An EMI shielding film is a specially film that can be used to cover electronic devices to protect them from electromagnetic interference. It prevents unwanted electromagnetic signals from interfering with the device’s operation, especially in flexible PCB and electronic components.
What is EMI?
Electromagnetic waves can interact with electronic components, and the interference they produce is called EMI (electromagnetic interference). For example, when a phone is close to a speaker during a call, the noise that occurs indicates that the received signal is interfered with.
When electronic devices are working, they do not want to be interfered with by external electromagnetic waves, nor do they want to emit electromagnetic waves to interfere with external devices. Therefore, electromagnetic shielding is needed to block the propagation path of electromagnetic waves. The attenuation of electromagnetic shielding mainly depends on the reflection and absorption principles of electromagnetic waves.
The main cause of EMI generation
Electromagnetic interference (EMI) is interference caused by changes in current and voltage from three sources.
1. Natural electromagnetic interference
In nature, lightning on Earth or high-energy cosmic rays from space can cause the generation of natural EMI. Here, humans are not involved in the generation of EMI, which occurs due to weather changes such as blizzards, rain, thunderstorms, and other natural phenomena such as solar radiation and cosmic noise. It is worth noting that natural EMI actually has a minimal impact on modern electronic devices.
2. Artificial EMI
All electrical devices manufactured by humans emit artificial EMI, which can cause interference when two signals are close to each other or when multiple signals meet at the same frequency. When the device comes into contact with EMI, it can hinder the normal operation of the device. This type of EMI mainly exists in equipment like generators, igniters, radio transmitters, power lines, telephones, and electric motors, and the EMI caused by such equipment and devices is usually relatively high.
3. EMI of electronic devices themselves
In the case of inherent EMI, electromagnetic interference released from the device itself can easily propagate internally, and the noise generated internally is usually caused by thermal agitation generated by current passing through electronic components such as resistors.
Principle of EMI shielding film
The principle of EMl (Electromagnetic Interference) electromagnetic shielding film is to suppress the propagation and interference of electromagnetic waves through the conductivity or absorption ability of the material. The following are several common EMI shielding film principles:
1.Reflection
Metal films reflect electromagnetic waves through conductivity. When electromagnetic waves hit the surface of a metal film, due to the high conductivity of the metal, the waves are reflected back, thereby reducing their interference with other electronic devices.
2. Absorption
The materials in the shielding film, such as nanomaterials and magnetic materials, have high absorption. When electromagnetic waves pass through the shielding film, conductive or magnetic particles in the material will absorb the electromagnetic waves, convert their energy into thermal energy, and thus consume the energy of the electromagnetic waves.
3. Scattering
Nanoparticles or fine structures in the shielding film can scatter electromagnetic waves, changing their propagation direction or path. This can achieve the dispersion effect of interference signals, reducing the intensity and range of interference.
4. Multilayer structure
Different thicknesses and materials in the multilayer shielding film structure can generate reflection, interference, and absorption effects between layers, thereby enhancing the shielding effect. The interface reflection and multiple interference between different layers help reduce the transmission and interference of electromagnetic waves.
Overall, EMI electromagnetic shielding film utilizes the material's conductivity, absorption ability, and scattering ability to prevent the propagation and interference of electromagnetic waves. By selecting appropriate shielding materials, structures, and thicknesses, the effect of suppressing electromagnetic interference can be achieved, protecting the normal operation of electronic devices.
Problems caused by EMI
Signal Degradation: EMI can distort or corrupt the signals transmitted through electronic circuits, leading to communication errors, data loss, or performance drops.
Fault: EMI may cause unpredictable system behavior, or even complete shutdown. In critical systems such as medical equipment or avionics, this may pose significant risks.
Audio/Visual Disruptions: In consumer electronic products, EMI can cause visible or audible interference, such as static noise in audio systems or flickering in video displays.
