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Protecting high impedance sections of PCB with metallic Sheets rather than metallic molds

Hello, In many sensitive devices/ instruments which have a sensitive high impedance section like a high impedance amplifier(10^12 Ohm or even higher) or other high impedance signals, I've seen they are protected by a metallic mold(mostly copper) that sits on top of sensitive parts and tracks and is usually screwed to the PCB. Such metallic molds which are specific to the shape of PCB seems very expensive. I wonder those of you that are familiar with such protection do you think it is necessary ? They are for protection against EMI but for instruments that have no signal/Clock faster than a few hundreds of MHz, the EMI wavelength should be fairly smaller than 30 Cm, So effective it would be to instead of designing a board specific metallic mold for each high impedance section, Just cutting copper or aluminum sheets approximately to the size of sensitive area and then mount them on PCB using 5mm metallic Spacers that are grounded. Well there is a 5mm gap but this method is much cheaper and simpler and to the extent of theory that I know it should stop all the EMIs that their wave length is higher than a few Cm and it is translated to about 10 GHz. Even harmonics of a few hundred MHz clock are very weak at such frequency.

What do you think ?

Regards.

Reply to
PureSine
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On Apr 18, 11:02=A0pm, PureSine wrote: ,

A bit of foil or sheet metal would work, of course, but it can also vibrate; that would make the stray capacitance into a microphone.

The sensitive parts of my old 10**-18 A electrometer are in a cast iron housing. Vibration control is why.

Reply to
whit3rd

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Search on slot antennas. Narrow gaps in shielding can form slot antennas at specific wavelengths which can provide better coupling to the sensitive elements than you'd get if there wasn't any shielding at all. I've not seen this happen but the risk is mentioned in most discussions of shield design.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

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I have used the sheet metal plate over the sensitive stuff method. Here is what I did:

1 The shield needs to connect to the "shield ground" trace that runs around the electronics in question. 2 The connection points need to be outside the area of the the sensitive stuff. 3 The sheet metal needs to extend out so that it nearly reaches the metal chassis. 4 All connections must be good connections or not at all. Poor connections can work as radio detectors.
Reply to
MooseFET

I've never seen these molded -- always bent up out of sheet metal. There are places that do this commercially, and enough of them that I can't imagine it being _too_ expensive.

Find the right keywords, and see if anyone makes stock sizes -- given a stock size, perhaps you can design your board to match the shield, instead of the other way around.

For low volumes, you can just darn well make them yourself -- bend them up out of beryllium copper, have them tin plated, and away you go (note that I'd do this for prototyping, but I'd have them made for production).

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Reply to
Tim Wescott

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Why not just make them out of a few scrap pieces of PCB soldered together? Shear narrow strips for the sides, cut a piece for the top and solder the seams.

al

Reply to
mickgeyver

y

,,,

Anybody remember "tinplate"? It's tin-plated steel, and solders like it's thirsty for the stuff. ;-)

I've seen shield boxes made of it; I think it's probably cheaper than copper, but more rigid, since it's steel. (Of course, it'd have to be bent into shape.)

Cheers! Rich

Reply to
Rich Grise on Google groups

We make irregularly shaped covers out of sheet metal, bent into sorta boxes, sometimes with mounting flanges that can be bolted hard against the pc board. In colors!

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For small stuff, Zero or equivalent deep-drawn aluminum covers work great.

ftp://jjlarkin.lmi.net/Ferrite.JPG

Flat sheets on spacers work pretty well at moderate frequencies, if you don't need things like air-current shielding too.

John

Reply to
John Larkin

This is the stuff we (try to) use to keep 2.4GHz out of our analogs.

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Once you get the hang of the SMT "balls" and the placement it works rather well.

Reply to
krw

On a sunny day (Mon, 19 Apr 2010 16:11:36 -0700) it happened John Larkin wrote in :

DOOO theeey alzo come wizz nozee piccsss?

Reply to
Jan Panteltje

Ok, Thanks everyone for useful inputs. I will use a flat aluminum sheet connected via spacer to PCB for my next design.

Regards.

Reply to
PureSine

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