Special Issue

Challenges to New Technologies and New Products〉〉New Products

Development of Thin Copper Foil DF-TSH
for Fine Pattern Circuit Boards
because common copper foils have such characteristicthat their strength gets lower after heating, thin copper
foil circuit boards have the defect of reduced rigidity after
heating in the layering process.
On the other hand, MSAP method has been alreadyspread for forming fine pattern circuits of L&S = 30/30 µm
or less, in which circuit patterns are formed by copper
plating by using thin copper foils with carrier film as aseed layer. The MSAP method has been mainly used for apackage board on which semiconductors are mounted.
However, recently, it has been adopted for a main board
as well along with higher density packaged mobile devices with enhanced performance and its application range
is spreading. However, the MSAP method had a problemof high cost although it exceled in forming fine patterncircuits. And when applying copper foils with carrier film,
extra time is needed for the peeling process when producing a circuit board.
We have successfully developed the copper foils whichenables further fine pattern circuits by the conventional
Subtractive method.

INTRODUCTION
1.1. INTRODUCTION
Recently, the width of circuit patterns and the space
between them have been getting thinner and thinneralong with increasing the density of circuits on printed circuit boards of mobile devices as a typical example, andthe demand for the fine pattern circuit board has alreadystarted to grow. In general, the circuit of fine pattern circuit board is formed with copper foils or copper plating.
Copper foils are roughly classified into electrodeposited
copper foils and rolled copper foils. In the application offine pattern circuit boards, the electrodeposited copper
foil is mainly applied, which can be used even for a large
size circuit board because it is excellent in productivity.
Built on the electrodeposited copper foil (glossy on bothside) technology, we have been producing the surface
processed copper foil - known as F-WS - for the printed
circuit board applications. This technology has an addedfunctionality offered by its surface treatment. We havereceived high marks from our customers on the F-WS.
It is effective to make the copper foils thinner against
finer circuit patterns for a circuit board built with copper
foils. In the Subtractive method, which has been used forprocessing a circuit board, 12 µm thick copper foils weremainly used in circuit forming process, in which resist
patterns were processed by photo-lithography andunnecessary copper was removed by etching. This is a
simple process to enable the low-cost production of circuit boards. In this process, L&S, which indicates thewidth of circuit pattern and the space between circuit
patterns, can be as small as 35/35 µm in production. For
producing further fine circuit pattern, copper foils with 9µm or less in thickness are necessary to be applied. Inthis case, because of the lower strength of copper foils,
problem occurs such as the copper foils may get wrinkled and curled making the handling very difficult intransfer process or layering process in which a sheet ofcopper foil is laid on a resin base board. In addition,

Table 1FEATURES
2.2. FEATURES
DF-TSH, our thin copper foils for fine pattern circuitboards is the thin copper foils with 6 µm in thickness,
which can be handled without any copper carrier film andis realized by our original electrolytic structure controltechnology to provide not only a high tensile strength but
also a high elongation. Typical mechanical properties andphysical properties of DF-TSH are shown in Table 1 incomparison with other copper foils.
(1) High tensile strength
The tensile strength of DF-TSH is approximately 1.5 times
higher than that of F2-WS, our present copper foil, however, its higher elongation which usually is the tradeoff fora higher tensile strength is as high as 3% which is a highvalue in comparison with other high tensile copper foils.

Mechanical properties and physical properties of DF-TSH and other copper foil.
Items for Evaluation

Thin Copper Foil for Fine Pattern Circuit
Boards, DF-TSH

Our Present Copper Foil,
F2-WS

Electrodeposited Copper
Foil, AThickness /µm

612 (for reference)

1212Tensile strength /MPa490490

320370Elongation /%3686Roughness /µm

2.0

2.0

2.0

3.0

Furukawa Electric Review, No. 50 201932

Development of Thin Copper Foil DF-TSH for Fine Pattern Circuit Boards

With these excellent properties of DF-TSH, the generationof wrinkles can be controlled even in either the transfer
process or the layering process of fine pattern circuitboards and can handle the thin copper foils with thickness as thin as 6 µm in the same way as usual.
(2) Excellent processing for fine pattern circuits by
conventional process
DF-TSH is excellent in etching characteristic to form finepattern circuits because of its thin thickness of 6 µm.
Figure 1 shows effects of copper foil thickness for a pattern trailing length D in the pattern circuits formed by theconventional process of the Subtractive method. In general, the cross section of pattern circuits formed by theSubtractive method has a trapezoidal shape as shown in

