PCB Technical - How to choose the right board material for flexible PCB manufacturing?

Flexible or flexible printed circuit boards (PCBs) are one of the popular types of circuit boards that meet the needs of flexible electronic circuits. Designed to replace traditional wiring harnesses, flexible PCBs can be easily shaped to suit a variety of complex electronic designs. Moreover, these panels provide design freedom while maintaining density and performance. Although the performance of a flexible PCB depends on many factors, the main material plays a key role in it. Choosing the best material is critical to the success of flexible PCB assemblies. Nowadays, there are a variety of materials available in different configurations to meet the needs of modern applications. This article focuses on different aspects of materials used in flexible PCB manufacturing.

Flexible core or substrate materials commonly used in flexible PCB manufacturing

Flexible circuit boards require adhesives and adhesive-free materials to attach their layers. Both materials provide a range of polyimide core thicknesses.

l Flexible adhesive materials:

These types of materials are the main body of flexible circuit board materials. As the name suggests, they use acrylic or epoxy-based glue to glue the copper to the flexible core. Some important advantages of sticky flexible cores include higher copper peel strength, lower material costs, and so on. Adhesive-based materials are commonly used in single-sided and double-sided flexible circuit board designs.

l Non-adhesive flexible material:

As the name implies, the copper of these types of flexible core materials is directly connected to the core without any adhesive. They can either cast a dielectric on copper or sputter copper on a dielectric film. Generally, they are preferred for flexible boards with a large number of layers. There are several reasons for the popularity of adhesive-free materials, including the elimination of bending thickness, improved flexibility, tighter possible minimum bending radius, higher potential temperature ratings, improved controlled impedance signal characteristics, and so on.

Conductor material of flexible PCB

Copper is the most preferred and easily available conductor material for flexible PCB assembly. The use of this material is due to its beneficial properties, including good electrical properties, high processability and so on. There are two types of copper foils configured with bendable materials: roll annealing (abbreviated as RA) and electrodeposition (abbreviated as ED). These foils come in various weights and thicknesses. Surface treatment of the foil before assembling the board. Combined with relatively low cost, electrodeposited copper foil has become popular in the market. Unlike ED copper foil, RA is quite expensive but has enhanced functions.

Flexible solder mask materials and overlays for flexible PCB manufacturing

The flexible PCB includes a cover layer, a solder mask or an outer circuit that combines the two. In the early days, OEMs used adhesives to bond the layers to the core. However, this method reduces the reliability of the circuit board. In order to solve this problem, they now prefer overlays due to their higher flexibility and reliability. Coverlay has a solid layer of polyimide with epoxy or acrylic adhesive. Likewise, solder mask is a commonly used material for rigid component areas with high-density SMT components. Good design practices include using overlays in flexible areas and solder masks in component areas to explore its function.