[SI-LIST] TDR on Package only
- From: "Porsh Shih" <porshs@xxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Mon, 19 Jul 2004 10:15:58 -0700
Hi,
1.
TDR a BGA package(PKG) with ball desoldered, and place probes on the =
signal ball pad and a ground ball pad next to the signal's (no test PCB =
involved)
2.
TDR the same BGA package thru the test PCB with SMA, PCB trace, PCB ball =
pad, and then the package itself
(PKG is soldered on PCB)
Assume the package substrate trace is 5mm and wire is 2.5mm long, TDR =
pulse 35psec.
Questions are:
Can we really get the impedance profile of the package trace+wire by =
using a 35psec Tr TDR?
Or should we use sub 10psec TDR?
The difference between 1 and 2 listed above as I can think of:
a.
Probe's ground, one is on PKG's and the other on PCB's
b.
The extra capacitance seen between PCB ball pad and the PCB ground
(0.13pF per SpiceLink)
Using ADS circuit simulator to TDR the cases above with and without =
adding a capacitor of 0.13pF at entry point of the package trace, we see =
when the cap is in place, the waveform gets a dip which masks out almost =
the entire impedance profile of the package's at no-cap situation.
Is it true that little cap between PCB and PKG interconnection dominates =
the impedance profile of PKG's?
Would like to get your opinions about package level impedance control =
and verification.
Thanks~
Porsh
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