Flexible Circuit Board Material Selection

Since printed circuit boards are used in almost all electronic gadgets and devices, such as smartphones, calculators, medical equipment, and defense systems, the demand for advanced PCB technology is at an all-time high.

Printed circuit boards offer miniaturized devices with advanced functionalities that enable manufacturers to develop multifunctional devices we use nowadays. Thus, the importance of flex PCB board material selection is essential. Material selection is determined by many factors, such as budget, electrical considerations, build-up, etc.

This article will discuss the benefits of flexible circuit boards and material selection according to the requirements.

Benefits of Flex PCB

Flexible PCBs are the new and improved version of conventional printed circuit boards. They are designed to reduce the size of devices further and increase component count and functionality.

• Flex PCBs are smaller and thinner than conventional circuit boards.
• The bending capability makes it easier to fit into corners and tight spaces.
• Reduces the weight of the device.
• Versatile and can be used in multiple applications.
• Low-cost assembly.
• It has improved speed and circuitry.

Flex pcb Material and Technical Terms

A flex PCB consists of 3 main layers:
1. Core
2. Copper
3. Soldermask

The choice of materials for flex PCBs is different from conventional circuit boards. Let’s discuss each material in detail.

Core - Adhesive vs. Adhesiveless
There are two major material types; adhesive-based and adhesiveless. Both have their pros and cons and differ in the method that attaches copper to the polyimide core. These materials also vary in their thickness and thus should be chosen based on the project's requirements.

Adhesive-Based
Adhesive materials use epoxy and acrylic adhesives to attach the copper layer to the core. The adhesive reduces material costs and has higher copper peel strength. But there are a few disadvantages, such as:
• It is thicker than adhesiveless materials
• It absorbs moisture from the environment and can cause cracks in rigid areas.

Adhesiveless
Adhesiveless materials enable manufacturers to cast the copper directly to the core, eliminating the need for an adhesive layer and thus reducing thickness. There are many advantages of adhesiveless PCB material, and thus it is most commonly used in flex PCB manufacturing.

• Reduced thickness improves flexibility and bending capabilities.
• Eliminates extra layers.
• Doesn’t absorb moisture from the environment.
• Can withstand high heat and operating temperatures.
• Improves the reliability of through-hole vias in a higher layer count PCB.

Based on this, there are 3 most commonly used core materials:

Polyimide
Polyimide is a widely used material for flex PCBs. It is cost-effective and heat and chemical-resistant. It has average electrical characteristics and good physical capabilities, thus suitable for flex PCBs. It is used for smartphones, calculators, and cameras.

Polyether Ether Ketone (PEEK)
PEEK is a core material that is mainly used in the medical industry. It is a substrate with excellent electrical performance and is heat-resistant. It can withstand high operating temperatures and doesn’t melt during soldering.

Polytetrafluoroethylene (PTFE)
PTFE is used in the automotive and defense industry. It is extremely stable and has a low dissipation rate. It can also withstand high heat and is used for material requiring high signal transmission, such as driving assistance.

Copper Type and Thickness
A copper layer, also known as a cladding component, is required to cover your flex pcb and keep it well-insulated to protect it from the external environment. There are many choices for copper layers, such as:
• Electro deposited: Simple and lower-cost material
• Copper-clad laminate: Doesn't soften on exposure to high temperatures and thus reduces the chances of board failure. This material stays flexible and elastic under extreme conditions.
• Rolled annealed: A slight modification from electrodeposited copper material. It has improved grain structure and bending capabilities. It is used in dynamic flex applications.
• Polyimide resin: Average thermal control and resistance. You also need to choose a high-quality film to improve peel strength and reduce moisture absorption.
Copper material is available in varying thicknesses, and the choice depends on your project requirements. For example, the most commonly used copper thickness is ½ and 1 oz. Any thicker or thinner than this can increase the manufacturing cost and weight of the device.
Copper thickness less than ½ oz is used for devices that require flexibility and high circuitry. In contrast, more than 1 oz copper thickness is used for devices with high current requirements.

Soldermask Material
You all must be familiar with the characteristic green color of circuit boards due to solder masks. Nowadays, solder masks are also available in different colors, such as blue, purple, etc. A solder mask is used to prevent liquid solder leakage.

The process involves applying a solder mask to the flex circuit board. Then molten solder is applied to the exposed copper surfaces. This occurs by heating the board at a high temperature where the solder becomes molten and components are attached to the assigned places. Later on, the solder is dried, and the components are fixed and soldered.

Cover Layer Material
Another material used to cover the circuit board is the cover layer. It can be used separately or in combination with a solder mask. Cover layers use a solid polyimide layer with adhesive to attach to the core under high heat and pressure. Coverlays are preferred over solder masks due to increased reliability and flexibility.
You can also use both solder masks and coverlays in the same circuit, such as solder masks in component areas and coverlay in flex areas.

Factors Affecting Flex pcb Material Choice

Many factors affect your flex PCB material choice, such as:
• Cost: Material prices and manufacturing costs can increase significantly based on the material used and project requirements. Materials such as blind and buried vias and gold tabs cost extra due to complex fabrication.
• Industry: Flex PCBs are used in the automotive, aerospace, medical, and consumer industry. Depending on the industry type, material selection can vary.
• Heat-resistance: The PCB manufacturing process requires the circuit board to go through high temperatures during lamination and soldering. Thus, you need to choose a material with high heat resistance.

It’s best to consult a professional PCB manufacturer, like Hemeixin, before deciding on the material for your flex PCB. They can offer expert guidance, free quote, and manufacturing and fabrication services.