Re: [SI-LIST] : BGA vs Leaded package electrical performance

Ron Miller (rmiller@Brocade.COM)
Sat, 06 Mar 1999 09:09:57 -0800

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> Vinu

Although the problem of grounds being modeled as a single point, in most
simulators,HP ADS
with the picosecond modeling suite has power and ground models which
include X and Y coordinates and which treats the PCB planes as matrixes.

> Maximum Frequency wrote:
>
> > Greg,
> >
> > Performance in the BGA can depend on the existence of pwr/gnd plane
> > layers or appropriately placed current return paths in the package.
> > Without such areas, the BGA is simply a low-profile funny leadframe
> > whose signal-pwr, signal-gnd, pwr-gnd, and signal-signal loop
> > inductuances are a function of the board onto which it is mounted. The
> > "BGA" must be designed to have better performance than the QFP, and it
> > is not necessarily true that it will. For example, even if the BGA has
> > a solid ground plane, if it is not part of the direct current return
> > path (but only indirectly through the ground's capacitance to the real
> > return path), a relatively increased loop inductance may result,
> > possibly worse than for some QFP.
>
> I have seen a few such BGA packages where little attention has been paid to signal returns. When a chip/package vendor runs a field solver that uses this BGA ground plane as a reference, the RLC matrices generated for the signals
> can be very misleading.
>
> > What really matters are the L1+L2+-2M
> > loops which may look like non-uniform ugly multiply-coupled "T-lines".
> > We want a high M between the signal and power "return" to minimize the
> > loop L by maximizing the M. (like twisting the +/- wires from a DC
> > power supply). In fact it may behoove us to have an equal loop
> > inductance from sig-pwrreturn as we have from sig-gndreturn so as match
> > 0-1 and 1-0 inductive loops.
>
> This would be true if signals routed on the board are as likely to be referenced to PWR as they are to a GND plane. If all signals on the board only referenced GND planes, it is best to assign all signal return bumps to GND.
>
> > The lonesome values of the partial lead
> > inductance matrix entries may be considered to be known only to within a
> > constant.
> >
> > If both TQFP and BGA are measured or simulated, port return currents
> > flow in the defined simulator or test-setup ground return. Inductances
> > measured or simulated in different environments end up different, even
> > for the same package. Comparison for a same package characterized in
> > different environments can be made by writing a super-node equation so
> > that the sum of all currents into the package leads totals zero and
> > "shutting off" the flow of current in the old simulator or measurement
> > return path which probably won't exist in the final application. Two
> > different packages are compared by looking at the loop values, NOT the
> > individual partial inductances, for given sig-gnd, sig-pwr, sig-sig
> > configurations.
> >
> > Practical measurements can be made with a network analyzer for example.
> >
> > Have fun with it ! Anyone disagree ?
> >
> > Fmax.
> >
> > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net

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Vinu
Although the problem of grounds being modeled as a single point, in most
simulators,HP ADS
with the picosecond modeling suite has power and ground models which
include X and Y coordinates and which treats the PCB planes as matrixes.
 
 
 
 
Maximum Frequency wrote:

> Greg,
>
> Performance in the BGA can depend on the existence of pwr/gnd plane
> layers or appropriately placed current return paths in the package.
> Without such areas, the BGA is simply a low-profile funny leadframe
> whose signal-pwr, signal-gnd, pwr-gnd, and signal-signal loop
> inductuances are a function of the board onto which it is mounted.  The
> "BGA" must be designed to have better performance than the QFP, and it
> is not necessarily true that it will.  For example, even if the BGA has
> a solid ground plane, if it is not part of the direct current return
> path (but only indirectly through the ground's capacitance to the real
> return path), a relatively increased loop inductance may result,
> possibly worse than for some QFP.

I have seen a few such BGA packages where little attention has been paid to signal returns. When a chip/package vendor runs a field solver that uses this BGA ground plane as a reference, the RLC matrices generated for the signals
can be very misleading.

> What really matters are the L1+L2+-2M
> loops which may look like non-uniform ugly multiply-coupled "T-lines".
> We want a high M between the signal and power "return" to minimize the
> loop L by maximizing the M.  (like twisting the +/- wires from a DC
> power supply).  In fact it may behoove us to have an equal loop
> inductance from sig-pwrreturn as we have from sig-gndreturn so as match
> 0-1 and 1-0 inductive loops.

This would be true if signals routed on the board are as likely to be referenced to PWR as they are to a GND plane. If all signals on the board only referenced GND planes,  it is best to assign all signal return bumps to GND.

> The lonesome values of the partial lead
> inductance matrix entries may be considered to be known only to within a
> constant.
>
> If both TQFP and BGA are measured or simulated, port return currents
> flow in the defined simulator or test-setup ground return.  Inductances
> measured or simulated in different environments end up different, even
> for the same package. Comparison for a same package characterized in
> different environments can be made by writing a super-node equation so
> that the sum of all currents into the package leads totals zero and
> "shutting off" the flow of current in the old simulator or measurement
> return path which probably won't exist in the final application.  Two
> different packages are compared by looking at the loop values, NOT the
> individual partial inductances, for given sig-gnd, sig-pwr, sig-sig
> configurations.
>
> Practical measurements can be made with a network analyzer for example.
>
> Have fun with it !  Anyone disagree ?
>
> Fmax.
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP.  si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

-- 
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017                 
     ==========0000-(_)0000=========== 
Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA  95131
rmiller@brocade.com,  rbmiller@sjm.infi.net
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