Special Issue

Challenges to New Technologies and New Products

Copper Foil for PCB, Simulation and Studyon High-speed Digital Circuits
Satoru Torimitsu*1, Takahiro Saito*2

Pursuant to a higher transmission speed with an increase of the information
amount such as Internet of Things (IoT) or big data, a higher frequency band
than the microwave band becomes used. The printed circuit board (PCB) which forms apparatuses/products requires low-loss materials in the high-frequency band. In this article, we
describe the manufacturing process and the feature of our electro-deposited copper foil, andalso explain, based on an actual case, how to model and utilize the electro-deposited copper foil
in a high frequency characteristic simulation which is essential for the design and the development of electronic devices.

ABSTRACTINTRODUCTION
1.1. INTRODUCTION
There are growing needs for higher speed and higher frequency of telecommunication equipment essential for
optical communication systems or mobile telecommunications using millimeter waveband. In order to meet suchneeds, manufacturers are developing electronic components and, as an example, a high performance PCB
which can have high multi-layers with low-dielectric constant and low-dielectric loss tangent is being developedby material manufacturers. The PCB made of polyphenylene ether (PPE)-type resin has less than 1/10 of dielectricloss tangent compared to the conventional one made ofglass epoxy-type material, and can have multi-layers
which have been difficult with polytetrafluoroethylene(PTFE)-type material which is enhanced in high-frequency
properties.
We have been developing and manufacturing an electro-deposited copper foil suitable to PCB for more than
40 years and were involved in making electronic devices
operating at higher speed and higher frequency.
In this article, we describe the manufacturing process
and the feature of our electro-deposited copper foil, andalso explain, using an actual case, how to model and utilize the electro-deposited copper foil in a high frequency
characteristic simulation which is essential for the designand development of electronic devices.

*1*2

THEMANUFACTURING
MANUFACTURING PROCESSPROCESS OF
2.2. THE
OF
THEELECTRO-DEPOSITED
ELECTRO-DEPOSITED COPPER
THE
COPPER
FOIL
FOIL
Most of the conductor layers of a PCB are processed tomake circuits after an electro-deposited copper foil or arolled copper foil is press-contacted to a resin basedmaterial. In this chapter, we describe the manufacturing
process of an electro-deposited copper foil which isespecially used at a high ratio.
2.1 The Manufacturing Process of the Raw Foil
Figure 1 shows a schematic diagram of the general manufacturing equipment for an electro-deposited copper
foil. The electrodes are configured with a drum-type cathode made of titanium or stainless steel and a concentrically opposing insoluble anode such as a noble metaloxide clad electrode or a lead electrode. Copper plating
is deposited on the surface of the drum-type cathode bycasting copper sulfate electrolyte between both electrodes from the bottom of the equipment and applying
electrical current. The drum-type cathode rotates at apredefined rate and the deposited copper plating is continuously peeled off from the surface of the drum-type
cathode and then rolled up as a copper foil.

Laboratories for Fusion of Core Technologies, R&D Division
Automotive Products & Electronics Laboratories, R&D Division

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits

Electro-deposited copper foil: Raw foil

Shiny surface

Matt surface

・Matt surface

Cathode・
Shiny surface
Anode

Figure 2 (a) A schematic X-section of a standard foil.

Matt surface

Copper sulfate solution
Figure 1 A schematic X-section of the manufacturing
equipment.

