[SI-LIST] Re: Decoupling capacitors for BGA
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "craig mathew" <craig.mathew@xxxxxxxxx>, si-list@xxxxxxxxxxxxx
- Date: Sat, 13 May 2006 09:05:38 -0700
If you have a 9 mil drill size on a 125 mil thick PCB, you will have great
difficulty getting adequate plating in the holes. That's an aspect ratio
of 14:1. Not many fab shops in the world that can plate these
successfully- maybe 2! Yes, many will claim they can or will say they will
try. Meantime, your design is at risk. Better keep the aspect ratio under
10:1 which means a drill size of 12 mils. IF that won't fit into the space
you have and still leave room for insulation, traces and drill wander,
might be good to stop the presses and rethink this design.
>
> Hi Avanindra,
>
> I have a similar question. We are using 0201 decaps at the bottom of
our BGA device for decoupling. We are using 9 mil drill size for via.
>
> Unfortunately we are up against two odds, our PCB is close to 125mil
thick and we have small via drill, This increases the via inductance. Is
there a way I can calculate to what frequency will my decap work
effectively?
>
> I may be really dumb here but more I think more it seems logical to
place decaps around the periphery of the device than at the bottom.
>
> Thanks
> Craig M
>
>
> ----text deleted-----
>
> Subbu,
> if your board is not thick, you may want to consider using 0201 caps
> directly underneeth the BGA aray, especially if GND and VDD are located
> directly diagonally across from each other.
> If you connect caps via individual traces to the pins, you will get alot
of
> resistivity.
> Which can be a plus if your goal is to dampen high-frequency noise.
However,
> if you make traces very thin and long, you may slow your response time
too.
> So calculate your resisitivity per square to figure out what is the
optimal
> cap connection for you.
>
> Dagmara
>
>
>
>
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