Furukawa Circuit Foil

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Copper Foils for the Printed Circuit Boards of the Future

As a material supporting the increasingly rapid trend
toward smaller size and higher performance of electronicequipment, electrodeposited copper foil is subject to thefollowing requirements in terms of properties:(1)Thinner foils with less surface roughness to enable finepatterning;(2)Improved flexibility to enable use in mobile phonehinges;(3)Surface finishing that supports adhesion even to highfunction special plastics;(4)Heat resistance to enable use with plastics requiringhigh-temperature processing;(5)Foils having functions for built-in components.
To meet these requirements, Furukawa Circuit Foil has
completed development of the following copper foils, andhas brought them to the mass-production stage.

2.

DT-GLD and DT-FLD Copper Foils for
Build-up Boards

Features: Low connection resistance with respect to conductive paste; contributes to reducing the size andupgrading the functionality of boards for mobile phonesetc.
DT-GLD is a double-side treated copper foil for use
with multilayer wiring boards having all-layer IVH structure. Figure 2 shows the condition of the surfaces.
Multilayer wiring boards having all-layer IVH structure
are manufactured by a method whereby micro via holes
are formed by laser in the prepreg, filled with copper

U-WZ Copper Foil for COF

Features: Flat, with low surface roughness. Outstanding
flexibility. Copper foil optimally suitable for COF applications, as well as FPC applications requiring high precisionand high flexibility.
Table 1 compares U-WZ foil with FO-WS, which offers
the best flatness of any of our current products. It is characterized in that, compared to the existing FO-WS foil, theroughness of the polyimide adhesion surface is muchless so that undulation is less.
With respect to visibility, the cast material using U-WZ
foil has been evaluated as comparable in every respect tothe metallized boards generally used in currently available
COFs.
Another advantage of U-WZ foil is its outstanding flex
resistance. Results of the IPC flexing test are shown inFigure 1.

Table 1UnitU-WZ

F0-WS

Surface roughness ofresin adhesion sideRaμm

0.110.32Rzμm0.60

1.21

Surface roughness ofreverse side

Raμm

0.120.26Rz

μm0.851.40

N/mm2339

313%

7.05.0Elongation

U-WZ foil
FO-WS foil
Rolled copper foil

10-2

10-3

104

105106107108

Life of FPC (Ne)

Figure 1

Flexural fatigue test results (taking flexural life as
110 % of initial resistance).

Characteristics of 9-μm foils.

Characteristic

Tensile strength

10-1

Maximum strain (E cu)

1.

In terms of the number of repetitions to failure, U-WZ
foil is superior both to F-WS foil, which has the best flexural resistance among our existing copper foils, and torolled foil. Its suitability is thus not limited to COF, but also
extends to FPC applications requiring high flexibility.

Shiny side
Figure 2

Matte side

DT-GLD double-side treated copper foil.

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2007

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Copper Foils for the Printed Circuit Boards of the Future

paste which then hardens during lamination, and electrical connection between the desired layers is arranged byIVH. These are known as ALIVH †, and at present only ourcompany’s copper foil is used in this method.
Since this method uses copper paste to provide connection between layers, it is necessary to keep down theconnection resistance between the copper paste and thecopper foil. For this reason granular copper is electroplated onto both sides of the copper foil, reducing the connection resistance between copper paste and copper foil
by means of the anchor effect.
Surface treatment that has dual-purpose of connection
resistance reduction and corrosion prevention is also carried out, thereby effecting overall reduction in the connection resistance between copper paste and copper foil.
†ALIVH is a registered trademark of Matsushita ElectricIndustrial Co., Ltd.

3.

