Re: [SI-LIST] : BGA vs Leaded - summary

greg kimball (gkimball@indra.com)
Wed, 10 Mar 1999 00:15:06 -0800

tomda wrote:
>
> Believe it or not there are techniques that can take advantage of bond
> wire parasitics to improve the bandwidth of the die to package
> interconnect. Tektronix used techniques like T-coils to match impedances
> between the die and the package in the early 70's. Just measuring distance
> between die and PCB is not adequate, you need to analyze the situation and
> apply good analog and RF design techniques to design a package. Obviously
> controlled impedance runs to the die pads is the optimal solution but there
> are other answers.
>
> Tom
>
> -----Original Message-----
> From: john lipsius [SMTP:jlipsius@cyras.com]
> Sent: Monday, March 08, 1999 2:46 PM
> To: si-list@silab.Eng.Sun.COM
> Subject: Re: [SI-LIST] : BGA vs Leaded - summary
>
> << File: jlipsius.vcf >> Greg,
>
> The regular BGA **does** have bond wires and this can be a problem when
> terminating hi-edgerate i/o, around 1GHz in my experience.
>
> The problem is that termination on the pcb can present itself as a resonant
> structure
> or a significant stub and must be fly-by. Also, the Ball locations should
> be closest to
> the die to minimize bond wire length in the pkg. and a 4 layer (with pwr,
> gnd) BGA
> pkg should be used to control the impedance and crosstalk of the critical
> io to/from the ball.
>
> Lastly, an excessive ball count pkg is typically required in order to (for
> an asic) obtain
> enough designer choice to achieve the optimum ball placements cited above.
>
> Flip Chip really improves this situation. The die pad are attached
> directly to the pkg.
> with attachments right on the die. But new asic physical design tools are
> required.
>
> -John
>
> greg kimball wrote:
>
> > All who have contributed.......thanks,
> >
> > Just wanted to summarize what i have read:
> > 1. That the BGA does have inherent lower parasitic L and C (no bond
> > wires and no packaging leads) and therefore better interconnect
> > transmission properties, - some details were given in the paper
> > suggested by Ravider at IBM "Noise computation in single chip...." IEEE.
> > (Thanks Ravinder)
> >
> > 2. Those advantages can easily be squandered by -
> > - long leads to the closest decoupling site
> > - long leads to the interconnect junctures
> >
> > 3. Moral of the story ??????
> > In order to take advantage of BGA, which does carry a
> manufacturing
> > cost premium, very careful interconnect strategies my be thought out
> > and put in place.
> > We have not talked about thermal aspects, but this is another
> tricky
> > area of consideration - underfills, number of balls to xfer heat out,
> > thermal gnd planes, etc, etc
> >
> > Any parting comments ????
> >
> > Regards,
> > Greg
> >
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> --
> Thanks,
> John Lipsius
> Member Technical Staff
> Cyras Systems, Inc.
> 46832 Lakeview Blvd.
> Fremont, CA 94538
> 510-623-6631
>
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Tomda,
Any references on BW enhancement techniques you mention?
An interconnect whose Zo is below Zload, produces a peaking response in
the frequency domain, and as such can be used to extend BW - is this the
ploy?
Regards,
Greg

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