PCB adhesives or glues are viscous liquids or gel-like substances used to bond different layers of PCB together. However, there are various options available, and users need to choose based on their specific requirements. For instance, when selecting adhesives for flexible circuit projects, flexibility and bendability to accommodate the bending and folding characteristics of flexible PCB should be emphasized.
Stages of Adhesive Application in Flexible PCB Manufacturing
Since our article focuses on adhesives for flexible PCB, let’s first examine the stages at which these adhesives will be applied in the flexible PCB manufacturing process.
Firstly, let’s introduce the lamination process. As we all know, the internal structure of a PCB is similar to a sandwich, and the lamination process is a crucial step in firmly bonding these layers together. In this process, adhesives are placed between different layers, and when subjected to high temperature and pressure, they are cured to form a strong connection, ensuring the stability and reliability of the PCB.
Sometimes, we may encounter design upgrades or troubleshooting situations where components are firmly fixed on the PCB, and for flexible PCB, disassembly can easily cause circuit damage. Therefore, specialized adhesives are usually designed to be reworkable and can be operated without damaging the circuits below during the operation process.
In addition to the manufacturing stage, due to the slightly insufficient structural strength and stability of flexible PCB, additional reinforcement may be required in certain applications. These reinforcements are firmly bonded to the circuit board with adhesives. These adhesives need to have sufficient adhesion and heat resistance to ensure a strong and reliable connection between the reinforcement and the circuit board while minimizing their impact on the flexible circuit board itself.
Flexible PCB Adhesive Types
Pressure-Sensitive Adhesives (PSA)
Usually appearing in the form of tapes, stickers, labels, etc., PSA’s main advantage lies in its ease of operation. It requires no additional heat, solvents, or chemical reactions; only a slight contact is needed for quick adhesion to the surface. Moreover, even after multiple attachments and removals, its bonding performance remains unaffected.
However, PSA has an obvious drawback, which is its low heat resistance. During the flexible PCB assembly, soldering operations are usually required, and PSA may degrade due to high temperatures, leading to a decrease in adhesive performance. Therefore, if PSA is chosen as the adhesive for your flexible circuit project, attention must be paid to controlling the soldering temperature.
Anisotropic Conductive Adhesives (ACA)
This is a special type of adhesive characterized by its conductivity in one direction and insulation in the other direction, which is due to the arrangement of microstructures in one direction. It should be noted that the preparation process of ACA is relatively complex, so the price is often higher, and it also requires specific process conditions during use. Therefore, this adhesive is usually used for the connection between components and FPCB.
Polyimide (PI) Adhesives
PI is a high-performance engineering plastic with excellent heat resistance, chemical stability, and electrical insulation properties. Compared with other adhesives, PI adhesives are more compatible with flexible PCB because some substrates and overlays are made of this material. However, it should be noted that the use of PI adhesives also has some drawbacks, such as the need for specialized equipment and technology to ensure that the adhesive can cure and bond the various layers of the PCB correctly.
Recommendations for Choosing Adhesives for Flexible PCB
Adhesives available in the market generally meet industry standards, so the key factor in selection lies in the compatibility between the adhesive and the characteristics and application of the PCB. For example, epoxy resin and acrylic are two common types of adhesives. Although both are reliable materials, they differ in their flexibility performance. If your flexible PCB needs to meet requirements for dynamic bending, it’s advisable to choose acrylic adhesives with better flexibility.
Additionally, when selecting adhesives, it’s essential to consider the application environment and the characteristics of the PCB. For applications prone to physical threats, adhesives that minimize the impact on the flexibility and mechanical stress of the PCB should be chosen to prevent delamination issues. For applications susceptible to signal interference, focus should be on the dielectric strength, insulation resistance, and low ion content of the adhesive. In high-temperature environments, it’s crucial to ensure that the adhesive possesses good thermal conductivity and high-temperature stability to prevent decomposition or failure.
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