Product Structure Of Flexible Copper Clad Foil Substrate

The flexible copper clad laminate (flexible CCL) substrate is a composite material composed of metal conductor materials and dielectric substrates. It is strip-shaped and can be wound into an axial shape without breaking in the metal conductor or the dielectric substrate.

Like the dielectric substrate material of rigid copper clad laminate, flexible dielectric substrate also plays a mechanical role in supporting electronic circuits and has a high dielectric insulation function. In addition to good mechanical and dielectric properties, flexible dielectric substrates must also have excellent flexibility, dimensional stability and heat resistance. Flexible CCL media substrates include film type, fiber paper type, glass cloth reinforced type and so on. In terms of film types, there are polyester (PET), polyimide (PI), polyesterimide, polytetrafluoroethylene (PTFE), polybutylene phthalate, liquid crystal polymer (LCP), olefin polymer (such as BCB) and other films. At present, the flexible film substrate for packaging is mainly polyimide film. The thickness of the film used is generally 12.5~125um. And a 25 um thick film is suitable in view of production cost. This thickness of film occupies a large part of the whole flexible CCL market. Considering the requirements for high dimensional stability and mechanical strength, some films with thickness of 50um, 75um and 125um are used.

The types of metal conductor materials used in flexible CCL include common electrolytic copper foil (ED), high ductility electrolytic copper foil (EDHD), rolled copper foil (RA), aluminum foil and copper Beryllium alloy foil, etc. Electrolytic copper foil or rolled copper foil with high ductility is commonly used as the flexible base material for packaging. In flexible CCL, the requirement for the flexibility of copper foil is higher than that of compressive CCL. Rolled copper foil generally has higher bending resistance and ductility than electrolytic copper foil. However, its volume is more expensive than ordinary electrolytic copper foil (taking 35um copper foil as an example, the price is twice higher). With the continuous development of electrolytic copper foil manufacturing technology, some new electrolytic copper foils (mostly low profile electrolytic copper foils) also have good flexibility. The extensibility of copper foil has also been greatly improved, even exceeding that of calendered copper foil. More importantly, this kind of electrolytic copper foil can be made very thin, which is more suitable for etching more fine circuit patterns in PCB manufacturing in recent years. Flexible CCL generally adopts 18um, 35um and 70um copper foil. The ultra-thin electrolytic copper foil of 12um and 9um has also been applied in flexible CCL manufacturing. Flexible CCL manufacturing technology has developed rapidly in recent years. In the manufacturing technology of adhesive free flexible CCL by electroplating, copper foil is no longer used as a metal conductor material, but is replaced by a copper conductor layer formed by direct electroless plating plating on a thin film substrate. It not only provides a new type of flexible substrate for tape borne packaging, but also opens up another technical way for the fabrication of packaging flexible substrate (PCB) (first forming through holes, then forming metal conductive layers).

In order to prevent the flexible circuit from being eroded by dust, moisture and chemicals, and reduce the impact of stress in the bending process, the flexible CCL substrate mostly adopts the surface coating structure. The traditional structure of the covering layer is to coat one side (non copper foil side) of the flexible dielectric film with an adhesive film, and then cover it with a removable protective film. This protective film is usually torn off only when the covering layer is aligned with the etched circuit. With the development of modern manufacturing technology, liquid photosensitive coating has appeared and been applied abroad. It has the advantages of reducing cost and simplifying process. It uses standard UV exposure and water-soluble developer for development. And then heated for post curing, eliminating the traditional lamination process. And improves the heat dissipation and flexibility of the flexible board. Through the mask process, a high quality photoinduced through-hole is made.