What is EMI Shielding in Flexible PCBs?

PCB Basics 
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Editorial Team - PCB Directory

Dec 2, 2022

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EMI shielding is done in flexible Printed Circuit Boards because flex PCBs are susceptible to electromagnetic interference, like any other electrical interconnect, which affects the circuits and can result in an increase in error rate or even a complete loss of data. Copper layers, silver ink, and specialised shielding films are used for EMI shielding on flex PCBs.

When selecting the shielding material, some additional factors like bend requirements and controlled impedance must be taken into account. The quality of shielding provided by each shielding technique affects the printed circuit boards’ overall thickness. Bending capability of a circuit board reduces with increase in thickness of the PCB.

Hence, as part of the design and material selection process, it is crucial to precisely define and evaluate the minimum bend radius and type of bend requirements for shielded designs. There are additional restrictions depending on whether the bend requirement is static (bend once to fit) or dynamic (repeated bending). The minimum bend capability of a dynamic bend flex PCB application is much higher than that of a static bend design.

The available shielding techniques are further constrained by the requirements for controlled impedance signals. To achieve the necessary controlled impedance values, the shield(s) must have electrical properties that meet the EMI requirements as well as serve as reference planes. Not all shielding techniques can meet both requirements.

Any EMI shield's effectiveness is dependent on the material used as a shield, and the type of circuit and frequency at which it operates dictate the material selection.

The shielding material also differs depending on the electrical and mechanical requirements of the PCB. Each shielding material has its pros and cons which are mentioned below:

Different types of shielding layers used in Flex-PCBs include:

  • Silver Ink Shields
  • Shielding Copper Layers
  • Shielding Films

Silver Ink Shields

Search in sidebar queryFigure 1: Flex PCB stack up with Silver Ink ShieldingSilver ink shields are made up of additional layers of silver conductive ink that are applied selectively to the surface of the coverlay that houses the circuit patterns. Selective openings in coverlays are stitched along the length of the part's outside edges, exposing the design's ground circuit.

Selective openings are filled with ink, which adheres and electrically connects to the ground. The silk screening technique is most frequently used to apply silver ink. The accuracy and range of the shield may be constrained by this technique. The silver ink layers are then laminated with additional layers of coverlay to encase and electrically isolate them.

Silver Ink Shields offer good flexibility, and excellent shielding ability and are inexpensive compared to other shielding materials. Silver Ink shields are not suitable for designs involving controlled impedance. A flex PCB shielded with Silver ink shields is 75% thicker when compared to the non-shielded variant.

Shielding Copper Layers

Copper layer shields are made up of extra etched copper layers that are added outside the structure to increase the number of design layers. Copper layers can either be crosshatched for greater flexibility or solid planes for better shielding. If ground stitching is needed, they require additional vias in the flexible area(s). 

Search in sidebar queryFigure 2: Flex PCB stack up with Copper Shielding

The copper layer gives highest level of protection and is perfect for controlled impedance designs solution. However, copper Layer shields are expensive and offer minimum flexibility. Additionally, stitched vias can reduce flexibility by acting as mechanical bend stress concentrators. For instance, a copper-shielded three-layer flex PCB is approximately 125% thicker than the non-shielded variant.

Shielding Films

On top of the coverlays, shielding film is made up of an additional layer of selectively specialised laminate. These films were created to satisfy the demands of the camcorder and cell phone markets. Similar to how silver ink creates interconnect points, the coverlay has specific openings that reveal the ground circuit. A metallic deposition layer, an isolation layer, and an electrically conductive adhesive make up the three layers of the shielding films. 

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Figure 3: Layers of Shielding Film

The friction-resistant isolation layer is black. These shielding films are laminated using pressure and heat. The electrical connection to the ground is made after the conductive adhesive adheres and flows into the coverlay opening. 

Figure 4: Flex PCB stack up with Specialized Shielding Film

Shielding films offer the highest flexibility and overall thinnest construction. These are cost-effective and have exceptional Shielding Capability. However, shielding films offer limited uses for Controlled Impedance Designs as it increases the tolerance value of PCB. The overall thickness of flex PCB shielded with the specialized film is only 15-20 % more than the non-shielded variant.

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