The surface of the electro-deposited copper foil whichfaces the drum-type cathode is called shiny (S) surface.
That is because the surface of the drum-type cathode isflat and shining and the copper foil deposited on and thenpeeled off from the surface of the cathode is also flat andshining. On the other hand, the opposite side of the
S-surface is called Matt (M) surface. That is because theM-surface of an early industrial electro-deposited copper
foil had basically large asperity and was matt. The thickness of the foil is mainly controlled by adjusting the rotation rate of the drum-type cathode, that is, by adjusting
the plating time, therefore the plating time is made shorter by higher rotation rate to make the foil thickness thinner. The 6-210µm of electro-deposited copper foil is currently mass-produced. The handling during manufacturing or using becomes difficult if the thickness is less than
6µm, therefore the foil is required to have its characteristic improved such as a higher strength as described in
the followings.
The electro-deposited copper foil has a feature bywhich its characteristic can be changed by adding various additives into the copper sulfate electrolyte. The
M-surface has been keeping its favorable asperity byusing organic system additives generally and by using
macromolecular organic compound such as animal glue
in early stages. The electro-deposited copper foil manufactured with such additive has the cross-section shape
shown in Figure 2(a) and is called the standard foil. The
manufacturing process in which the M-surface can haveflatter and more shinning surface than the S-surface byusing several additives is developed afterward. The electro-deposited copper foil manufactured by such process
has the cross-section shape shown in Figure 2(b) andbeen called a double side shiny foil.

Shiny surface
Figure 2 (b) A schematic X-section of a double side shiny foil.

The Manufacturing Process of the Surfacetreated Foil
Depending of each product, a surface treatment is added
onto the raw foil obtained. The electro-deposited copper
foil can be categorized as for PCB application and forbattery application. Here, we explain a general manufacturing process for the former use.
After cleaning the surface of the raw foil, a roughening
particle forming is done as the first process. Specificallythe roughening particles which consist of copper particles
are formed by plating the surface of the raw foil using acopper sulfate electrolyte with a low copper concentration and a low solution temperature. The roughening particle forming is an important process to obtain adequate
adhesiveness with the resin of the PCB by an anchor
effect which occurs when the particles dig into the resinof the PCB.
Next, the process to give the foil chemical resistance,heat resistance and/or rust-prevention is done.
Describing particularly in the general example, on theroughened side where the roughening particles areformed, that is, on the side where the resin will be adhered,
the chemical-resistant and heat-resistant layer which
consists of, such as, nickel, zinc and/or cobalt is formed
and also rust-preventive layer which mainly consists ofchromium for rust-prevention is formed. Last, the silane
coupling treatment layer as an adhesion-improving layer
is formed. Silane coupling agent is so selected in a manner to be suitable for the resin adherence. The schematicof the copper foil for PCB, which has the surface-treatment construction described above, is shown in Figure 3.
2.2.

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits

Adhesion-improving layer

Roughened surface (Resin adhesion side)

Rust-preventive layer

Chemical-resistant& Heat-resistant layer

Roughening particles

Copper foil : Raw foil

Heat-resistantRust-preventive layer
Figure 3 A schematic X-section of the copper foil for PCB.

Subsequently, the surface treated foil is cut out into theshapes and sizes, such as sheets or rolls, according to
the user’s requests and shipped out.
Since the network communications traffic increased
recently, a demand of a high frequency PCB for servers,routers and mobile equipment which handles increase indata processing is increasing and enhanced high-frequency characteristics is required for the foil of the PCB,
therefore the double side shiny foil which has a roughening particle forming on the M-surface has been developed. The M-surface of the double side shiny foil is amore flat surface than the S-surface and the roughening
particles are formed uniformly and finely, therefore the foil
meets the requirement at higher frequency characteristic(Figure 4). This type of the copper foil is superior in thecircuit linearity (providing the characteristic if the circuitedge is cut linearly with the sight from the direction of thesurface after etching) and the etching typified represented
as such as an etching factor, therefore the foil meets therequirement of a high-density circuit forming which has
an increased demand in recent years.

Figure 4 (a) M att surface of double side shiny foil:
Untreated.

