other hand, is directly applied on the surface of the laminate. Before choosing the type of material for the cover coat, the developers take into consideration factors like materials used, manufacturing process, and potential applications of the board.
7. Blanking the Circuit Board: Cutting or blanking each printed board from the main production panel warrants extreme caution of the manufacturers. Engineers prefer using the hydraulic punching method for producing high-volume of flexible PCBs, but it is costly. Contrary to it, developers recommend using a blanking knife for prototyping flexible PCBs in small production as it is cost-effective.
8. Testing and Verification: Finally, when the whole assembly process is completed, the boards underdo electrical testing and verification to assess their performance, continuity, and quality corresponding to the design specifications. These tests are carried out in compliance with per IPC ET-652. Mostly, the developers use the flying probe and grid methods for testing flexible PCBs.
To sum it all, Flex-PCB manufacturing is a tough job and warrants utmost care and accuracy in every step from planning at the start to material selection, designing, assembling the components, and testing.