[SI-LIST] Re: BGA vias outside the package

I don't think the microstrip formula can be used for the 1.8V and 3.3V traces, 
since the return current does not flow through the underneath plane.  The 
return current flows through the ground trace, so I suggest that an evaluation 
of the loop inductance would give more accurate results than the microstrip 
analysis.
 
Cosmin Iorga,
NoiseCoupling.com
--- On Wed, 1/28/09, V S <for_si2003@xxxxxxxxx> wrote:

From: V S <for_si2003@xxxxxxxxx>
Subject: [SI-LIST] Re: BGA vias outside the package
To: si-list@xxxxxxxxxxxxx
Date: Wednesday, January 28, 2009, 1:17 PM

Scott,

First I will complement your answer by adding -

1. Your have 4 pins on 1.8V rail. Assuming the four traces coming from the four
balls have no coupling the total inductance will be equivalent to the parallel
combination of the inductance.

2. Find the change in current dI expected in your 1.8V rail. IF dV is the
maximum change in the voltage that your 1.8V rail can tolerate then the maximum
tolerable impedance Z is given by dV/dI.
 
3. Make sure that the impedance across all the frequencies as calculated in the
Scott's suggestion is less than this Z.

You will need to repeat the things for your 3.3V rail. For calculating dI, take
into account the maximum current consumption and the take into account current
due to charging the all IO pins simultaneously. If C is the capacitive load, and
if there are N IO pins, then this switching current can be gives as NCdV/dT.. 
You
should be able to find C on the datasheet. dV/dT can also be found on the
datasheet. N is the maximum number of the IO pins. This switching current should
be applicable to 3.3V supply. I am not sure about 1.8V. I believe, 1.8V is only
the core voltage with not IO.  

This will give you a pass or fail answer - or a ball park number. If you can
achieve the result with you existing design that is good. Otherwise, increase
the number of capacitors, use reduced series inductance capacitors. Recalculate
the check if you are meeting the required impedance target. 

Vikas Shukla

--- On Wed, 1/28/09, Scott McMorrow <scott@xxxxxxxxxxxxx> wrote:

> From: Scott McMorrow <scott@xxxxxxxxxxxxx>
> Subject: [SI-LIST] Re: BGA vias  outside the package
> To: 
> Cc: si-list@xxxxxxxxxxxxx
> Date: Wednesday, January 28, 2009, 12:43 PM
> Ryan
> 
> Use a 2D field solver to compute the impedance of the trace
> you'll use 
> to connect the outer layer planes to the balls.  If
> microstrip, prop 
> delay is approximately 150 ps/in.  Once you know your trace
> impedance 
> you can compute the incremental inductance.
> 
> Z = sqrt(L/C)
> tpd = sqrt(LC)
> 
> solving for Inductance
> L = Z x tpd
> 
> For 50 ohm microstrip that would be 7.5 nH of inductance
> per linear inch 
> of trace.
> For 40 ohm microstrip = 6 nH/inch
> For 30 ohm microstrip = 4.nH/inch
> For 20 ohm microstrip = 3 nH/inch
> 
> You can use this, along with available publicly published
> analytical 
> formulas for planes and vias to compute the total
> inductance in the 
> power delivery system up to your ball pads, to determine
> you power 
> system impedance vs. frequency.
> 
> regards,
> 
> Scott
> 
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
> 
> http://www.teraspeed.com
> 
> Teraspeed® is the registered service mark of
> Teraspeed Consulting Group LLC
> 
> 
> 
> Ryan Sequeira wrote:
> > Would like to understand the effects of supplying
> power and ground to the
> > BGA by connecting
> > vias to the VCC/GND plains and traces outside the BGA
> package. 
> >
> > Pros: Allows for larger vias outside the package.
> > Cons: Compromised noise performance through trace
> length added to VCC/GND
> > trace length.
> >
> > Is there something else that needs to be taken care
> of....
> >
> > The BGA device is a CPLD, IO=3.3V, Core=1.8V, Fmax > 180MHz, VCCO(3.3V) - 11
> > pins, VCC(1.8V) - 4 pins, GND - 15 pins
> > The device is a 132csBGA, 0.5mm pitch, 0.3mm pad. So
> cannot afford to drop
> > the vias within the BGA area. Microvias would be too
> expensive...
> >
> > Ryan
> >
> >
> >
> >
> >
>
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