Regulatory Standards for EMI
EMI and EMC (Electromagnetic Compatibility) standards ensure that electronic devices do not interfere with other equipment and can operate correctly in specific electromagnetic environments. Different countries and regions have their own regulatory standards, including:
International Standards: Such as CISPR and the IEC 61000 series, widely used in many countries.
EU Standards: Including CE marking and the EMC Directive, with requirements like EN 55032.
US Standards: Such as FCC Part 15, FCC Part 18, and military standard MIL-STD-461.
Chinese Standards: GB/T 9254 and CCC certification.
Other National and Regional Standards: Such as Japan's VCCI certification, Korea's KC certification, Australia's RCM mark, and Canada's ISED standards.
These standards regulate the electromagnetic emission and immunity of devices to ensure global compliance.
Factors to Consider in EMI Shielding for Flex PCBs:
Thickness of Shielding Layer: The thickness of the shielding layer directly affects the shielding effectiveness. Thicker layers provide better EMI attenuation but can reduce flexibility.
Material Selection: The choice of conductive material impacts both shielding effectiveness and weight. Especially in consumer electronics and aerospace applications, the weight of additional shielding layers is a concern. Lightweight alternatives like aluminum or conductive films can help minimize the impact on weight.
Cost: Shielding materials like copper and silver can increase the overall cost of the flex PCB. Alternatives like conductive adhesives or polymer coatings may offer a more cost-effective solution, but might not provide the same level of shielding.
Flexibility vs. Shielding Effectiveness: Adding additional shielding layers, ground planes, or vias increases the effectiveness of EMI shielding but can reduce the flexibility of the flex PCB. Designers must strike a balance between shielding performance and maintaining the flex PCB’s required bending radius.
Operating Environment: The level of EMI shielding required depends on the environment in which the flex PCB will operate. Industrial and automotive environments typically require more robust EMI shielding due to the presence of motors, RF communication systems, and other sources of interference.
What are the types of EMI shielding films used on flexible PCB ?
The shielding film applied on flexible circuit boards is mainly used to reduce EMI and ensure signal integrity. Due to the need for flexible circuit boards to maintain a certain degree of bending and flexibility, shielding film materials suitable for Flex PCB not only need to have good electromagnetic shielding effects, but also need to be lightweight and flexible. The following are common types of shielding films used in flexible circuit boards:
1. Copper foil shielding film
Copper foil has good electrical conductivity and shielding effect, and is one of the most common shielding materials in flexible PCB. By adding copper foil to the surface or middle layer of the flexible circuit board, external electromagnetic interference can be effectively shielded. It is commonly used in high-frequency signal transmission circuits, smart devices and other devices requiring high shielding efficiency.
Advantages: Strong conductivity and good shielding effect.
Disadvantages: The copper foil is thick and heavy, which may affect the flexibility and bending performance of the Flexible circuit board.
2. Aluminum foil shielding film
Aluminum foil is lighter than copper foil, but its electrical conductivity is slightly lower than copper. It is usually used with an insulating layer to form a composite shielding film, suitable for applications that are sensitive to weight and cost requirements. It is commonly used in Flexible PCB designs that do not have such strict shielding requirements, such as consumer electronics, automotive electronics, etc.
Advantages: Lightweight, low cost, good flexibility.
Disadvantages: The conductivity is not as good as copper foil, and the shielding effect is slightly inferior.
3. Conductive fabric shielding film
Fiber cloth (usually polyester fiber cloth) is pre treated and coated with electroplated metal to give it metallic properties and become conductive fiber cloth. It can be divided into nickel plated conductive cloth, carbon plated conductive cloth, nickel plated copper conductive cloth, and aluminum foil fiber composite cloth. There are distinctions in appearance between plain weave and mesh; The most basic layer is highly conductive copper, and the outer layer combined with nickel has corrosion resistance; Polyester fiber cloth coated with nickel/copper/nickel provides excellent conductivity, shielding effectiveness, and corrosion resistance to meet various requirements in different ranges, with a shielding range of 100K-3GHz.