the Figure and the pitch of pattern circuits can be narrower when the trailing length is shorter. The trailing length ofa foil thickness 6 µm is reduced by 40% in comparison
with that of a foil thickness 12 µm at L&S = 30/30 µm.
Furthermore, the pattern width dispersion is improved tobe 50% or less as shown in Figure 2 We have alreadyconfirmed in the actual manufacturing process of finepattern circuit boards that the L&S = 30/30 µm could beformed by using DF-TSH of foil thickness 6 µm by theSubtractive method. With this process, compared withthe conventional MSAP method, the cost of the patterncircuit forming process can be drastically reduced.
Further, the extra time to peel the copper carrier film canbe saved.

7DF-TSH 6 µm

6DF-TSH 12 µm

D/µm

543D210102030L&S/µm

4050（a）

50 µm（b）

50 µm

Dispersion of width of circuit pattern, 3σ/µm

Figure 1 Effect of the copper foil thickness for pattern trailing length D by the Subtractive process.

87（c）

6543210DF-TSH6 µmDF-TSH12 µm

Figure 2 Fine pattern formation on L&S = 30/30 µm by the Subtractive process.
(a) DF-TSH 6 µm(b) DF-TSH 12 µm(c) Effect of copper foil thickness for a pattern width dispersion by the Subtractive process.

Furukawa Electric Review, No. 50 201933

Development of Thin Copper Foil DF-TSH for Fine Pattern Circuit Boards(3) High thermal stabilityFigure 3 shows the heat softening curves of 1-hour heating under various heating temperature. In general, in thelayering process of pattern circuit boards, the hot pressing process is carried out under 170ºC to 220ºC. Heatand pressure of the press melt resin and thermoset tostick the copper foil onto the resin base tightly. DF-TSH
keeps its tensile strength even after heating at 200ºC atapproximately 1.5 times higher than that of our present
copper foils. And, it kept its tensile strength even after
heating at 200ºC at approximately 1.8 times higher thanthat of the electrodeposited copper foil B whose tensile
strength before heating is equivalent to that of theDF-TSH. Because of this high thermal stability, DF-TSH
keeps its high tensile strength to control the defectcaused by lowered rigidity of pattern circuit boards evenafter being exposed to the heat in the layering process ofpattern circuit boards.

600

CONCLUSION
3.3. CONCLUSION
DF-TSH, a copper foil for fine pattern circuit boards, has
higher tensile strength in comparison to conventional
electrodeposited copper foils as its featured property.
With this property, the generation of wrinkles can be controlled even when either transferring or layering in themanufacturing process of pattern circuit boards and canhandle the thin copper foil of thickness as thin as 6 µm
without a carrier copper film in the same way as usual.
Furthermore, DF-TSH is excellent in forming fine patterncircuits because of thickness as thin as 6 µm and canform the pattern circuits of L&S = 30/30 µm by the conventional process of Subtractive method. Now, we have
DF-TSH of 6 µm in thickness in addition to that of 9 µm inthickness in our line-up and package them either in a rollor in a sheet for shipping.
We offer the optimum material constructed for customer’s requirement specifications based on our manufacturing technology acquired over many years of experiencesuch as with or without anti-corrosion processing andsurface roughening processing.

Tensile strength, Ts/MPa

500
400300200
10000DF-TSH
F2-WS
Electrodeposited copper foil B
100200300400
Heating temperature (1 hour), T/℃

For more information, please contact
Second Sec. Copper Foil Marketing Dept.
Copper Foil Div.
TEL: +81-3-3286-3725 FAX: +81-3-3286-3289
Contact Form
https://www.furukawa.co.jp/srm/form/index.php?id=copper

500

Figure 3 Thermal stability of DF-TSH compared with otherelectrodeposited copper foils.

(4) Supply of fine copper foils in the form of sheets
Improving the tensile strength of our copper foils, wehave realized the supply of 6 µm thick copper foil in theform of sheets, which was very difficult in the past. Thishigh tensile copper foil can be used in the conventional
manufacturing process of pattern circuit boards without
any changes.

Furukawa Electric Review, No. 50 201934