Flexible Circuit Board Manufacturing – How to Choose the Right PCB Material?
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Types of Flexible Core or Substrate Materials Used in Flexible Circuit Board Manufacturing
The core materials of flexible circuit boards are made of adhesive-based and adhesive-free materials. Both provide a range of polyimide core thicknesses.
Adhesive-Based Flexible Materials: Adhesive-based materials form the main body of flexible circuit materials and are typically used in single-sided and double-sided circuit board designs. As the name suggests, they use epoxy- or acrylic-based adhesives to bond copper to the flexible core. Adhesive-based flexible materials offer various advantages, including higher copper peel strength and reduced material costs.
Adhesive-Free Flexible Materials: This type of flexible core material is manufactured by sputtering copper onto a dielectric film or casting dielectric onto copper. They are commonly used when designing rigid or higher-layer flexible structures. In both cases, adhesive-free flexible cores provide superior quality and reliability. These materials are widely used for several reasons, one of the primary benefits being the elimination of the adhesive layer, resulting in a more flexible and thinner structure. Other advantages of adhesive-free flexible materials include a smaller possible minimum bending radius, higher potential rated temperature, and more.
Conductor Materials Used in Flexible Circuit Board Manufacturing
Using thin, fine-grain, low-profile copper foil enables high levels of flexible circuit board manufacturing. There are two main types of copper foil configurations used in flexible materials: Electro-Deposited (ED) and Rolled-Annealed (RA). Both adhesive-based and adhesive-free materials begin with electro-deposited copper; however, in the rolled-annealing process, the grain structure transitions from the vertical ED orientation to the horizontal RA copper structure.
Due to its relatively low cost, ED copper foil has gained immense popularity in the market. RA foil is significantly more expensive but offers improved bending capability. Additionally, RA foil is the standard material required for dynamic flexing applications.
Coverlay and Flexible Solder Mask Materials Used in Flexible Circuit Board Manufacturing
Flexible printed circuit boards consist of outer-layer circuits that are encapsulated with flexible solder mask, coverlay, or a combination of both. In the past, PCB manufacturers used adhesives to bond these layers, which compromised circuit board reliability to some extent. To address these issues, most manufacturers now prefer coverlay due to its enhanced reliability and flexibility. Coverlay features a solid layer of polyimide with acrylic or epoxy adhesive. Similarly, flexible solder mask is the preferred material for rigid component areas using high-density surface mount technology (SMT) components. A good design practice is to use solder mask in component areas and coverlay in flexible areas to maximize their respective functionalities.
Over the years, flexible circuit boards have gained significant popularity and have found widespread applications in complex circuits. Choosing the right flexible PCB material is crucial, as it not only affects the circuit board’s performance but also its overall cost.