Advantages: Good flexibility, light weight, suitable for designs that require high flexibility.
Disadvantages: The shielding effect is usually not as good as metal foil, and the price is relatively high.
4. Conductive adhesive film
Conductive adhesive film is made by dispersing conductive particles (such as silver, nickel, etc.) in a flexible polymer matrix, which not only maintains flexibility but also provides electromagnetic shielding and conductive connection functions. Conductive adhesive film is usually used for local shielding or as a conductive grounding layer.
Advantages: Good flexibility, easy processing, suitable for fitting complex shapes.
Disadvantages: The shielding effect and conductivity are slightly inferior to metal foil.
5. Conductive coating shielding film
By coating conductive coatings (like silver, nickel, carbon, etc.) on the surface of flexible circuit boards, a thin and lightweight shielding film is formed. Conductive coating shielding film can provide basic EMI shielding while maintaining the thinness of the circuit board. Often used in Ultra thin electronic devices such as mobile phones, tablets, and displays.
Advantages: Thin and lightweight, simple process, suitable for lightweight design.
Disadvantages: The shielding effect may not be as good as metal foil, and the wear resistance is poor.
6. Multi layer composite shielding film
Multi layer composite shielding film is composed of various materials, such as metal foil, conductive adhesive, conductive fabric, etc. By stacking different shielding materials, the shielding effect can be effectively improved while maintaining flexibility. It is usually used in flexible circuit boards in high-performance and complex environments such as automotive electronics, aerospace, and medical equipment.
Advantages: Good shielding effect, able to provide comprehensive EMI protection while maintaining a certain degree of flexibility.
Disadvantages: Complex process and high cost.
The Commonly Used EMI Shielding Film at Flex Plus - HSF6000-2 EMI Shielding Film
Item | Carrier film | Insulation layer | Metallic layer | Conductive adhesive layer | Protective film |
Thickness (μm) | 50 | 5 | 0.2 | 10 | 50/100 |
Material | PET | Black Ink | Alloy | Resin | PET |
Specification of HSF6000-2 EMI Shielding Film
Features excellent shielding efficiency (60dB)
High flexibility
Chemical resistance, support reflow-soldering
Achieve consistent impedance control design
Environment friendly (Halogen-free, RoHS etc)® UL94 VTM-0
Properties of HSF6000-2 EMI Shielding Film
Item | Typical Value | Test Method |
Shielding efficiency | 60 dB | GB/T 30142-2013 |
Grounding resistance | <1Ω（GNDΦ1.6mm） | JIS C5016 1994-7.1 |
Flexibility | >25 times | IPC6013A 3.6.1 (Static bending, R 1mm) |
Soldering | Pass | JIS C 6471 1995-9.3 |
Peel strength | >7N/cm | IPC-TM-650-2.4.9 (D3450) |
Acid and alkali resistance | Pass | JIS C6471 1995-9.2(10% NaOH, 2NHCI immerse 5 min) |
Print character | Pass | IPC-TM-650-2.4.1.1 |
RoHS | Pass | 2011/65/EU |
REACH | Pass | No 1907/2006 |
Halogen-free | Pass | BS EN 1458 2:2007 |
Operation step of EMI Shielding Film
Step 1: Cutting
Step 2: Drilling hole
Step 3: Punching
Step 4: Pre-fixing (Remove the protective film, fix and quick press to fit)；
Quick press: 80℃-120℃ above 50kg/c㎡ above 10 Sec)
Step 5: Fast-press machine (Pre-hot0-1 sec, Press 180-190℃, above 180 sec,
100-150 kg/c㎡) ；Press(Keep-temperature 150-170℃, above 60min, 20-35kg/c㎡)
Step 6: Curing at 160-180℃,1.5-2h, removing carrier film.
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