FEATURESOF
OFTHE
THE ELECTRO-DEPOELECTRO3.3. FEATURESDEPOSIED
COPPER
SIED
COPPER
FOIL FOIL
3.1 Crystal Structure
Figure 5 shows X-section crystal structures of the copper
foils. The crystal structure of the ordinary state (Figure 5(a)) shows that the rolled foil is constructed with a layered
structure which extends in the rolling direction whereas
the standard electro-deposited copper foil shown in
Figure 5 (b) is constructed with a pillar structure which
extends in the thickness direction of the plate deposited
direction. On the other hand, the double side shiny foil,
shown in Figure 5 (c), of the electro-deposited copper foil
which is manufactured by using special additives is constructed with a random granular structure and its crystal
structure has growth and coarsening by a thermal loadwhen it is press-contacted to a resin based material.

Figure 4 (b) After treatment.

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits（a）Rolled copper foil
As received

After 210℃×1 hour（b）Electro-deposited copper foil: Standard
As received

After 210℃×1 hour（c）Electro-deposited copper foil: Double side shiny
As received

After 210℃×1 hour

Figure 5 X-section crystal structures of copper foils.

3.2 Mechanical Characteristics
Except high-strength type used special additives, theelectro-deposited copper foil has a tensile strength ofnearly 300-400MPa at the ordinary state of both standard
foil and double side shiny foil. Since its crystal structure isisotropic, it has small anisotropy of an elongation and its
elongation rate is higher because the crystal structure iscoarser compared to the rolled copper foil. It also
improves its crease performance because its crystal
structure becomes coarser by thermal load when it ispress-contacted and this characteristic is preferred for a
flexible printed circuit.
3.3 Introduction of the Typical Products
The variety and characteristics of our typical products ofhigh-frequency compatible electro-deposited copper foil
and schematic X-sections of their types are shown in
Table 1 and Figure 6. DGTSEU2-MP is called RTF foil
which forms roughening particles on the S-surface of thestandard foil shown in Figure 2 (a) and classified in Very
Low Profile (VLP) as the class of the high-frequency compatible electro-deposited copper foil. F2-WS and FV-WS
form roughening particle on the M-surface of the forenamed double side shiny foil and both surfaces including
the resist surface are flat, therefore they can be suitable
for the application which require enhanced high-frequency properties. The FV-WS is especially classified in H-VLP
which is a higher class of VLP and has superior adhesiveness with a resin consisting of a polyphenylene ether(PPE), therefore it is a product proud of its high reliability.
FZ-WS is a product developed for the application which
requires superior high-frequency properties with maintaining adhesiveness to a resin by a contriving process of
the roughening particle forming. Since FZ-WS, FV-WS
and F2-WS have, as a base material, a double side shiny
foil which is superior in a crease performance, they are
preferred for not only a high-frequency substrate but also
for the flexible printed circuit board which requires highfrequency performance.

Table 1

Typical products of high-frequency compatible
electro-deposited copper foil.
Product/Profile

Roughness Resin adhesionside (µm)
Rz(at 18 µm) Resist Side (µm)
TensileStrength(at 18 µm)
Elongation(at 18 µm)

FZ-WS/ FV-WS F2-WS
HVLP2 /HVLP /VLP
1.1

1.2

1.8

DGTSEU2-MP/VLP(RTF)
3.0

1.2

1.2

1.2

3.5

RT (MPa)

310

310

310

320

at 180℃ (MPa)

180

180

180

170

RT (%)

9

9

9

12

at 180℃ (%)

12

12

12

14

High-Frequency Substrate

〇

〇

〇

〇

Flexible Printed Circuit

〇

〇

〇

Semiconductor Package〇〇

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits

THEEXTRACTION
EXTRACTION OF THE
4.4. THE
THECONDUCTORCONDUCSURFACEROUGHNESS
USINGUSINGTHE
TOR
SURFACEROUGHNESS
S-PARAMETERTHE
S-PARAMETER

