Protecting high impedance sections of PCB with metallic Sheets rather than metallic molds
in [Design]
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From: Rich Grise on Google groups on 19 Apr 2010 17:51 On Apr 19, 7:50 am, Tim Wescott <t...(a)seemywebsite.now> wrote: > PureSine wrote: > > 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 ,,, > 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). > 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
From: John Larkin on 19 Apr 2010 19:11 On Mon, 19 Apr 2010 10:32:29 +0430, PureSine <Green.Tech.Coder(a)gmail.com> wrote: >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. 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! http://www.highlandtechnology.com/DSS/V470DS.html http://www.highlandtechnology.com/DSS/V450DS.html 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
From: krw on 19 Apr 2010 23:45
On Mon, 19 Apr 2010 16:11:36 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Mon, 19 Apr 2010 10:32:29 +0430, PureSine ><Green.Tech.Coder(a)gmail.com> wrote: > >>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. > >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! > >http://www.highlandtechnology.com/DSS/V470DS.html >http://www.highlandtechnology.com/DSS/V450DS.html > > >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. This is the stuff we (try to) use to keep 2.4GHz out of our analogs. http://www.gore.com/en_xx/products/electronic/emi/snapshot/index.html Once you get the hang of the SMT "balls" and the placement it works rather well. |