What Are the Features of Newly Developed FPCs?
We have developed FPCs with excellent heat resistance. This achievement is attributed to improvement of the adhesive material used in the FPCs.
FPCs consist of a conductor made of copper foil and copper plating, either in single-sided or double-sided construction, and insulator films made of polyimide film and adhesive, which isolate the above conductor (Figure). Conventional FPCs used for electronic devices under an operating environment of 150°C or lower employ an epoxy- or acrylic resin-based adhesive as cover lay film adhesive. However, the above adhesive compositions are vulnerable in high temperature environments exceeding 150°C; the constituent materials are subject to faster aging degradation, resulting in significant less adherence and insulating capacity.
Our newly developed FPCs employing heat-resistant polyimide in adhesive materials mitigate the performance deterioration of insulator films in high temperature environments and significantly improve the basic performance of FPCs, including heat resistance and durability especially in high wet heat conditions.
To confirm the advantages, we conducted durability tests in high temperature environments*1 under (1) high temperature storage at 150°C ambient air temperature, (2) wet heat storage (high temperature and high humidity), and (3) immersion storage in oil at 150°C. Through these evaluations, we confirmed that the adhesive strength of the cover lay film satisfied the requirements for FPCs (JPCA standard, 3.4 N/cm or more), both in the initial conditions and after 3,000 hours of storage.
*1:Durability test in high temperature environments
In the automotive cable standard (ISO 6722), the heat resistance standard varies from Class A to Class H. The temperature resistance to 150°C is equivalent to that of Class D. The procedure for the long-term heat aging test is specified as “Test pieces shall be left in a rated temperature environment for 3,000 hours, and shall be checked for damage in the insulation layer in a coiling test.” No such specific heat resistance standards have been established for FPCs. So, as a reference, we conducted the durability tests in accordance with Class D (as described above) at 150°C × 3,000 hours.