ryu wrote:
>
> I saw the term in the IPC-610.  I think it has something to do with the
> design of a product for cross talk elimination.  But can someone tell me
> if there is a minimal or maximum requirement for the distance, is there
> any number a designer needs to follow? if the answer is no, then how the
> designer comes up with the number ?  Thanks.

<sorry this is a little long>

I can answer this from a compliance engineering point of view.

First is in regard to safety testing - Underwriter's Labs stuff.
Clearance is specifically the straight line distance between
two points on a board, or between any two points that have
air between them.   UL and other safety agencies have tables
that specify spacing dependent upon the voltage, the board
material, and the environment.  One place where these tables
are listed is in UL-1950.  A broad standard that covers the
UL testing of specific types of equipment.  But mainly the
clearance issue with UL has to do with high voltage.

Another term in the same vein is "creepage".  Whereas clearance
is with regard to "how the crow flies", creepage is with regard
to how the crow walks".  In other words it's along the surface
of the board.  UL also has tables for this.

Second is in regards to EMC or signal integrity stuff.
As others have mentioned, the closer the traces, the more
crosstalk between them.  The return current in the ground plane
beneath the trace is not confined to the specific area directly
beneath the trace.  It spreads itself out much like a normal
curve; stronger directly underneath growing weaker the further
it gets away from the trace.  Below, the dotted line is the
amplitude of the return current in relation to the trace.

             trace
             +-----+
             +-----+
             .......
           .         .
        .               .
    .                        .
   ---------------------------
           ground plane

Two traces in close proximity to one another end like -


             trace1            trace2
             +-----+           +-----+
             +-----+           +-----+
             .......           .......
           .         .       .         .
        .               . .               .
    .               .   CT    .                .
   --------------------------------------------
                   ground plane

The area marked "CT" is where and how crosstalk
occurs between traces.  The "mixing" of the return
current of trace1 with the return current of trace2.
The further apart the traces are, the better.

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