🚀 go-pugleaf

> (paraphrase) how do I measure pingrid package inductances?

Flippant answer: It helps to work at an instrument manufacturer :-)

Real answer:  An approximate inductance suitable for low frequency
modelling could be found by grounding one end the pin, driving the
pin from a 50 ohm source, and looking at the voltage signal from 
the pin to ground on a terminated 50 ohm line into the scope.

At low frequencies (less than 100 MHz) a resistive source will look
more or less like a current source driving the pin.  You may want to
stick some attenuators between the signal source and the circuit under
test to remove any effects of the signal generator.  When you wire on
the coax for the measurement, keep the loops small, less than 0.2
inches or so.

Keep in mind that the behavior of neighboring pins on the package
(whether they are shorted, terminated, or whatever) will affect your
measurement.

-----
I assume you are measuring package inductance to see how it will
affect a digital circuit and not an RF circuit.  You will probably
want to do all this with a good, fast rise pulse generator.  Using
a sine source makes some of the calculations easier, but won't give
you as good a feel for what will happen when you put silicon in the
package.


(Pulse Gen) ===> trigger ==========>(Oscilloscope)
 |      |                            |   |
 |   (Atten)-C-----+------C--(Termination)
 |      |    O     |      O   |
 |      |    A  (D.U.T.)  A   |
 |      |    X     |      X   |
 +------+----------+----------+  Ground

First, look at the test fixture with the package not connected, and
make sure the signal is making it through cleanly.  

If the pulse takes a long time ( > 3 nsec) to flatten out,
find a better signal generator and scope, and tighten up your setup.
Ideally, you would like to be working with 100 psec or so pulses,
but you would need sampling gear and tunnel diode pulsers and such.
We normally use a device called a TDR (for time domain reflectometry)
for such measurements, but then, we are usually pushing GHz signals
around here.  (See your local Tektronix field office, plug, plug!).
The slow risetime makes the measurement harder but not impossible.


Put in the package.  What you will see will probably be a short
pulse, much smaller amplitude than the unshorted pulse, and perhaps 
a little ringy.  After that, there will be a residual DC component due
to the resistance of the run in the package under test.

Now the first hard part:  take a scope photo, and with eyeball or
planimeter INTEGRATE the area of the pulse on the screen.  This gives
the number of Volts*Seconds "stored" in the inductor.  Be sure to
count the negative rings as negative area.

Now, assume that the residual resistance is constant (not a great
assumption because of skin effect) and subtract the area of the
residual voltage times the time the unshorted pulse would be on
screen (say the residual is 20 mV, and the pulse is on screen for
4 nsec, the residual is 80 picovolt-seconds).  


If you started with, say a 2 volt pulse into the scope, you should
have a number on the order of 1000 pico-volt seconds.  Use the following
formula:

     L = (Volt seconds)*(25 ohms)/(Original step voltage)

Then, short the test fixture with a short, fat wire (less than 0.1 inch)
and see how much residual signal makes it through.  There shouldn't
be much;  you should only be seeing the inductance of the short wire.
You will want to measure this and subtract it from the measurement on
the package, by same technique.

------
This technique should give an approximate number (20%???)  for package
inductance.  As I am extrapolating from measurements we normally
perform with much faster gear, there may be some factors I am ignoring.

Good Luck!









-- 
Keith Lofstrom   ...!tektronix!vice!keithl   keithl@vice.TEK.COM
MS 59-316, Tektronix, PO 500, Beaverton OR 97077  (503)-627-4052