What Types of Adhesive in Flexible PCB?

In flexible PCB manufacturing, The function of adhesive is to bond together all the layers of the circuit board. The main types of flexible PCB adhesive are epoxy, acrylic, thermoplastic PI, pressure-sensitive adhesive, and conductive adhesive. Different adhesives are applied in different processes. They all have their own unique characteristics.

1. Lamination Adhesive

Lamination adhesives can bond the copper foil to the polyimide film or join multiple flexible printed circuit layers. Below are 4 commonly used types of laminating adhesives:

• Epoxy adhesive: Strong bonding and good heat resistance, but not very flexible.

• Acrylic adhesive: Better flexibility and temperature performance, ideal for bending areas.

• Modified epoxy-acrylic adhesive: Combines both strength and flexibility for high-end applications.

• Adhesiveless structure: Instead of using glue, copper is directly laminated or deposited on PI film. This makes the circuit thinner and more stable.

Lamination adhesives are the foundation of any flexible PCB adhesive system, ensuring the layers stay bonded even under repeated bending and heat exposure.

2. Coverlay Adhesive for Flexible PCB

The coverlay is a protective film applied to the surface of a flexible PCB, leaving only solder pads exposed. The adhesive under the coverlay determines how well it sticks to the copper and base material.

• Acrylic adhesive: The most common and cost-effective option.

• Epoxy adhesive: Stronger and more heat resistant, suitable for industrial or automotive electronics.

• Thermoplastic polyimide adhesive: Used in high-temperature FPCs, can withstand up to 300°C.

A good flexible PCB coverlay adhesive not only protects the circuit surface but also prevents cracking or delamination during bending.

3. Stiffener Adhesive in Flex PCB Assembly

Stiffeners, made of PI, FR4, or stainless steel, are added to connector areas to increase strength and support.There are 2 main adhesives are used for this purpose:

• PSA (Pressure Sensitive Adhesive): The 3M 467 or 468 series are very polular. Easy to apply, no heat curing required. Of course, there are other brands to choose from as well, depending on the customer's needs.

  

  3M-467 Series Adhesive

• Thermosetting epoxy adhesive: Provides higher strength but needs heat curing.

The right stiffener adhesive helps prevent connector damage and keeps the flexible PCB stable during assembly.

4. Other Flexible PCB Adhesives

Besides lamination and stiffener bonding, other adhesives are also used in flexible PCB production, such as:

• Conductive adhesive: Contains silver or other conductive particles for EMI shielding or grounding. It is commonly used on the flexible PCB pads of automotive PDLC applications.

• Encapsulation adhesive: Used to seal and protect chip areas during SMT or COB assembly.

  

  Encapsulation adhesive for COB assembly

• PSA tape: By attaching it to the back of the FPC, and removing the release paper during installation, the FPC can be fixed onto the panel or the machine casing. For subsequent operations.

These special adhesives expand the application range of flexible PCBs, improving both mechanical and electrical performance.

Summary of Flexible PCB Adhesive Types

Application	Adhesive Type	Main Features
Lamination	Epoxy, Acrylic, Adhesiveless	Bonds copper foil and PI film
Coverlay	Acrylic, Epoxy, Thermoplastic PI	Protects circuit surface
Stiffener	PSA (3M 467/468), Epoxy	Adds mechanical strength
Shielding	Conductive Adhesive	EMI and grounding protection
Encapsulation	Epoxy, Silicone	Protects chip area

FAQ: Common Questions About FPC Adhesives

Q1: Why are different adhesives used in flexible PCBs?

Different adhesives have different applications — lamination adhesives bond copper to PI film, coverlay adhesives protect circuits, and PSA or conductive adhesives support assembly and EMI functions.

Q2: What is the most commonly used adhesive in FPCs?

Acrylic adhesives are the most widely used type.

Q3: What is an adhesiveless flexible PCB?

An adhesiveless flexible PCB is a type of flexible circuit that doesn’t use adhesive to bond the copper foil to the base film. Instead, the copper is directly laminated onto the PI substrate through a casting or sputtering process, resulting in thinner, lighter, and more heat-resistant circuits.

Q4: Are PSAs suitable for high-temperature applications?

Standard PSAs are not recommended for long-term high-temperature exposure; in such cases, our technicians recommend that you choose a thermosetting epoxy adhesive or a thermoplastic polyimide adhesive.