FWL-WS Copper Foil for High HeatResistant Resins and High-Frequency
BoardsFeatures: Strong adhesive force even with special resins
where adhesion is hard to obtain. Highly suitable for highfrequency boards and liquid crystal polymer boards.
FWL foil is a copper foil developed based on WS foil
(glossy on both sides) for liquid crystal polymer applications. Liquid crystal polymers have superior high-frequency characteristics, but on the other hand show a tendency
to lower strength of adhesion to copper foil. In the case of
liquid crystal polymers, the structural formula means thatpeel strength cannot effectively be raised by chemicalbonding, so peel strength improvement was achieved

Table 2Surface roughness and peel strength.
Surface roughness (μm)

Peel strength (kN/m)

FWL

1.8 ~ 2.5

0.7 ~ 0.8

Standard foil (GTS-STD)

7.0 ~ 10.5

0.7 ~ 0.8

F2-WS

1.9 ~ 2.8

0.25 ~ 0.3

using the anchor effect.
For the FWL foil, the matte surface treatment is differently configured from that of F2-WS, and despite the factthat its surface Rz is of the same order as F2-WS, it has aconfiguration with high peel strength. Table 2 shows thesecharacteristics.
Figure 3 shows the relationship between frequency andtransmission loss. From this it can be seen that FWL is acopper foil that combines high peel strength with outstanding high-frequency characteristics.

4.

Features: A superthin (3~5 μm) copper foil that is peelable even after heating to 250~300°C, and can be usedeven with resins such as polyimide that are subjected tohigh-temperature processing.
We market F-WS foil as a copper foil for fine patterning
applications, but 9 μm is its limit in terms of foil thickness,anything thinner being difficult for customers to handle.
Given the objective of fine patterning, those with respect
to requirements for which there is a limit with the subtractive method, a semi-additive method is adopted, whereby
superthin foil is laminated onto a resin board, and onto
this is applied a process comprising resist → plating →resist peeling → flash etching.
As a copper foil for this sort of application, we market
F-DP foil, made by a process in which a peeling layer isprovided on a carrier foil, onto this is formed a superthin
foil of 3 to 9 μm, and, after lamination onto the resinboard, the carrier foil is peeled off.
Since F-DP foil assumes laminating onto an epoxy
resin board, peeling of the carrier foil and the superthin
foil becomes difficult at laminating temperatures in excessof 220°C. We have therefore augmented our product lineup with high-temperature peelable U-HP and FWL-HP
foils as superthin foils with carrier that can stand up torecent high-Tg package boards or polyimide boards thatare cast or laminated at 300°C or more. Table 3 showstheir characteristics.

5.

FR-WS Copper Foil with Resistive Layer

Features: Has a 50~100Ω/□ characteristic capable ofreplacing resistance chip components, holding promisefor reducing the size and improving the functionality ofcircuit boards.
A copper foil with resistive layer is a one with an electri-0Transmission loss (dB/100 mm)

U-HP and FWL-HP High Heat-ResistantSuperthin Peelable Copper Foils-1-2-3-4Table 3

LCP/LP copper foil

-5

LCP/ standard copper foil
PI/LP copper foil

-6

PI/ standard copper foil

-70510Results for transmission loss, without coverlay.
LP copper foil: FWL foil
(By courtesy of Japan GORE-TEX INC.)

Surface roughness ofcarrier foil ( μm)

Peel strength of carrier whenheated to 330°C (kN/m)

F-DP

1.2~2.0

Can not be peeled off

U-HP
FWL-HP

1.2 or less〃0.02~0.03〃

15Frequency (GHz)

Figure 3

Characteristics of superthin peelable copper foilswith carrier.

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Copper Foils for the Printed Circuit Boards of the Future

LengthWidth

Insulating resin
portionResistive portion

Conductor portion

Base WS foil
Roughened layer
Resistive layer

Figure 4

FR-WS copper foil with resistive layer.

cally resistive layer formed on the foil (see Figure 4). This
makes it unnecessary to have an external resistor as isused with conventional boards.
The resistive circuit is formed by etching the copperand electrically resistive layer, followed by selective etching of the copper.
Since our copper foil with resistive layer uses F-WS foil
as the base foil, it has outstanding properties not found in
the foils of other manufacturers that make possible finepatterning and, with respect to flex resistance, make possible its use in FPC applications.
At present we have completed the development of copper foils with sheet resistance of 25Ω/□ and 50Ω/□, andhave begun marketing them.
Concept of sheet resistance:• Area of resistive material = sheet resistance of resistivematerial;• Resistance value = sheet resistance x (length/width)• Sheet resistance = resistance value / (length/width)
For more information, please contact:
Sales Dept., Furukawa Circuit Foil		 TEL: +81-288-22-3496 FAX: +81-288-22-3479

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