DGTSEU2-MP

F2-WS

FV-WS

As described in Chapter 1, a high frequency characteristic simulation is increasing in its importance in the designand development of the electronic devices and it makes it
possible to reduce the number of trial productions and tohandle rare events and can significantly improves thequality of the products.
Figure 7 shows the measured attenuation (S21) of the
strip-line transmission line generated on the PCB and theresult which is calculated with a relative dielectric constant of 3.6 and a dielectric loss tangent of 0.004 for the
resin material while the condition of the electric conductivity of the conductor is assumed as infinite. As seenfrom Figure 7, the difference between the measured value
and the calculated value becomes larger as frequency is
increasing. This difference is attributed to the determination of the conductor at the simulation therefore the conductor surface roughness and the skin depth which
depend on the frequency need to be expressed appropriately. In this chapter, we calculate high-frequency characteristics in consideration of the surface roughness of aconductor model based on the Hemispherical model
which quantifies the conductor surface roughness. There
are two ways to determine parameters of this conductor
model; one way is to determine by observing the surface
roughness of the foil with an electron microscope, andthe other way is to determine by making a PCB actuallyand using S-parameter obtained by a vector network
analyzer. In this chapter, in consideration of user convenience, we use the latter way to determine parameters of
conductor surface roughness model by using the
S-parameter.
0-2Figure 6 Schematic X-sections of high-frequency compatible
products.

-4
S21, dBFZ-WSCalculated-6
Measured

-8

-10
-12
0

5

10

15

20
25
freq, GHz

30

35

40

Figure 7 Comparison of the calculated and the measured
attenuations of the strip-line transmission line.

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits

0

Calculated-5-10
S21, dB

Figure 8 shows the X-section of the PCB for
S-parameter measurement. Figure 9 shows the
S-parameter measurement result of the PCB shown in
Figure 8. Using this S-parameter, parameters are determined by fitting to the transmission line model which iscompatible with the conductor-surface-roughness. The
parameters of our electro-deposited copper foil FV-WS
are shown in Table 2. Figure 10 shows the results of themeasured value by generating strip-line on the PCB using
FV-WS and extracting the parameters, and the calculated
value of the simulation. It shows that the measured value
and the calculated value of the Hemispherical model simulation are extremely matching.

Measured

-15
-20
-25
-30
0

5

10

15

20

25

30

35

40

freq, GHz
Figure 10 Comparison of values calculated and measured forthe electro-deposited copper foil FV-WS.

Resin

STUDYOFOFHIGH-SPEED
HIGH-SPEED DIGITALDIGITAL CIR5.5. STUDYCIRCUIT
CUIT
0.38 mmCu foil
0.15 mm

In this chapter, we introduce the transmission characteristics with setting values of the high-speed digital simulation shown in Table 3, using the conductor surface roughness extracted in the previous chapter.

Figure 8 X-section of the PCB for S-parameter measurement.
Table 3

S21, dB0

Parameters of high speed digital (HSD) simulation.
Parameter

Value

-5

Tx, PRBS Bit rate

28Gbps-10

Tx, PRBS Register length

7

Tx, AMI Modulation

NRZ

Tx, AMI
Pre-shoot De-emphasis

0dB

-15
-20

EncoderNo-25

Rx, AMI CTLE

Disable-30

Rx, AMI CDRDisableRx, AMI DFEDisable

0

5

10

15

20

25

30

35

40

freq, GHz

PRBS (Pseudo Random Bit Sequence)
AMI (Algorithmic Modeling Interface)
CTLE (Continuous Time Linear Equalizer)
CDR (Clock Data Recovery)
DFE (Decision Feedback Equalizer)

Figure 9 Measured S-parameter S21.

