flex circuit
EMI (Electromagnetic Interference) is a disturbance that affects the operation of a circuit, causing it to degrade its performance or even stop functioning altogether. Fortunately, there are a variety of methods and materials available for preventing EMI from absorbing or radiating off a flex circuit.
Generally speaking, the substrate material that a flex circuit is constructed from determines its EMI performance. Some materials have lower dielectric constants than others, meaning that signals travel through them more quickly and with less attenuation. This can help to reduce delays and skew between traces, and may result in better data integrity.
Another important factor in determining a flex circuit EMI performance is its layer thickness. Thicker materials tend to have higher impedance and a more difficult time accommodating high-speed data transmission. For this reason, a design engineer should carefully consider the material thickness required for a particular application.
How does flex circuit affect electromagnetic interference?
In addition to the type of material used for a flex circuit, its construction method also plays an important role in minimizing EMI. A flex circuit can be produced using a variety of techniques, including both polymer thick film (PTF) and rigid flex PCBs. PTF flex circuits use a single conductor layer printed on a flexible polymer base film, while rigid flex PCBs use several layers of copper and other components on multiple different insulating films.
A flex circuit’s EMI performance can be further improved by adding shielding layers to the construction. These shielding layers can be made from a variety of materials, including copper foil and metal laminates. Shielding layers are typically connected to the ground plane through vias plated into the conductive layer. However, these vias can also act as mechanical stress concentrators in the flex area of the circuit, reducing its flexibility. The optimum solution is to avoid placing vias in the flex area of the circuit, or at least to cross-hatch the copper shield layers to improve the flexing capabilities.
One of the most common and effective EMI shielding methods for a flex circuit is to add a layer of copper directly over the Polyimide base layers. This provides superior EMI dissipation functionality and impedance control, but it restrains the circuit’s bending capabilities as it increases the total thickness of the board. A better alternative is to use a crosshatched copper shield layer which is still mechanically superior to a plain copper layer, but does not restrain bending capability as much.
Another method of EMI shielding is to use silver ink. In this case, liquid silver is applied through a silk screening process to the outside surfaces of a coverlay that encapsulates the circuit patterns. Openings are stitched into the coverlay on the outer edges of the flex circuit, and the silver ink flows into these openings to encapsulate and electrically connect to the ground shield. This technique is cost-effective and easy to fabricate, but it doesn’t provide the requisite impedance control needed for modern flex circuits. In addition, the silver ink layers add to the overall thickness of the circuit, reducing its bending capability.
