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April 2006

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From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, [log in to unmask]
Date:
Tue, 25 Apr 2006 22:48:44 EDT
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Hi JaMi,
I do not know where you asked your questions, but obviously not at the right
places [at least for some of them.
Your questions are very pertinent, but incomplete--you also need to ask for
physical properties such as modulus and ductility.
Lets get the easy one out of the way:
1) Common rolled sheet copper, when annealed, can be relied on to have the
'book values' given for copper both in terms of the electrical and physical
properties.
If it is not annealed, there will be gross differences in the machine and
cross-machine directions in terms of strength and ductility due the needle-like
structure in the rolling direction--I do not know what the electrical
properties are in this case. Perhaps Olin <[log in to unmask]> can be of help.
2) I do not know of values for electroless deposited copper--I would suspect
it might behave similar to electrolytic copper, but I am not sure.
3) Electrolytic copper as plated in a PCB Fab house will be somewhat
different from electrolytic copper as you find in commercial electroplated foil--the
difference comes from significantly different plating chemistries and plating
current densities. You certainly should look at IPC-4562, Metal Foil for
Printed Wiring Applications, as well as IPC-TR-484, Results of IPC Copper Foil
Ductility Round Robin Study.
What they have in common is, that in both cases organic and other plating
bath additives get co-deposited during the plating processes.
The consequence is that the density of these deposits is lower than the
copper 'book value' as well as the rolled copper foils--typically in the range of
8.8 to 9.0 g/cm3. That is the underlying reason why copper foil is sold in
oz/sq-ft rather than a thickness. A further consequence is that the modulus of
elasticity is about 30% lower than the Cu 'book value,' typically at 12e6 psi;
still another is that the resistivity is higher--it varies from 0.181 to 0.155
ohm-gram/sq meter depending on thickness; as well as a higher etch rate.
Because the amounts of co-deposited organics varies, these properties have ranges
rather than a fixed values.
In fact, IPC-4562 is specifying electrolytic copper foil so liberally, that
3-sigma production outliers can be shipped meeting the spec. When that happens,
larger than normal amounts of organics are co-deposited with the following
effects on properties happening all at once: a) reduced strength, b) reduced
ductility, c) reduced density, d) increased resistivity, and e) increased etch
rate. You combine d) and e) and your current carrying capacity goes down quite
rapidly.
No idea about effects on CTE, but given the inaccuracies I see with the
resins going into PCBs, any variation in E-Cu CTE would be small potatoes.
Whether the interfaces between electroless Cu and Cu foil on the one hand and
electroless Cu and electrolytic Cu form some barrier is a very good
question--alI I can say is that these interfaces are implicated for 2 out of the 3
inner-layer separation [formerly post separation] failure modes.

Regards,
Werner Engelmaier
Former Chairman of the IPC Copper Foil Committee
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com


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