Electromagnetic interference (EMI) is something generated by the power supply, which affects the circuit through electromagnetic induction, electrostatic coupling or conduction. EMI can significantly degrade the performance of FPC, or even make it unable to work properly.

Radio frequency interference (RFI) refers to the mutual interference during the transmission of electromagnetic waves. It causes the receiving end incorrectly identify information. RFI will generate useless signals on the radio frequency, thus affecting normal operation of the instrument.

Shielding can prevent electromagnetic and radio frequency interference from external signals. It can also suppress high-frequency signals from leaking out and interfering with surrounding circuits. This is achieved by using a metal barrier to absorb electromagnetic interference transmitted through the air.

FPC: How to shield EMI & RFI

FPC is susceptible to EMI and RFI. The negative effects of them range from increased error rates to complete loss of data. When talking about the shielding for FPC, it's a challenge in design. All shielding methods will increase the thickness and cost of FPC. The increase in thickness leads to violations of bending standards, which causes mechanical failure and additional cost.

It is necessary to consider bending requirements and controlled impedance. Every shielding method will increase the overall thickness of the FPC. Since the minimum bending ability related to thickness, it will limit or reduce the bendability of FPC.

As part of the design and material selection, the minimum bend radius and bend requirements for shielding design must be accurately determined and evaluated.

EMI and RFI shielding is further limited by the controlled impedance signal requirements in FPC. The shielding must have electrical features that meet both EMI and reference plane standards to obtain the appropriate regulated impedance value. Not all shielding technologies meet both requirements.

To prevent the interference in FPC, copper layer, silver ink, and specialized shielding films are popular.

1. Copper layer

In this structure, copper layer as a solid or cross-hatched plane is connected to the ground through stitching vias. The signal layer is sandwiched between the shielding layer. This solution is also used in PCB design. Solid copper layer provides a high degree of shielding. If shielding is to be performed without reducing flexibility of FPC, we use a cross-hatched layer.

2. Silver ink

Silver ink shielding is a traditional choice. Silver ink offers better flexibility and lower cost than copper layer. The ink is more flexible than copper, but it still requires an additional coverlay to encase and hold the silver ink.

This type of shielding is thinner and cheaper than copper layer shielding. Flex circuits are only 75% thicker than unshielded variants. Silver ink is applied in selected perforations in the primary coverlay. These perforations expose the ground layer, and allow for electrical interconnection. The silver ink flows into these holes, and creates a connection between the shield and the ground/signal layer.

3. Specialized shielding films

Electromagnetic shielding films are the most common choices. They are suitable for dynamic bending applications, and allow the use of the smallest possible structure. Shielding films are very effective, and can be used in a variety of sensitive applications.

In this structure, conductive adhesive, metal deposition layer, and outer insulating layer are laminated together. They are then bonded to the surface of the cover layer by heat and pressure. The ground circuit is connected in the same way as that of silver ink. Conductive adhesive can be bonded and electrically connected through selective holes in the cover layer. The thickness of the double-sided shielding film is only 15-20% of the non-shielding film.