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

Maximum Frequency (fmax@att.net)
Thu, 04 Mar 1999 20:38:07 -0600

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. 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. 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.

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