Selection of Copper Foil for Flexible Circuit Board
As we all know, material selection is very important in the fabrication process of flexible circuit board (FPC). There are specific requirements in material thickness, solderability, melting point, conductivity, resistance welding and so on. Here we focus on the selection of copper foil.
I. Summary:
2. Electrolytic Copper and Calendering Copper Material Processing Technology:
Electrolytic copper foil is formed by precipitation of acidic copper plating solution on bright stainless steel rolls, forming a uniform copper film, which is stripped and rolled continuously. Calendered copper foil is formed by repeated calendering and annealing of copper ingots or blocks with a certain thickness (20cm).
3. Microstructure of copper foil materials:
Due to the different processing technology, the atomic structure of copper in calendering material is irregular layered and strong crystal when the cross section of material is observed under 1000-fold microscope.
For recrystallization after heat treatment, it is not easy to form cracks, and copper foil material has better bending performance; while electrolytic copper foil material shows columnar crystalline structure in thickness direction, which is prone to crack and fracture when bending.
Similarly, although columnar crystals are dominant in the section observation of high-ductility electrolytic copper foil after heat treatment and other special processing, layered crystals are formed in the copper layer and are not easy to break when bending.
4. Bending properties of copper foil materials:
Most flexible circuit board products require higher bending performance, which leads to the preference of calendering materials in most manufacturers. In fact, there are many blind selection factors. As mentioned above, calendering materials have their own characteristics. At the same time, they also have many shortcomings, which should be applied properly. The following data are the bending test results of various copper foil materials under the same conditions.
The bending property of calendered copper foil is four times that of common electrolytic copper foil, but its price is also more expensive. Therefore, for products with low flexibility requirements (such as keyboard, module board, 3D static flexible circuit, etc.), we can choose to use high-calendering electrolytic copper foil instead of calendering copper foil material. Of course, in the case of high reliability requirements (such as sliding cover mobile phone plate, folding mobile phone plate, etc.), it is better to use calendering copper foil material.
5. Development Trend of Copper Foil Materials:
With the increasing miniaturization of electronic consumer products and aerospace, the reliability requirements of military and civil high-tech products are becoming stricter and stricter, and the requirements for flexible circuit board processing and physical properties of materials are becoming higher and higher.
(1) High bending calendering copper foil, under the same conditions, bending is seven times as much as that of conventional calendering material.
(2) Electrolytic copper foil material is used for fine and ultra-fine pattern making. For the production of high bending fine conductors of COF products, the lines are more uniform after etching under the same conditions, and the residual copper is less. The process is to spray a thin layer of copper on the polyimide film by spraying method, and then electroplating the ultra-thin copper foil with a copper layer of about 9um.
(3) Calendered alloy copper, alloy copper foil material is mainly characterized by good electrical conductivity, close to pure copper, mechanical properties, thermal stability are better than conventional calendering materials.
