Flex PCB Manufacturing Process: Materials Selection, Etching, & Lamination

What is a Flexible Printed Circuit?

Flexible PCBs (FPCB) or flexible circuits are a type of printed circuit board that is very flexible in nature unlike the traditional rigid PCBs they are bendable to a great extent which makes them very useful in applications where space constraints are a major factor. Flex PCB manufacturing is very complex. The flexible PCB is capable of being folded dynamically which makes them desirable in modern electronic devices such as foldable smartphones, laptops, and tablets where the circuit requires repeated bending this is possible because this type of circuit is made up of substrates that are flexible in nature such as polyimide and polyester film.

So now we have an idea about what is a flexible circuit. The purpose of this post is to understand the flex PCB manufacturing process and what are the aspects that need to be considered while selecting the materials for the circuit. We are going to look what are the different parts of a flexible circuit and what are the different materials from which these parts can be made. Then we will have a look at all the techniques involved in the flexible printed circuit manufacturing process along with the precautions that are being taken during the flex circuit manufacturing process.

Material Selection in Flexible PCB Manufacturing Process

During the flex PCB manufacturing process, material selection should be done first. A FPCB is mainly made up of many layers depending upon its complexity but typically it consists of three primary layers. Those primary layers and the available material options are.

Substrate Material: A substrate is the base material upon which conductive traces and components are laid. It provides the FPCB with distinct properties such as flexibility, thermal stability, and mechanical strength. Hence to achieve optimum performance substrate material should be selected carefully. Different materials used in the flexible PCB manufacturing process are compared under various parameters in the following table.

Each substrate material has its strengths and weaknesses which makes them suitable for different applications thus selecting the right material depends on specific requirements such as flexibility, thermal resistance, mechanical strength, cost, etc.

Conductive Materials: In the flex circuits manufacturing process, the electric paths are formed using conductive materials to connect various components on the circuit. Choosing the conductive material is very crucial to flex PCB fabrication as it directly impacts the flexibility as well as the overall performance of the circuit.

The following table discusses different conductive materials on the basis of different parameters.

During the flex PCB manufacturing process, each material offers various properties that make them suitable for specific applications. Copper has good electrical conductivity and can stand high temperatures which makes it the most commonly used substrate material. On the other hand, silver ink has much better conductivity than copper but it is more expensive. Carbon ink is a good alternative for low-power applications. Conductive polymer offers versatility. Hence choosing the right material for the substrate depends upon the specific requirements of the Flexible circuit board.

Adhesives and Overlays: Adhesives and cover layers are crucial components of flexible circuits as they provide protection to the circuit from the environment as well as maintain mechanical stability. The material selection criteria for Adhesives and Overlays are as follows.

• Thermal Properties: Adhesives and Coverlays should be able to withstand the temperature range of the FPCB.
• Chemical Resistance: They should resistance against various chemicals.
• Flexibility: Adhesives and Coverlays should not hinder the flexibility of the Flexible circuit.
• Adhesion: They should be able to maintain good adhesion between the different layers.

Etching process

Etching is a step in flex PCB manufacturing where a desired circuit pattern is created on a copper-clad laminate. The steps involved in etching are as follows

• Cleaning: In this initial process the copper-clad laminate is cleaned using chemical cleaners to clean any impurities such as dust, oil, or oxidation layer from the surface.
• Photoresist Application: It is a layer of material that is applied over the circuit design so that it remains intact after etching.
• Exposure: After the application of the photoresist, the assembly is then exposed to UV light. Due to the exposure of the UV light hardens the exposed areas of the Photoresist leaving the unexposed area soft.
• Etching: to remove the unprotected copper, and create the circuit pattern the laminate is immersed in an etchant solution which dissolves the exposed copper photoresist-covered circuit pattern. Ferric chloride (FeCl3), and cupric chloride (CuCl2) are the most used etchants.
• Stripping: After the etching process the remaining Photoresist is removed by treating the laminate with a stripping solution which leaves the etched copper pattern intact.
• Rinsing and drying: The laminate is thoroughly cleaned with a neutral solution to remove any etchant and stripping solution and then dried.  

Lamination is a very important step in the flex PCB manufacturing process. Once all the layers the different layers are prepared, they are stacked over each other in the correct order and bonded by the application of heat and pressure which solidifies the adhesive. Lamination provides mechanical stability as well as protection from the environment. Due to lamination, the mass flex PCB manufacturing becomes streamlined. The following are the steps involved in the lamination of flexible circuits.

• Cleaning: Before the lamination begins, all the PCB layers, especially the copper laminate, and substrate, are cleaned thoroughly with isopropyl alcohol to remove any contaminates on their surface.
• Stack up: after cleaning all the layers are stacked over each other in the correct order.
• Pressing and Heating: Once all the layers are stacked up they are subjected to a temperature of around 375°F (190°C) and pressure of between 275 to 400 psi. this temperature and pressure is applied gradually
• Colling: In this stage, the PCB stack is cooled down gradually while maintaining the pressure. Once the stack is cooled pressure is also released slowly. The gradual release of temperature and then pressure is important in flex PCB fabrication because it prevents internal tension and warpage.

Quality Control

Quality control is very crucial in flexible circuit board manufacturing process because it ensures that the final product meets the required standards and specifications. An overview of the quality control in flexible printed circuit manufacturing process is as follows.

• Design Verification: The design should be verified and should meet all the specifications.
• Component inspection: All the components should be placed in their correct position.
• Solderability test: All the components should be able soldered without any defects.
• Surface Inspection: The surface of the circuit is inspected thoroughly to ensure that there are no damages such as scratches and cracks on the surface.
• Micro inspection: Microscopes are used to inspect the circuit with typical magnification of 5x to 30x.

Except for the above processes, we also need a surface finish process to protect the PCB pads, and also a silkscreen process to print components notations. But for double-sided and multilayer flexible circuit board manufacturing process, drilling and plating are also needed.

Thus, flex PCB fabrication requires a meticulous selection of materials, precise etching, and lamination. This ensures that the final product meets the quality standards of modern electronic devices. The advancement in flex PCB manufacturing techniques and new materials will solidify the role of flexible circuits in the future.