«

"Whiskers" <> wrote in message
news:2302p3-...

> Insulating materials, such as circuit boards, can hold substantial 'static
> charges'; conducting materials, such as metal screws and bolts, let the
> electrons move freely and harmlessly so that no 'static' can build up.
> Otherwise the gap between the circuit board and the case could become a
> very large and unpredictable capacitor.

So what about the plastic clips - you know, the ones that bend over, break,
and won't let you remove the board without breaking. How're they going to
earth the board?

Dan





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On 2006-07-19, Toolman Tim <> wrote:
> Whiskers wrote:
>> On 2006-07-19, Paul <> wrote:
>>> Whiskers wrote:
>>>
>>>> On 2006-07-19, Paul <> wrote:
>>>>> Whiskers wrote:
>>>>>
>>>>>> On 2006-07-19, Paul <> wrote:
>>>>>>> Dan Evans wrote:
>>>>>>>
>>>>>>>> The mounting holes are isolated from everything else.
>>>>>>>>
>>>>>>>
>>>>>>> I'd get yourself a multimeter and check that before making stupid
>>>>>>> statements.
>>>>>>
>>>>>> I'd try to work out the difference between 'static' and 'current'
>>>>>> electricity. Ever got a spark off a door-handle after walking
>>>>>> across a nylon carpet in your rubber-soled shoes? You do not
>>>>>> want your circuit-board to have a different 'static charge' from
>>>>>> the case.
>>>>>>
>>>>>
>>>>> ?????
>>>>
>>>> <http://www.sciencemadesimple.com/static.html>.
>>>>
>>>> Insulating materials, such as circuit boards, can hold substantial
>>>> 'static charges'; conducting materials, such as metal screws and
>>>> bolts, let the electrons move freely and harmlessly so that no
>>>> 'static' can build up. Otherwise the gap between the circuit board
>>>> and the case could become a very large and unpredictable capacitor.
>>>>
>>>
>>> Yes. That's exactly why the mounting holes are connected to earth,
>>> both internally through layers in the circuit board, and externally
>>> through the case.
>>
>> As far as I'm aware, there are no conducting layers within a PCB; the
>> only conducting part is the 'tracks' on one or both surfaces.
>
> Look again. There are MANY layers in modern printed circuit boards with
> pleny of other traces. Just do a quick search on Google for 'multilayer pcb"
> and you'll get a hint.

snip

That's a new concept to me, but having looked I think that the conductors
on the inner layers are still part of the electronic circuits, they are
not an 'anti-static device'. In essence, the multilayer PCBs are one or
two-sided PCBs layered together so that you can achieve a higher density
of tracks and minimise the number of places where insulated conductors
need to be soldered in place where you want tracks to cross each other
without making electrical contact. Obviously there will be places where
there are holes or conductors passing through the outer layers so as to
allow components to be connected to tracks on the inner layers.

I still don't think it would be a good idea to rely on any sort of
electrical contact being made using a mounting screw or bolt.

--
-- ^^^^^^^^^^
-- Whiskers
-- ~~~~~~~~~~

On 2006-07-19, Dan Evans <> wrote:
>
> "Whiskers" <> wrote in message
> news:2302p3-...
>
>> Insulating materials, such as circuit boards, can hold substantial 'static
>> charges'; conducting materials, such as metal screws and bolts, let the
>> electrons move freely and harmlessly so that no 'static' can build up.
>> Otherwise the gap between the circuit board and the case could become a
>> very large and unpredictable capacitor.
>
> So what about the plastic clips - you know, the ones that bend over, break,
> and won't let you remove the board without breaking. How're they going to
> earth the board?
>
> Dan

I don't think I've ever encountered such a method for mounting a computer
motherboard - but some plastics are electrically conducting, and layers of
conducting foil are certainly sometimes used for 'shielding' and for
averting static build-up. Sometimes what looks like plastic may in fact
be cheap metal alloy.

--
-- ^^^^^^^^^^
-- Whiskers
-- ~~~~~~~~~~

Whiskers wrote:

