Introduction:

In recent years, the miniaturization and reduced thickness of portable electronic products have led to a decrease in available mounting space inside devices. As a result, the demand for flexible circuit boards (PCBs) has been steadily increasing. The primary applications of flexible PCBs in electronic products include:

• Installation of driver chips for LCD, plasma displays, and other flat-panel displays.
• Smartphones, computer hard disk drive modules, optical signal receiving modules, among others.

The field of flexible PCB applications is expanding rapidly as technology continues to advance.

Flexible Copper Clad Laminate (CCL):

Flexible Copper Clad Laminate (CCL) is typically available in two main types:

• A three-layer structure with epoxy resin as the adhesive.
• A two-layer structure without adhesive.

The structure without adhesive exhibits superior heat resistance and dimensional stability, as it contains no halogens, making it an ideal choice for flexible CCL. However, the lack of adhesive can cause difficulties in ensuring proper adhesion between the polyimide film (used as the insulating layer) and the copper foil during PCB production.

Halogen-Free Coating Materials:

Coating materials are films applied to the surface of flexible PCBs to protect the circuitry. These materials are essential for ensuring the longevity and reliability of the boards. In recent years, with growing environmental concerns, several halogen-free coating materials have been introduced. However, these alternatives often do not match the original halogen-based coatings in terms of insulation reliability and peel strength. To address these challenges, researchers are focusing on the following areas:

• The need to determine the role of halogens in the coating.
• Ensuring that halogen-free coatings offer sufficient insulation reliability and peel strength.
• Maintaining the softness of the coating material.
• Controlling the fluidity of the coating to achieve the desired thickness on the circuit surface.

Technology for Halogen-Free Solutions:

To achieve flame retardancy while maintaining the necessary softness of the coating, elastomers are often added to the epoxy resin. However, this can negatively impact flame retardancy. To mitigate this, researchers have explored the following strategies:

• Using phosphorus-based flame retardants, such as phosphate ester, which promote dehydration and carbonization during combustion, forming a protective carbonized layer that cuts off oxygen and reduces flammability.
• Introducing nitrogen-containing flame retardants that generate inert gases during combustion, reducing the amount of oxygen available to sustain a fire.
• Incorporating inorganic flame retardants, such as magnesium hydroxide or aluminum hydroxide, which release water during combustion to lower the temperature and inhibit flame spread.
• Using special resin structures, such as epoxy resins combined with phenyl alkyl groups, which enhance flame resistance.

Conclusion:

To improve the adhesion between copper foil and the polyimide film, advanced surface treatment technologies for copper foil and pressing techniques for CCL were developed. This has led to the creation of flexible CCL with superior adhesion. Furthermore, new halogen-free coating materials were developed that maintain the softness required for effective protection while ensuring flame retardancy. These innovations allow the production of fully halogen-free flexible PCBs.

Halogen-free PCBs have gained significant popularity due to increasing environmental and safety concerns regarding traditional PCBs that contain halogenated materials (such as bromine and chlorine). The primary driver for the adoption of halogen-free PCBs is the demand for more environmentally friendly and less toxic electronic products.

Chapters:

• 00:22 What is halogen-free?
• 01:36 What is the replacement for halogen?
• 02:24 Advantages of Halogen-Free PCBs
• 03:35 Drawbacks of Using Halogen-Free PCBs

If you have any questions about PCB or PCBA, feel free to contact us at info@wellcircuits.com.