materials are used in flexible PCBs

A flexible printed circuit board offers a vast array of flexibility and adaptability in modern electronics. The materials used in the fabrication of a flexible PCB are at the core of its functionality and durability, dictating the types of applications it can serve and environments it can thrive in. The following article examines the three most common materials in flexible PCBs: the substrate, copper foil and protective film (coverlay).

Unlike standard rigid PCBs, flex PCBs have a unique insulating base that is naturally flexible. The base material is typically a combination of Polyimide or Kapton (Kapton being the most popular brand) and one to multiple layers of copper. The copper is bonded to the substrate using a resin such as PI Resin, PET resin or modified epoxy resin.

This binding layer is essential to the strength of a flex circuit and enables mechanical processing such as drilling, routing and laser processing. The resulting material is also highly tolerant of solder reflow cycles and expansion and contraction due to temperature fluctuations, making it a key element in the performance of a flexible pcb.

The copper layer is the conductor of the flex circuit and represents the actual circuit traces. The copper must be of the highest quality and grade for a flex circuit to perform well. The higher-grade rolled annealed (RA) copper is ideal for a flex circuit as it can be stretched further before fatigue cracking occurs. RA copper is also springier in the Z deflection direction – ideal for a flex circuit that will be continuously bending and moving.

What materials are used in flexible PCBs?

PI is the most popular substrate material for a flex circuit, but other polymer films such as PET (polyester) or Liquid Crystal Polymer (LCP) are suitable alternatives. These films are generally thinner than PI and offer better resistance to humidity and tears. However, their UL spontaneous combustion rating is lower than PI.

In addition to the insulating film, the flex circuit also requires an adhesive for the layers to attach to. Manufacturers have shifted away from using only glue for this purpose as the process increased the risk of via breakage during assembly. To mitigate this issue, they developed an adhesiveless PI that allows the copper to adhere directly to the insulating film without an intermediate adhesive.

In addition to the insulating and copper film, a flex circuit requires a protective film called coverlay that acts as a shield for the exposed copper surfaces. Coverlay is created with a layer of adhesive on one side of the base material and a photoresist ink that can be coated with either bright or matte oil to protect the surface from damage. The flex circuit is then laminated to the coverlay and is ready for the etching process. The coverlay is a vital component in the reliability of a flex circuit and should be carefully selected for your application.

The burgeoning wearable technology market heavily relies on flexible PCBs. They are integrated into smartwatches, fitness trackers, smart garments, and healthcare wearables. Flexible PCBs enable comfortable, lightweight, and durable wearable devices that can adapt to the body’s contours.