EMI/RFI Shielding Management Tips and FAQs
1. What are the most important factors that effect EMI/RFI shielding?
The following components comprise an effective EMI/RFI management solution:
- A complete Faraday Cage over the EMI/RFI transmitting components that are tied to a true ground.
- The EMI/RFID shielding cans or lids are the last resort and can always be relied on if the circuit design cannot take care of the EMI/RFI noise.
- A good EMI/RFI gasket made of compressible metal springs or a conductive rubber is the solution when a large area inside a cabinet must be addressed.
- If a see through window is required, a blackened mesh over a transparent window is a good solution.
- If there is still EMI/RFI leakage, a good conductive caulk will take care of it.
- Conductivity of the shielding materials is directly related to the shielding effectiveness of the solution, then followed by the thickness of the shielding materials.
- Ventilation holes along the top surfaces are currently the only solution when heat must be removed for the components inside the shielding can.
Some of the potential pit-falls after implementing the solutions listed above:
- There is always a potential resonance at some higher frequency that is generated by the size and shape of the shielding can or lid that can put the compliance out of order.
- Potentially oxidation and galvanic corrosion can occur whenever there are two different metals used (including filled conductive gasket, adhesive, coating and caulks).
- Soldering of the lids while providing the best and most consistent shields, can be costly to remove to rework any of the components.
2. Besides soldering the EMI/RFI lid or cover, is there a better way?
- With the lead-free soldering requirements on most commercial devices, the EMI/RFI shielding cans/covers must be soldered at the same time as the other components are being attached. This process makes the subsequent visual inspection almost impossible.
- Since the processing temperature is as high as 250-300°C, making repairs by removing the covers/cans that much more difficult and detrimental to the components on the board.
- AIT has a solution that can be directly placed on the shielding cans/lids without soldering. The lids or cans are pre-coated with a pressure-sensitive conductive adhesive that forms both mechanical attachment bonding and electrical grounding using the same geometric shape.
3. What can I do if I need to remove more heat from the components inside the EMI/RFI cans/lids besides adding more ventilation holes and fans?
- Adding ventilation holes and fans outside of the EMI/RFI shielding can/cover can only achieve a basic level of heat removal of less than a watt. Higher power chips that are now used in communication may require much better thermal management along with EMI/RFI shielding.
- AIT has a patented thermal solution of a compressible phase-change pad that can be pre-applied onto the EMI/RFI shielding can/cover before attachment to the circuit board. For best results, heat the lids to above 60°C before placement onto components or modules that need both thermal and EMI shielding requirements. This temperature permits good flow to minimize any trapped air gaps.
4. Is dispensing and curing a conductive rubber caulk or gasket compound the most cost effective method for covers of cellular phones and other devices?
- Traditionally the form-in-place method of placing a gasket is the most cost effective method in terms of materials but may be more costly in terms of manufacturing.
- AIT can provide a pre-form shaped-gasket to specific engineering drawings with conductive pressure-sensitive adhesive as a high volume alternative to the form-in-place EMI/RFI gaskets.
- Use of pre-form gaskets can be extremely cost effective and faster in manufacturing for larger volumes.
5. What can I do when traditional copper tape shielding relaxes and does not make proper electrical contact lossing the shielding effectiveness?
- The problem when using most of the copper tapes on the market today is that they provide short-term compliance and are a temporary solution to the problem but lack long-term reliability.
- Conductive EMI/RFID caulking material can be applied over the shim areas that are currently in place.
- If you are doing it new or designing for a new project, you may want to use an intrinsically conductive pressure sensitive copper tape such as those available from AIT. AIT uses a proprietary Z-axis conductive pressure sensitive tape to provide better Emi/RFI shielding. The XY discontinuity may help minimize the chance of residual resonance noise that may occur.