1/ 3

October 7, 2013

Sumitomo Electric Develops FPCs with Excellent Heat Resistance
Ensuring long-term reliability at 150°C by using a new adhesive material
Sumitomo Electric Industries, Ltd. has developed flexible printed circuits (FPCs) with significantly
improved long-term reliability in the environment of 150°C or higher (up to 175°C) and started shipmentof sample products. This achievement is attributed to the enhancement in insulator film adhesion andinter-circuit insulation performance by improving the adhesive material contained in the FPCs.
FPCs are primarily used in electronic devices including mobile devices. In recent years, FPCs have been widelyused even in LED lamps and in-vehicle products due to its thinness, flexibility and component-mountablefine-pitch circuits. While the service temperature of FPCs for electronic device applications is generally 100°Cor less, those used in lighting and in-vehicle applications are required to have high heat resistance of 100°C or

more in many cases.
In the case of conventional FPCs, constituent materials are subject toaging degradation especially if used in a high temperature environment at 150°C or more, resulting insignificant loss of adhesiveness of insulator films. Sumitomo Electric has succeeded in significantly enhancing
the basic performance in a high temperature environment (durability in a high-temperature andhigh-humidity environment) by improving heat resistance of the adhesive (one of constituent materials).
The

newly

developed

FPCs

areapplicable to powertrains*1 in vehicles
and LED lighting equipment relay
cablesreliability(whereishigh-temperaturerequired),andcomponent-mounting circuit boards.
Sumitomo Electric will promote sales
in these areas to help customers
enhance design flexibility by using theFigure1: Cross section of heat-resistant FPCs
(Single-sided / double sided FPCs)

FPCs as wiring materials, achieve
reduction in size and weight of electronic components, and increase reliability and durability.

【Outline of FPCs】
Printed circuit boards (FPCs) are flexible and foldable, and allow mounting of components. Notably, FPCs
enable three-dimensional wiring in small spaces. Thus, FPCs have been increasingly used in small electronic

2/ 3

devices such as mobile terminals, HDDs, DVD drives, and game consoles.
FPCs are available mainly in two different structures: single-sided FPCs (having single-layer conductors) anddouble-sided FPCs (having double-layer conductors) (Fig. 1).
In FPCs, insulator films (base film: polyimide film, cover lay film: polyimide film and adhesive) are used toelectrically insulate conductors (copper foil and copper plating). The newly developed FPCs are characterized
by enhanced long-term reliability in a high-temperature environment by improving the cover-lay filmadhesive.

【Features of the new FPCs】In electronic devices, the operating environment isless than 150°C, and thus, conventional FPCs use

Table 1: Adhesive strength in a high temperature
exposure test (150°C)

adhesive materials made from epoxy and acrylic
resins as cover lay film adhesives. However, when

Test condition

used in an environment at 150°C or more, theadhesive materials deteriorate, resulting insignificant loss of adhesiveness and insulation
performance (Table 1).

Conventional
product

Newlydeveloped
product

150°C × 3,000

0.1 N/cm> 5 N/cmhours

（NG）

（OK）

(JPCA standard: cover lay film adhesive strength
measuring 3.4 N/cm or more)

Containing new heat-resistant polymers, the newly developed adhesive material helps minimize
deterioration in a high temperature environment, significantly improves the basic performance of FPCs,
and mitigates the performance deterioration of insulator films.
We conducted durability tests in high temperature environments*2 to perform evaluations on (i) hightemperature storage at 150°C in the atmosphere (Table 1), (ii) wet heat storage at 85°C/85%RH*3 (hightemperature and high humidity), and (iii) immersion storage in oil at 150°C. We confirmed that, in all ofthese evaluations (in the initial conditions and after 3,000-hour storage), the cover lay film adhesivestrength (JPCA standard, 3.4 N/cm or more) required of FPCs is guaranteed.
*1: A system that transmits driving force from a power source, such as an engine, to terminal parts through gears andshafts.
*2 : In the automotive cable standard (ISO 6722), the heat resistance standard ranges from Class A to Class H. The
temperature resistance to 150°C is equivalent to that of Class D. As regards a long-term heat aging test, there is thefollowing description: “Test pieces shall be left in a rated temperature environment for 3,000 hours, and shall bechecked for damage in the insulation layer in a coiling test.” No such heat resistance standards have been establishedfor FPCs. As a reference, durability tests are conducted in accordance with Class D (as described above) at 150°C ×3/ 3

3,000 hours.
*3: Relative humidity (HR) is the ratio of the amount of water vapor in an air-water mixture to the saturated vapor
density at a prescribed temperature.

【Outline of Sumitomo Electric Printed Circuits, Co., Ltd.】Business
Development and manufacture of FPCs for electronics products
Head Office
30 Hinokigaoka, Minakuchi-cho, Koka, Shiga, Japan
Representative
Junichiro Nishikawa (President and Representative Director)
Established
April 2000
Capital
1.5 billion yen (100% owned by Sumitomo Electric)