EMI
Shielding Tips and FAQs
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.
03/09/2010 09:06
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