Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!pyramid!prls!mips!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: sci.electronics
Subject: Help with L measurement
Message-ID: <1872@mips.mips.COM>
Date: 15 Mar 88 19:57:51 GMT
Lines: 71
Keywords: inductance of chip ground


 { line eater ? } 
  
I have a problem.  I'm trying to measure the inductance of a
gate-array (IC pin + package trace + bondwire), i.e. the total
inductance between the ground plane of my printed circuit board and
the on-chip ground metallization inside my gate-array IC.  Here are
some of the particular constraints I'm working under:

	1.  I don't have the mechanical drawings of the package so I
		can't "solve" the 3D Laplace E&B-field equations.
	2.  My test equipment consists of a 500MHz scope, 100MHz
		signal generators, VOM, etc.  I.e. no vector impedance meter.
	3.  The gate array vendor won't get any more precise than an
		estimate of 5-15 nanoHenrys.
	4.  There are about 10 ground pins/traces/bondwires on the part
		and a measure of any of them will suite me just fine.
	5.  Trusty Mr. Fluke VOM tells me that from one ground pin to another
		ground pin on the same edge of the chip, is 1.2 ohms (@ DC).
	6.  I can build a smallish jury-rigged test fixture if necessary
		(see below).

FIRST ATTEMPT:
I tried to measure the L by constructing an L-C tank circuit and measuring
the resonant frequency.  The "L" was from one ground pin, through the pkg
trace, bondwire, onto the chip, out through another bondwire, another
package trace, and another pin (i.e. 2X the L of one ground pin).

The test fixture placed a 1000pF ceramic cap across the "L", and arranged
to pull 100mA of DC current through "L".  Then the current is suddenly
(3ns falltime) cut off ... LC should ring like a Banshee.  Worked great
with a 1000 nH test inductor purchased at Fry's.  But substitute the
gate-array ground-pin "L" and, voila, no ringing at all.  Bah.

Here's the circuit; the switch is really 3 74F244 drivers in parallel.
Someone can translate it to EDIF if desired :-)

            SW                  ---------          ---------
 GND....o<-------o...../\/\/\...-1N4148 +......x...|   L   |...x....+5.0V
                       50 ohm   ---------      |   ---------   |
 +5.0V...o                                     |               |
                                               |------| |------|
                                               |     1000 pF
                                               |
                                               |
                                           Oscope probe


Now, I ask the Old Farts of the net (and the Young Turks as well), what
next?  The fixture pictured above produced negligibly small waveforms
at the scope probe point (i.e. NOT sinewaves).  The whole thing was built
over a PC ground plane.

REQUESTS
If somebody has already faced and solved this problem, I'd love to
hear about it.  In particular, a Measurement Method is highly prized,
rather than a bunch of calculations based on assumptions.  I don't
know the diameter of the bondwire, the length of the package trace,
the dielectric coefficient of the packaging material, etc.  All I
really DO know is that I have 20 of these suckers on hand, and I can
burn em out if necessary.

	Idea #1: the nonzero resistance of the test "inductor" causes
	poor oscillatory behavior.  So build an active circuit
	rather than a passive tank.

	Idea #2: Measure impedance vs. frequency.  For the expected L
	values (20 to 30E-9 Henry) this is under 10 ohms at 50 MHz.  Yuck.

 -Mark Johnson	*** DISCLAIMER: Any opinions above are personal. ***	
 UUCP: {decvax,ucbvax,ihnp4}!decwrl!mips!mark   TEL: 408-991-0208
 US mail: MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086