[SI-LIST] Re: 0.8mm BGA routing

Bob,
 
I'm confused as to why the propogation velocity would change as a result
of varying the line-to-antipad spacing. I can understand a path length change
because of return path irregulatities. I can also understand a change in the
effects caused by capacitive parasitics as impedance changes. What am I
missing?
 
TIA,
 
Dennis
Robert Haller <rhaller@xxxxxxxxxx> wrote:
Jack,
.Istvan is absolutely correct that this can not be ignored. A while ago 
I did a bench study to examine some of the SI effects.

I examined impedance and propagation velocity at various edge rates 
for signals passing through a BGA field with varying anti-pad 
clearances. You must also be aware that in the PWB manufacturing process 
there are additional contributors (layer to layer mis-alignment) that 
effects the line to antipad space.

The surprise was how much the propagation velocity varied especially for 
signals that serpentine under the BGA field (A practice that should be 
avoided but is sometimes encountered). Accurate control and prediction 
of propagation velocity is critical in Source Synchronous designs.

regards,
bob


-- 
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15


Istvan NOVAK wrote:
> Jack,
> 
> The impact of anti-pad on signal routing should not be ignored.
> The plane perforation has consequences on both the signal
> integrity of traces referencing the perforated plane and on the
> power distribution network using the perforated plane.
> 
> In my message below I just mentioned the extreme possibility
> of completely cutting the plane with anti-pads. In a separate
> thread recently, Zhangkun asked about the impact of perforation
> on trace signal integrity.
> 
> As usual, whether perforation is a problem, it depends on:
> both the increase of impedance and increase of crosstalk will
> have a time duration equaling the time of flight through the
> perforated area. If this time of flight is much less than the
> signal transition time, the impact is minimal. For fast edges and
> long paths over perforated areas, the impedance increase
> will create extra reflections, and crosstalk also goes up.
> You can try to compensate for the impedance change either by
> narrowing the trace over non-perforated areas, thus increasing
> the overall trace impedance (you may need to adjust the termination
> values accordingly), or you can try to decrease the trace impedance
> over perforated areas by making it wider (this, however, will further
> increase the already higher crosstalk and will further limit routing
> density).
> 
> I hope this helps.
> 
> Regards,
> Istvan
> 
> 
> ----- Original Message -----
> From: "mediwheel_js" 
> To: ; 
> Cc: "Tayyab Jamil" ; 
> Sent: Monday, December 29, 2003 9:25 PM
> Subject: Re: [SI-LIST] Re: 0.8mm BGA routing
> 
> 
> 
>>Istvan,
>>
>>One thing that I did not see mentioned was the anti-pad, and
>>its effect on signal routing thru the via array. Should this
>>aspect be ignored or is there someway to compensated for
>>this???
>>
>>thanks,
>>
>>Jack Stone
>> 


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