I don't think it is totally due to measurement error, but rather the
assumption that the trace manitains the exact same cross-section area
profile, as measured, throughout the entire length of track. Last year I
tried to do a similar study on resistivity.
I measured some test coupons from which I had some cross-section measurement
data of stripline geometry. I used 4-point Kelvin probing technique when
measuring. In one case, for example, I found with just 2.5 um of difference
in measured thickness,I measured a resistivity of 1.72e-8 Ohm-m (5.81e7 S/m)
vs 2.05e-8 Ohm-m (4.87S/m). I also found that trying to measure an 8 mil
wide, 1/2 oz copper, with trapazoidal edges is difficult, especially for
rough copper. I tried this on several different designs. N4k13 Rough Copper,
Meg6 HVLP, N4k13-ep VLP.
In the end, I pretty much came to the same conclusion as Doug. Personally I
am not yet convinced that the resistivity of ED copper is significantly
higher than that what is published.
Bert Simonovich
Signal/Power Integrity Practitioner | Backplane Specialist | Founder
LAMSIM Enterprises Inc.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jeff Loyer
Sent: October-27-15 4:29 PM
To: dbrooks9@xxxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Thermal measurements on a test board *** thanks
Personally, I've seen too many independent sources come to the same
conclusion to conclude there are measurement errors to blame. But,
resolution will have to wait until someone finds a strong reason to
reconcile the difference between theoretical and actual PCB "copper". The
thing that's most surprised me since uncovering the difference is the lack
of reaction from the power delivery community. It seems like a 20%
difference would be significant enough to influence their DC power delivery
simulations considerably. The impression I currently have, however, is that
power delivery shapes are so conservatively drawn (much larger than
necessary) that they can accommodate a 20% error. I welcome others'
insights.
Jeff Loyer
Signal & Power Integrity Product Manager Altium US
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Doug Brooks
Sent: Monday, October 26, 2015 5:26 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Thermal measurements on a test board *** thanks
Thanks all
I believe my resistance measurements and temperature measurements are pretty
close. And the thermal coefficient of resistivity calculations are right
what would be expected (.0038, within measurement accuracy.) But the thermal
coefficient of resistivity measurement does not require a calculation of
resistivity (as long as you have resistance and
temperature.)
I am concluding that a normal test bench setup (which is not a high accuracy
lab setup) is simply not sufficient to measure resistivity.
Some of the variables needed are not available or can't be determined with
enough precision. And sensitivities are such that even small errors in some
variables (like width and thickness) can get magnified in the calculations.
Added to that are the uncertainties associated with pads and connections.
I am concluding that resistivity calculations at the small trace level on a
normal PCB (i.e. not one specifically designed to measure
resistivity) simply aren't practical without a very high-end lab.
Thanks again for all your insights.
Doug
Doug Brooks wrote:
I am looking for some thoughts on some measurements I have just taken
I have a new test board I have done some preliminary measurements on,
and have come up with the following measurements:
Resistivity 0.8666 micro-ohm in (expected 0.67)
Thermal coefficient of resistivity (alpha) .0048 (expected .0038)
The "expected" values are the published values for "copper", not
processed copper on a PCB. Have any of you done similar measurements
and come up with similar values?
Doug
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