I agree that fabricators can do things to the copper that make it worse. My
fudge factor accounts for that.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jeff Loyer SI
Sent: Thursday, July 2, 2015 1:28 PM
To: 'Lee Ritchey'; 'SI-LIST'
Subject: [SI-LIST] Re: Resistivity of copper
Sorry, Lee, but I disagree. I don't think that's where the discrepancy
comes from (impure copper from the ED vendors). My understanding from the
ED copper foil vendors I've spoken with is that they measure the
conductivity to be very close to that of copper. IPC spec. 4562A section
3.8.1 is also very clear about the purity and resistivity of incoming
copper. Something happens after it's received by the laminate vendors, I
believe.
The values below represent data I've measured and gathered from other
sources. Doubling the resistivity of copper seems overly conservative.
Jeff Loyer
Signal Integrity Consulting
-----Original Message-----
From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
Sent: Thursday, July 2, 2015 12:24 PM
To: jeff.loyer.si@xxxxxxxxxxx; dbrooks9@xxxxxxxxxxxxxxx; 'SI-LIST'
Subject: RE: [SI-LIST] Re: Resistivity of copper
Most foil copper used in PCBs is plated on a drum using scrap wire copper.
You can guess how impure that might be! Because of this, I take a
pessimistic view of its conductivity, doubling what pure copper is. Yes,
that is a guess, but so is the foil data you get.
Some world we live in! That's what you get when you drive the system with
price instead of cost!
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Jeff Loyer SI
Sent: Wednesday, July 1, 2015 9:45 PM
To: dbrooks9@xxxxxxxxxxxxxxx; 'SI-LIST'
Subject: [SI-LIST] Re: Resistivity of copper
Hi Doug,
FYI, my experience (and others') agrees with Gert's, that PCB "copper" isn't
pure. Our measured conductivities were:
2.1uohm-cm (4.76e7 ohm-m conductivity) for stripline
2.4uohm-cm for microstrip
Copper = 1.7uohm-cm (5.88e7 ohm-m)
We measured the temperature coefficient as 0.35%/degree C (interestingly
there was a difference between U.S. domestic and foreign vendors').
Coming from the copper foil vendors, IPC specs. ensure it is very "pure",
with conductivity specified. It appears the processing of the laminate
suppliers affects the conductivity significantly, though we've never
understood exactly what part of the process does that. I suspect treatments
diffuse into the copper, reducing its conductivity.
The data Ive seen indicates packages copper behaves more like copper.
Jeff Loyer
Signal Integrity Consulting
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Doug Brooks
Sent: Wednesday, July 1, 2015 8:44 PM
To: SI-LIST
Subject: [SI-LIST] Re: Resistivity of copper
Thank you all for your help.
Turns out I was on the wrong track! First, I have been under the assumption
that the resistivity curve was non-linear over this wide a range. Turns out
that the normal equation still works here. ( Rho-subT
= Rho-sub20*(1 + Alpha-sub20*(T-20)) This leads to about 5.3 or 5.1
depending on what assumptions you make. My 4.3 number, it turns out, was
irrelevant!
So, I apologize for the exercise. But thanks for the support.
Doug Brooks
Doug Brooks wrote:
I would like to know the resistivity of copper at 540 degrees C,
assuming it is 1.72 * 10^-8 Ohm*m²/m at 20 degrees C. The temp
coefficient of resistivity is .00393 at 20 degrees C but using this I
get a result of around 5.31 at 540 degrees. But a problem I am solving
says it is closer to 4.30. I am having trouble resolving the difference.
Can anyone confirm the (approx.) 4.3 number for me?
Thanks for any help.
Doug Brooks
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