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topic 10460

Ground contacts from PCB to EMI/RFI/EMC shield


(2001)

I am shielding for EMI/RFI/EMC on a PCB. In this application I am placing a lid (made of conductive material) over some circuitry on the board. If I place some solder bumps on the board as grounding points between ground plane and lid, I am concerned that the solder bumps surface resistance will increase over time.

Another option would be to have pick-n-place contacts placed on the board, but how costly would this be? Any other ideas?

Barry Aho
circuit board mfgr. - Colorado Springs, Colorado, USA


(2001)

Just to make contact with solder bumps doesn't sound reliable enough. How about the usual BeCu springy material used to make contact in so many other applications?

tom pullizzi monitor
Tom Pullizzi
Falls Township,
   Pennsylvania 


(2001)

Why not solder it in place? Or, maybe a well placed conductive fastener might do the trick?

Dale Woika
- Bellefonte, Pennsylvania, USA


(2001)

A typical simple solution for small quantity is screw a die-cast lid over the circuits. You may need a conductive adhesive wire braid or conductive plastic/rubber gasket to make a good connection. These gaskets/braids are normally self-adhesive. Shop around! The die-cast shielding lids are made in bulk and come in off-the-shelf standard sizes. You need to design your board to fit them.

For higher quantities you can specify a folded tinned-brass lid with lots of holes in it (typically 5 mm diameter). The lid is placed over the SMD components before the reflow oven. The holes allow heat through to solder the components to the PCB. The lid also is soldered to the board. The size of the holes is small compared to the wavelengths that you are trying to block, so nothing gets through.

If you need multiple compartments then it is possible to have a custom extrusion made that end-on matches the size and shape of the compartments. The extrusion is cut into 3 mm lengths, and soldered with everything else by the reflow oven. A flat lid fits on top, with a wire braid or conductive plastic/rubber gasket to give a good connection. The lid is typically held down by the mechanical design of the equipment case.

There are also varieties of RF absorbent sponge foam rubber, which are self-adhesive. These can be stuck to the inside of the lid to get rid of resonant RF standing waves inside the cavity, but they're expensive.

Better to think about the dimensions of the screened cavity and the frequency its operating at before you start. Don't make it a 1/4 wavelength in size (or any other sub-multiple)! Obvious advice is a good ground all round, or at least small distances between connections to ground. (Small in comparison with wavelength, tat is).

Long term environmental problems may occur if the board gets humid or wet, as aluminium directly against tinplate/copper creates a battery effect, and one or the other will eventually rot, but this may take 10 years.

John Maddison
- Cambridge, UK


(2002)

Some SMT finger may be useful for PCB contact to ground plane, you can contact LAIRD CO. for finding a suitable finger or EMI STOP CORP. For smaller size finger usually used at cell phone or notebook grounding.

Max Weng
- Taipei Hsien, Taiwan, R.O.C


(2003)

SMD Finger is used for letting grounding system work better

Jack Lo
- Taipei, Taiwan


(2007)

SMT Finger will be a good solution for grounding application in tiny size equipment,like NB,GPS,Cellular phone,it also can be used in signal connector or battery contact.

Jairo sue
- Sinjhuang City, Taipei, Taiwan



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