Josh-

The following is an application note from the Merix website
(www.merix.com) that helps explain the difference.  Feel free to visit
the website for more information.

Paul Formiller
Program Manager
Merix Corporation
(503)-992-4511 F(503)359-2614
[log in to unmask]



Electrodeposited vs. Rolled Copper 
Circuit substrate suppliers offer a choice of rolled (R) or
electrodeposited (ED) copper foils laminated to microwave dielectric
materials in a range of weights. Questions about the difference between
the two types of copper are common. The terms "rolled" and
"electrodeposited" refer to the way in which the copper foil is
manufactured, and not the way in which it is laminated to the base
dielectric material. R and ED copper foils are laminated to the
substrate in the same way. The difference in the manufacture of the
foils creates different foil properties that are of interest to
microwave design and process engineers. 

Rolled copper is made by the successive rolling of a copper billet
between polished parallel rolls. This process gradually reduces the
copper thickness, producing a continuous coil with a flat, smooth
surface. Mechanical constraints limit the minimum finished thickness to
about 1/2 ounce per square foot, or about 0.0007 inches (0.018mm). 

Electrodeposited copper is made by an electrolytic process in which
copper is plated from a sulfate solution into a slowly rotating
stainless steel drum. The thin layer of electrically deposited copper is
then peeled off the drum in a continuous sheet. Handling problems limit
the minimum thickness of ED copper to 3/8 ounce per square foot, or
0.0005 inches (0.013mm). However, by depositing the copper onto an
aluminum foil "carrier" (CAC foil) instead of the steel drum, the plated
copper can be made as thin as 1/8 ounce per square foot, or 0.00018
inches (5 microns). 

Copper foils bond differently to different material types. Copper foils
are directly bonded to polytetrafluoethylene (PTFE) substrates. Under
heat and pressure, the polymer will flow into the treatment on the
copper surface. When cooled, the PTFE solidifies. The solid
thermoplastic forms a mechanical bond, unlike thermosetting resin
substrates (such as epoxy) that cure and form a chemical bond, in
addition to a mechanical bond. 

To enhance the bond, it is necessary to roughen the copper foil surface.
This gives the copper a tooth for the PTFE or other material types to
"grab" onto. This is necessary with both R and ED copper types, and is
done using an electrolytic treatment in which one side of the foil is
plated with a coarse granular copper deposit. 

Copper crystals "grow" from the drum side of ED copper so that their
long axes are at right angles to the plane of the foil. The drum side is
as smooth as the drum, but the outer side is rougher due to the end of
the crystals. The secondary electrolytic plating treatment adds to this
roughness. 

For R copper, the crystals are flattened and oriented in the plane of
the foil as the copper is forced through the rollers. Since both sides
of the foil are as smooth as the rollers, the surface roughness is much
less than that of ED copper, even after treatment. 

The surface roughness of the copper foil controls two important
properties: bond strength and insertion loss. Bond strength is directly
related to surface roughness; the rougher the surface, the stronger the
bond. For example, one ounce per square foot ED copper with a surface
roughness of 90 to 100 microinches on the treated side has a typical
copper peel strength of 12 to 16 pounds per inch on RT/Duroid ® 5880.
This compares to peel strength of 8 to 10 pounds per inch for R copper
with a treated side roughness of 50 to 60 microinches on the same
material. 

The second important property affected by surface roughness is the
insertion loss for a microwave signal in a copper conductor. (Insertion
loss is the loss of signal power as it travels through the conductor.)
For fairly low microwave frequencies of one or two gigahertz (Ghz), the
loss in the conductor is small compared to the losses caused by the
power dissipation factor of the dielectric material itself. However, at
higher frequencies, say 10 Ghz, the conductor losses become more
significant. In these applications, you can save 15% or more in total
losses by using R copper instead of ED copper. 

The crystalline structure of the foils also contributes to the
differences in conductor losses. A signal propagating along an ED foil
conductor will "see" more crystal boundaries than in an R copper
conductor because of the crystal orientation. Each crystal boundary
represents a slight, but significant, discontinuity to the signal,
resulting in a minuscule but measurable increase in the resistivity.
Also, the mechanical crushing in the manufacturing process of rolled
copper makes it slightly denser, with more intimate contact between
individual crystals. As a result, the bulk resistivity is about 20% less
than that of ED copper. 

Crystal structure and orientation also affect mechanical properties. The
tight-knit, horizontally-oriented crystals of rolled copper enable it to
stretch and accommodate dimensional changes. However, the structure
tends to crack along the crystal faces when it is stretched to its
elastic limit. 

Thermal cycling of a circuit will alternately stretch and compress the
copper conductors as the dielectric thermally expands and contracts.
Rolled copper will withstand this stress much better than ED copper, and
we strongly recommend specifying laminates clad with R copper for
circuits that will see large temperature changes (e.g., avionics and
space applications). 

For high bond strength and for copper weights less than 1/2 ounce per
square foot, the choice must be ED copper. Rolled copper gives the
lowest insertion loss and best thermal stress crack resistance. Look to
the Merix Application Engineering group for help in choosing the
appropriate copper for your PCB design. 




> ----------
> From:         Josh Moody[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Josh Moody
> Sent:         Monday, February 09, 1998 3:00 PM
> To:   [log in to unmask]
> Subject:      [TN] electrodep Cu foil vs. RA foil
> 
> Technet,
> 
> Can someone out there explain the difference between
> rolled/annealed copper foil and electrodeposited copper
> foil?  also what are the cost and availibility issues
> with both, specifics are not needed only multipliers
> (i.e. RA foil is half the price ED foil)
> 
> Thanks,
> 
> 
> Josh Moody
> Materials Quality Engineer
> Hewlett-Packard - Richardson (HPSD)
> ph# (972) 497-4617
> [log in to unmask]
> 
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