Shielding Flex PCBs from EMI

 In PCB

Rush PCB Inc makes all types of Printed Circuit Boards (PCBs), including Flexible PCBs. Like any other types of PCBs, flexible circuits are also susceptible to interference such as RF and Electromagnetic (EM). The negative effect of EMI and RF interference can be devastating malfunctions, leading to a loss of data and increases in error rates. We address these interference issues in our flex circuit boards by using EMI shielding using various methods.

What Causes EMI?

Electrical sources typically generate EMI or electromagnetic interference that may affect electronic circuits indirectly through electrostatic coupling, electromagnetic induction, or directly by conduction. If a circuit is susceptible to EMI, its performance may degrade, and in severe cases, the circuit may even stop functioning. High frequencies can cause components and circuit traces to act as antennas, receiving or emitting radiations.

EM radiations can interfere with other components on the board and affect their performance. Therefore, it is necessary to take steps to control the generation of EMI and make the circuit more compatible to EM.

What is Shielding?

Shielding effectively reduces the amount of external EMI and RF interference affecting the circuit. Simultaneously, shielding also inhibits leakage of high-frequency signals from a circuit and prevents them from interfering with neighboring circuits.

The simplest shield is a metallic barrier that can absorb the EMI entering or leaving a circuit. With the metallic screen completely surrounding the transmitting or receiving circuit, it acts as a Faraday cage.

For instance, it is possible to introduce a Faraday cage around a multi-layer board, by placing several ground points on the board edges. As all routing signals are encased by the ground points, it effectively shields the circuit from leaking or receiving EMI.

Challenges in Flex Shielding

When subjecting flexible circuits to shielding, designers come across many challenges. One of the major challenges is the increase in thickness caused by introduction of shielding. Although the flex PCB cost does increase to some extent because of additional shielding, increase in thickness is of greater concern, as this leads to a violation of the flex board’s bend criteria.

Flex circuits are made to allow bending and twisting. With the addition of shielding to meet EMI/EMC requirements, the thickness of the flex circuit increases, leading to a difficulty in fulfilling both mechanical and electrical design requirements. For our flex circuits, we use various options to minimize the EMI/EMC effects on one hand, while meeting the mechanical and electrical design specifications, on the other.

Shielding Considerations for Flexible PCBs

EMI/EMC considerations for flexible circuits must take into account two major requirements, such as bending capability and controlled impedance.

Bending Capability

Adding any type of shielding increases the total thickness of the flexible circuit. This reduces the bending capability of the flex circuit, its minimum bending capacity being inversely proportional to the board thickness. To achieve the minimum bend radius and bend type, the designer must evaluate and select proper materials that allow both bending and shielding.

Bending capability also depends on the type of bending the board must undergo. The application may demand a static type of bending, requiring the board to bend only once during its installation. Other applications may need dynamic bending, requiring the flex board to bend repeatedly when functioning. Dynamic bending demands a greater bending capability from flex circuits than static bending does.

Controlled Impedance

High-speed operation of electrical signals in a flex circuit demands control impedance requirements. Achieving control impedance requires selection of special substrate material, close control over substrate thickness, and presence of copper layers acting as reference planes around the track carrying the high-speed signals.

Shields introduced on the flex circuit must not only meet EMI/EMC requirements, but also preserve the controlled impedance characteristics.

Types of Shielding for Flexible PCBs

We typically use three types of shielding on flexible PCBs for guarding them from EMI/EMC:

  • Copper Planes
  • Silver Ink
  • Special Shielding films

Selection of the shielding type depends on the performance required, its influence on bending capability, and its effect on controlled impedance.

Copper Planes

In this design, we use copper planes linked to the ground reference using stitched vias. The copper plane may be entirely solid or cross-hatched. The shield layers or planes enfold the signal layers. While solid copper planes provide better shielding, it reduces the flexibility of the board and adversely affects its bending capability. To retain the flexibility of the board, designers often resort to using a cross-hatched pattern for the copper plane. However, this leads to poorer EMI shielding.

Although the copper plane arrangement offers efficient shielding, the disadvantage is the increase in PCB cost and its design thickness. The extra thickness limits the board’s bending capability when bending it into its desired shape, leading to a reliability problem. However, this arrangement supports controlled impedance designs.

For a single layer flex board, adding a two-layer copper plane for improving its shielding, may increase the cost substantially. This is because the addition of the two copper shielding layers changes the type of the board from a single layer to a three-layer board. The additional copper layers, added for the shielding, contribute to the overall expense.

Another drawback in adding copper shielding layers is their requirement of grounding, as this means adding vias in the flex area. IPC 2223 standards do not recommend using vias in the flex area, as these vias reduce flexibility, while functioning as concentrators for mechanical stress.

Silver Ink

Shield layers made of silver ink are a favorite option when compared to copper layers. This is because silver ink offers substantially higher flexibility and lower costs compared to copper planes. However, shield layers made of silver ink require an additional coverlay to encase and protect them. The silver ink shield layer adds only about 75% additional thickness to a non-shielded flex board.

The design of shield layer made of silver ink requires specific perforations in the main coverlay of the board for electrical connection to ground planes. As the silver ink flows into these holes in the coverlay, it establishes a connection between its parent shield and the ground layer.

Compared to other EMI shielding materials, silver ink is considerably inexpensive. The number of material layers is thinner than copper layers, while the manufacturing process steps are lower. However, this is not a feasible option when considering EMI/EMC shielding for flex PCBs with controlled impedance designs.

Special Shielding Films

Another common option for providing EMI/EMC shielding in flex boards is by using special shielding films. They allow the thinnest possible shielding, suitable for dynamic bend applications. We use them for a variety of sensitive applications as they are extremely effective.

Special shielding films have a laminated three-layer structure—the inner layer is an electrically conductive adhesive, followed by a metallic deposition layer. The exterior is an insulating layer. The film requires bonding to the surface of the coverlay on the board with heat and pressure. Perforations in the coverlay are necessary to make the electrical connections to the ground, using a conductive glue.

Of all the three methods, a two-sided special shielding film adds only about 15-20% additional thickness to a non-shielded flex board.

Conclusion

Flex boards, similar to any other type of PCB, require shielding to protect them from EMI/EMC. At Rush PCB Inc, we provide a dependable, cost-effective, and functional shielding. For this, we thoroughly examine all the mechanical and electrical design factors before suggesting the best option possible. If you have a flex board that you would like our help in reducing EMI/EMC, do not hesitate to contact us today.