> On 2006-07-19, Toolman Tim <> wrote:
>> Whiskers wrote:
>>> On 2006-07-19, Paul <> wrote:
>>>> Whiskers wrote:
>>>>
>>>>> On 2006-07-19, Paul <> wrote:
>>>>>> Whiskers wrote:
>>>>>>
>>>>>>> On 2006-07-19, Paul <> wrote:
>>>>>>>> Dan Evans wrote:
>>>>>>>>
>>>>>>>>> The mounting holes are isolated from everything else.
>>>>>>>>>
>>>>>>>>
>>>>>>>> I'd get yourself a multimeter and check that before making stupid
>>>>>>>> statements.
>>>>>>>
>>>>>>> I'd try to work out the difference between 'static' and 'current'
>>>>>>> electricity. Ever got a spark off a door-handle after walking
>>>>>>> across a nylon carpet in your rubber-soled shoes? You do not
>>>>>>> want your circuit-board to have a different 'static charge' from
>>>>>>> the case.
>>>>>>>
>>>>>>
>>>>>> ?????
>>>>>
>>>>> <http://www.sciencemadesimple.com/static.html>.
>>>>>
>>>>> Insulating materials, such as circuit boards, can hold substantial
>>>>> 'static charges'; conducting materials, such as metal screws and
>>>>> bolts, let the electrons move freely and harmlessly so that no
>>>>> 'static' can build up. Otherwise the gap between the circuit board
>>>>> and the case could become a very large and unpredictable capacitor.
>>>>>
>>>>
>>>> Yes. That's exactly why the mounting holes are connected to earth,
>>>> both internally through layers in the circuit board, and externally
>>>> through the case.
>>>
>>> As far as I'm aware, there are no conducting layers within a PCB; the
>>> only conducting part is the 'tracks' on one or both surfaces.
>>
>> Look again. There are MANY layers in modern printed circuit boards with
>> pleny of other traces. Just do a quick search on Google for 'multilayer
>> pcb" and you'll get a hint.
>
> snip
>
> That's a new concept to me, but having looked I think that the conductors
> on the inner layers are still part of the electronic circuits, they are
> not an 'anti-static device'. In essence, the multilayer PCBs are one or
> two-sided PCBs layered together so that you can achieve a higher density
> of tracks and minimise the number of places where insulated conductors
> need to be soldered in place where you want tracks to cross each other
> without making electrical contact. Obviously there will be places where
> there are holes or conductors passing through the outer layers so as to
> allow components to be connected to tracks on the inner layers.
>
> I still don't think it would be a good idea to rely on any sort of
> electrical contact being made using a mounting screw or bolt.


<point> missed </point>

Whiskers wrote:
> On 2006-07-19, Dan Evans <> wrote:
>>
>> "Whiskers" <> wrote in message
>> news:2302p3-...
>>
>>> Insulating materials, such as circuit boards, can hold substantial
>>> 'static charges'; conducting materials, such as metal screws and
>>> bolts, let the electrons move freely and harmlessly so that no
>>> 'static' can build up. Otherwise the gap between the circuit board
>>> and the case could become a very large and unpredictable capacitor.
>>
>> So what about the plastic clips - you know, the ones that bend over,
>> break, and won't let you remove the board without breaking. How're
>> they going to earth the board?
>>
>> Dan
>
> I don't think I've ever encountered such a method for mounting a
> computer motherboard -

I have - mostly on older (PC, XT, 286, 386) boards. But EVERY ONE of them
also had one or more metal standoffs as well, for two reasons: screw it down
so it won't move, and provide chassis grounding to the board.

--
When I was a child, I remember my Mom telling me, "Son, when you grow
up, you can marry any girl you please." When I became a young man, I
learned the sad fact was that I could not please any of them.

Paul wrote:
> Whiskers wrote:
>>
>> I still don't think it would be a good idea to rely on any sort of
>> electrical contact being made using a mounting screw or bolt.
>
>
> <point> missed </point>

<g>

The mounting screw connections are PRIMARY grounding points on every device
I've ever assembled. That's WHY they have the copper traces and solder
there - to create a GOOD grounding point. Those grounding points are
significantly MORE RELIABLE for the circuitry than connecting a wire from
the chassis: 1) wires can be missed during assembly, 2) wires can weaken
over time due to movement (example: mouse cords quite often fail right at
the point where they enter the mouse due to the frequent flexing at that
location).

In my opinion, there is very little need to worry about static electricity
or capacitance developing from a stationary board screwed into place. The
real issue is earth/chassis/grounding for electrical protection.

--
When I was a child, I remember my Mom telling me, "Son, when you grow
up, you can marry any girl you please." When I became a young man, I
learned the sad fact was that I could not please any of them.

X-No-Archive: yes

On Tue, 18 Jul 2006 19:49:53 +0100, "Trevor Kenworthy"
<> wrote:

>A very simple question I hope. I'm about to install a motherboard into a
>case, all screws are supplied but there are also some red fibre washers. I
>thought there had to be conductivity between the motherboard and the chassis
>for earthing/anti static purposes. Am I correct or should I be using these
>washers and, if so, where?

Dan Evans is right. The red fibre washers go between the screw head
and the mobo - that is first assemble the washer to the screw and then
fix them.

--
Sandy Archer
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