Table 2

Simulation parameters of FV-WS.
Parameter

Value (µm)

L1Rough

0.55

L1Bbase, L1Dpeaks

1.1

L2Rough

0.36

L2Bbase, L2Dpeaks

0.72

L1Rough：
Value of protrusion height (Level1)
L1Bbase, L1Dpeaks： Value of protrusion base width, peak distance
between two adjacent protrusions (Level1)
L2Rough：
Value of protrusion height (Level2)
L2Bbase, L2Dpeaks： Value of protrusion base width, peak distance
between two adjacent protrusions (Level2)

To see the difference of our electro-deposited copper
foils, FZ-WS and F2-WS are added as the comparison
objects besides FV-WS described in Chapter 3. Figure 11
shows high-speed digital transmission characteristics at
28Gbps. It shows the eye patterns when the length of the
strip-line is changed as 300mm, 400mm and 500mm.
Seeing the eye patterns of 500mm transmission line,
the eye pattern of the F2-WS is closed. It is found out
that, changing the copper foil of the PCB to FV-WS andFZ-WS, the eye patterns will be improved.

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Copper Foil for PCB, Simulation and Study on High-speed Digital Circuits

300 mm

0.4

0.2

0.2

0.0

-0.6

0

0

0

30 40 50 60 70 80
time, psec

-0.6

0.0
-0.2
-0.4

0

0

0

0.6

-0.6

30 40 50 60 70 80
time, psec

0.4

0.2

0.2

0.2

0.0

0.0
-0.2

-0.4

-0.4

-0.6

-0.6

0

0

0

0.6

30 40 50 60 70 80
time, psec

Voltage

0.4

-0.2

0

0

0

-0.6

30 40 50 60 70 80
time, psec

0.2

0.0
-0.2
-0.6

0

0

0

30 40 50 60 70 80
time, psec

Voltage

0.2

Voltage

0.2

-0.4

0

0

0

30 40 50 60 70 80
time, psec

0

0

0

30 40 50 60 70 80
time, psec

0.6
0.4

-0.6

30 40 50 60 70 80
time, psec

-0.4

0.6

-0.4

0

0.0

0.4

-0.2

0

-0.2

0.4
0.0

0

0.6

0.4
Voltage

Voltage

FV‒WS

0.0
-0.2
-0.4

0.6

Voltage

Voltage

0.4

0.2
-0.2

500 mm

0.6

0.4

-0.4

F2‒WS

400 mm0.6

Voltage

Voltage

FZ‒WS

0.6

0.0
-0.2
-0.4

0

0

0

30 40 50 60 70 80
time, psec

-0.6

Figure 11 28 Gbps high-speed digital simulation results.

The design of PCB which is actually applied for thehigh-speed digital transmission line is more complicatedthan this example and has a large influence to high-frequency characteristics. It is assumed that we can contribute to the design and development of high quality highfrequency circuits by establishing high-frequency characteristics of the foil as a conductor which configures thewiring.

CONCLUSION
6.6. CONCLUSION
The material characteristics of the electro-deposited copper foil for a PCB, which we have been developing andmanufacturing, and the example of high-speed digital circuit design by using parameters of conductor surface
roughness are explained.
The manufacturing process of the raw foil and the surface processing which is important for the PCB application are explained and crystal structures and mechanicalcharacteristics of our electro-deposited copper foil are
explained.
It makes it possible to consider high-frequency characteristics of a copper foil as a conductor comprising PCB,
through extraction of the conductor surface roughness
parameters by using the S-parameter and using it in ahigh-speed digital circuit. In order to meet the needs
which are becoming more sophisticated in the future, weare going to fuse and improve our electro-deposited copper foil technology and the high-frequency technology
and make a contribution to advancement of PCBs andelectronic devices which uses those PCBs.

REFERENCES
1) “MATERIAL STAGE” Vol.18 No.4 (2018), p. 69. Technical
Information Institute Co., Ltd.
2) JIS C 6515:1998 “Copper foil for printing”
3) S. Hall, S. G. Pytel, P. G. Huray, D. Hua, A. Moonshiram, G. A.
Brist, and E. Sijercic, “Multigigahertz causal transmission line
modeling methodology using a 3-D hemispherical surface roughness approach”, IEEE Trans. MTT, Vol. 55, No. 12 (2007), pp 26142624.

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