Custom flex PCBs may be able to withstand high temperatures but remain bafflingly susceptible to Electromagnetic waves and Radio Frequency Interference. Although these are not enough to damage a typical Flex PCB, they can cause many problems. The most common of these is an increase in the error rates, which can cause constant shutdowns.
To keep your custom flexible PCB safe from the EMI and RF waves, you need to cover it with EMI shields. The most common materials used for EMI shielding are copper layers, aluminum sheets, and silver ink. However, each material has its advantages and limitations, which you need to know before choosing.
However, before going into that, we need to define EMI and RF shielding and how it works.
What is EMI and RF shielding?
EMI shielding typically involves building a protective cover around an EMI or RF-sensitive components like circuit boards to prevent the penetration of external signals. Although it’s not the primary purpose, EMI shielding also prevents high-frequency units within the PCB from leaking out, which is a crucial cause of attenuation.
EMI shields are based on the same principle used in the Faraday Cage, which is essentially an enclosure used to keep certain electromagnetic radiation inside or outside. Therefore, when selecting an EMI shield, you need to consider; the bend requirements and controlled impedance. These are described further below.
A custom flexible PCB will not be of much use if it’s not fully bendable. So, it is essential to go with shields with a high minimum bend radius and bending capacity.
Notably, how much a particular shield can bend depends on its thickness. As such, you may need to compromise in that regard if you want a highly bendable shield.
Also, consider the type of PCB you’re working with. Dynamic flex PCBs are, for instance, more bendable than static ones.
This refers to the standardization of the natural impedance level of both the receivers and feeders within the board.
The level of control over the impedance and the corresponding hardware and signal requirements can affect the working of an EMI and RF shield.
So, before choosing a shield, make sure it possesses the electrical properties needed to obtain the required impedance values.
Best Materials for EMI and RF Shielding
Copper is the most common – and reliable – material for EMI shielding. In addition, copper is highly flexible, meaning it can be fabricated into various shapes and sizes.
Secondly and most importantly, copper is highly conductive, making it very effective in weakening magnetic and electrical waves.
Although not as conductive as copper, Silver Ink has an advantage in that it is much cheaper than all other shielding materials. It is also remarkably flexible and only increases the thickness of the flex circuit by 75%.
Additionally, you can make some perforations in the Silver ink coverlay, which allow you to make necessary electrical interconnections without interfering with the signal attenuation.
Custom Shielding Films
Custom shielding films are typically made from a metallic sheet, conductive adhesive, and insulating sheet. The three materials are bonded together and then stuck on the PCB’s coverlay.
The most remarkable feature about these films is their low thickness relative to the materials mentioned earlier. For example, covering your PCB with a two-sided shielding film will only make it 20% thicker.
Additional Tips to Prevent EM and RF Interference in Custom Flexible PCB Boards
- Connect all your split ground planes at a single point
- Keep the current return path as short as possible
- Avoid materials with sharp bends as they can affect the impedance and signal reflections of various components
- Always keep the high-speed signals isolated from the low-speed ones
- Place your vias on grid-like structures, fully aligned with their routing components
EM and RF shielding are increasingly becoming an essential part of PCB manufacturing processes, especially when it comes to the rather sensitive Flex PCB boards.
However, to effectively shield your device, it is crucial to examine both the mechanical and electrical aspects to develop a method that works for your PCB.
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