Polyimide film applications and challenges in mobile phones!
Dec 08, 2024
1. What Is Polyimide Film?
Polyimide (PI) refers to a group of aromatic heterocyclic polymers containing imide groups in their molecular structure. Known for its exceptional properties, PI is regarded as one of the most heat-resistant polymers commercially available today.
Main Applications:
Polyimide is widely used in advanced technology fields in various forms, including films, coatings, plastics, composites, adhesives, foams, fibers, separation membranes, liquid crystal alignment agents, and photoresists.
Key Application Fields:
Aerospace: Insulating materials for spacecraft.
Marine Equipment: Protective materials for naval electronic systems.
Electronics and Electrical Industry: High-temperature insulation films and flexible circuit board substrates.
Among these, polyimide film is the most commercially successful product and is widely utilized across high-tech industries due to its outstanding comprehensive performance.
2. Differences Between Tape and Protective Film
Thanks to its exceptional performance, polyimide film has been adapted into various forms, including tapes and protective films.
Tape
Tape consists of a base material and an adhesive layer. It bonds two or more objects together by applying adhesive to the base material's surface. It's commonly used for securing, sealing, and insulating.
Protective Film
Protective film is designed to shield delicate surfaces from scratches, contamination, or corrosion. For instance, it protects electronic device screens during manufacturing, transportation, and storage.
Key Difference:
Protective film is a subset of tape, but the primary distinction lies in their purpose:
Tape focuses on bonding and securing.
Protective film emphasizes surface protection.
3. Applications of Polyimide Film in Smartphone Components
Polyimide film plays a crucial role in various smartphone components, such as:
1. Flexible Circuit Boards and Reinforcements
Flexible Circuit Boards (FPC)
Flexible circuit boards are printed circuits made from flexible insulating materials like polyimide film. These boards typically feature a multilayer structure consisting of copper foil, an insulating substrate, and a cover film. Polyimide film serves as the insulating substrate for circuits, ensuring flexibility and reliability.
Reinforcement Materials
Polyimide film is also used to reinforce flexible circuits, providing additional strength to areas that experience high mechanical stress while maintaining the circuit's flexibility.
As smartphones evolve to become thinner and more multifunctional, polyimide film has become an essential material due to its superior heat resistance, excellent electrical insulation, and flexibility.
iPhone's Approach to High-Frequency Communication
For high-frequency communication, black LCP antennas are commonly used in iPhones. Beneath the antenna, a reinforcement material is required to support its structure, with a thickness specification of approximately 0.225–0.25 mm. PI (Polyimide) is generally preferred for this purpose due to its excellent thermal and dielectric properties.
Challenges with PI Thickness
However, achieving this level of thickness with PI presents significant challenges:
Manufacturing Difficulties: Producing PI at such a thickness while maintaining its quality is technically demanding.
Dielectric Constant: Ensuring a stable dielectric constant at this thickness is another critical hurdle.
Composite Methods and Their Issues
To address these challenges, some manufacturers have adopted composite methods, combining epoxy resin or acrylic hot-melt adhesives with PI. While this approach meets the thickness requirement, it introduces several problems:
Delamination: The composite layers are prone to separation under mechanical or thermal stress.
Low Rigidity: The resulting material lacks the necessary stiffness, affecting performance and durability.
Glue Overflow During Die Cutting: During the die-cutting process, adhesive often overflows, leading to uneven edges and potential contamination of other components.
These challenges highlight the need for further innovation to enhance material performance and processing methods in high-frequency communication applications.
Graphite Sheet
High-Thermal-Conductivity Graphite Film: This material is produced by carbonizing polyimide (PI) film followed by graphitization at temperatures ranging from 2800°C to 3200°C. It is also known as artificial graphite film, as opposed to natural graphite film.
