Re: [SI-LIST] : Traces Over Plane Clearances

Douglas McKean ([email protected])
Mon, 09 Aug 1999 11:29:22 -0700

Dave Hoover wrote:
> I have a customer who has a 2000+ I/O BGA with
> tight pitch. They are using 2 track routing to
> escape the device on the PCB. At the device,
> the traces cross over the edge of the reference
> plane clearances. The microprocessor and
> clocks currently run at ~1 GHz (headed above 3 GHz).
> The distance across that clearance would be
> less than .020" in length. There is a copper
> web of around .008" between the plane
> clearances (thinking adequate signal return path).
> My question is that the customer is afraid the short
> distances without the reference plane will impact
> his electrical performance properties.
> The problem I face is currently the clearance size
> is reduced to the point that it can't be manufactured.
> Faced with a dilemma, so I went to the pros.
> David Hoover

Hi Dave,

Well, I wouldn't consider myself a pro, but I'll
add 2 cents worth in addition to what you've had
as replies. There will be an impact electrically
whenever a discontinuity is encountered as you
probably already well know. Exactly what that
impact would be, I don't know.

What I would point out is:

1. If the traces cross perpendicular to the edge
of the slot, there might be a chance of return
currents being transposed from common mode to
differential mode of return as the primary return
path, i.e. through the other traces. The amount
of diff-crosstalk, I suspect, would be dependent
upon the width of the gap - wider gap, more coupling
between the traces, thus more diff-crosstalk.

2. Also, the return path will focus itself out at
the ends of the slot. Something like this ...

| trace
| Return |
| Plane |
+-----------|--------+ return
| | current
Gap | | will focus
+-----------|--------+ out here
| |
| |

So, if you've got other traces near the end
of the slot, you may get crosstalk that you
don't expect. Here I suspect the amount of
return current out at the end is not really
dependent upon the width of gap.

3. If the traces run parallel to the edge of the slot,
then inductance of the return plane will obviously
be higher near the edge than toward the middle of
the return plane.

Regards, Doug

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