Challenges with Graphite and Protective Layers
Both artificial and natural graphite are prone to delamination, which necessitates edge-sealing treatments with PI or PET protective films (acrylic-based adhesive tapes). However, over time and with increased operating temperatures, the adhesive tends to degrade, significantly reducing the lifespan of the graphite and increasing the risk of conductive short circuits caused by graphite detachment.
Improved Process for Enhanced Performance
To address these issues, we developed a process where a liquid PI coating is applied to the graphite surface and then imidized to form a film. This approach offers several key benefits:
Enhanced Adhesion: The imidized PI penetrates into the micro-gaps of the graphite, forming a strong bond and significantly improving durability.
Extended Lifespan: The lifespan of the graphite is greatly increased, with a service life of up to 10 years easily achievable.
Improved Temperature Resistance: The operational temperature of the graphite can be elevated from approximately 80°C (limited by the acrylic adhesive) to over 260°C, dramatically expanding its range of applications.
This innovation not only enhances the reliability of graphite materials but also ensures safer and longer-lasting performance in high-temperature environments.
Graphite as a Base Material for Specialty Tapes
Graphite can serve as a base material for adhesive tapes. With specialized surface treatments such as coating, compositing, PVD magnetron sputtering, or chemical plating, it can be tailored for unique applications in specialized fields.
We have integrated the latest PPCVD (Plasma-Polymerized Chemical Vapor Deposition) technology to apply a protective layer over the metal surface. This protective layer enhances the metal's resistance to oxidation and scratches without compromising its conductivity, unlocking new possibilities for advanced applications.
Graphite as a Base Material for Specialty Tapes
Graphite can serve as a base material for adhesive tapes. With specialized surface treatments such as coating, compositing, PVD magnetron sputtering, or chemical plating, it can be tailored for unique applications in specialized fields.
We have integrated the latest PPCVD (Plasma-Polymerized Chemical Vapor Deposition) technology to apply a protective layer over the metal surface. This protective layer enhances the metal's resistance to oxidation and scratches without compromising its conductivity, unlocking new possibilities for advanced applications.
Coverlay Film and Lithium Battery Edge Sealing
Coverlay Film: Protection for Flexible Circuits
Coverlay films are designed to shield flexible circuits from heat (high temperatures), moisture, contaminants, and corrosive gases, offering robust protection in harsh environments. For FPC black PI film, the primary specification is 12.5μm thickness with specific CTE (Coefficient of Thermal Expansion) requirements.
The choice of black matte coverlay films and tapes in smartphone interiors serves several purposes:
Aesthetic Simplicity: Black offers a clean and minimalist appearance.
Intellectual Property Protection: It prevents competitors from copying internal designs.
Internal Metal Protection: Black minimizes light-induced damage to internal metal components during production.
Optical Inspection Accuracy: Matte black reduces light reflection, mitigating errors during optical testing.
Design Elegance: Black enhances the perceived quality of the product's appearance.
Edge Sealing in Lithium Batteries
Since aluminum-plastic composite films are thin, gaps inevitably remain at the seals during battery packaging. To ensure safety, these seals require a specialized high-temperature-resistant adhesive tape to prevent electrolyte or gas leaks, which could harm the device or lead to combustion and explosions upon electrical contact.
Kapton tape, made from polyimide (PI), is ideal for edge sealing due to its:
High-temperature resistance
Chemical and mechanical strength
Flame-retardant and halogen-free properties
These features make PI tape the first-choice material for sealing and edge-wrapping
Material Comparisons for Edge Wrapping
PET (Polyethylene Terephthalate):
Advantages: High rigidity due to biaxial stretching.
Disadvantages: Prone to lifting in narrow wrapping areas, reducing long-term reliability.
NOMEX Aramid Paper:
Advantages: Best coverage, suitable for thicker areas and deeper grooves.
Disadvantages: Lower voltage resistance, higher dyeing costs for black coloration, and susceptibility to moisture.
Application: Primarily used at positive and negative electrode interfaces.
By leveraging the distinct properties of these materials, modern lithium battery packaging ensures optimal safety and functionality.
