In recent years, due to the miniaturization and thinness of portable electronic products, the mounting space inside the machine and the mounting area of the substrate are becoming smaller and smaller, and the demand for flexible circuit boards is becoming more and more urgent. The application of flexible circuit board in electronic products mainly includes the following aspects. The first is the LCD, plasma display and other planar display driver chip installation applications. Secondly, smart phones, computer host hard disk drive module, optical signal receiving module and other applications. And the application field of flexible circuit board is still expanding.
Flexible Copper Clad Laminate:
Flexible CCL is usually divided into two types that are “three layers structure substrate with epoxy resin as its adhesive” and “two layers structure substrate without the adhesive”. Among these, the heat resistance and dimensional stability of the substrate without adhesive are excellent due to no halogen in its structure, which means that it is an ideal structure for flexible CCL. However, since no adhesives are used, the adhesion of polyimide film (as an insulating layer) and copper foil can be problematic when making printed circuit boards.
Halogen free coating material:
The so-called coating material is a film forming material that is coated on the surface of the graph of flexible PCB to protect it, which is an indispensable basic material for flexible printed circuit board. In recent years, due to the requirements of environmental protection, a number of different varieties and types of halogen-free coating materials have been listed, but in terms of insulation reliability, peeling strength and other aspects are not as good as the original (that is, halogen as a flame retardant agent) coating materials. In view of the current situation of coating materials used in flexible printed circuit boards, researchers have determined the following research directions: first, the need for halogens; Secondly, as a coating material to ensure that it has enough insulation reliability and enough strength; Furthermore, as a coating material, it must be kept soft; In addition, its fluidity must be controlled to ensure the desired thickness on the circuit surface.
Technology of Halogen-Free:
As an important characteristic of the coating material, it is necessary to maintain sufficient softness while ensuring the flame retardancy. In order to ensure this softness, elastomer is usually introduced into the epoxy resin. However, doing so will have a more serious consequence, that is, the flame retardancy will become worse. Therefore, to solve this problem, researchers usually take the following measures.
(1) Phosphorous flame retardant is adopted. For example, the use of phosphate ester flame retardant, in the combustion of it can promote dehydration carbonization reaction, so that the surface of the material formed a layer of carbonization film, cut off the contact with oxygen, so that it is difficult to burn.
(2) Adopt nitrogen-containing flame retardant. Nitrogen atoms are introduced into the molecular structure of the resin to generate an inert gas to oxygen during combustion.
(3) Adopt inorganic flame retardant. For example, the addition of magnesium hydroxide, aluminum hydroxide and other inorganic filler, they release crystal water in the combustion, reduce the combustion temperature, thereby inhibiting the combustion.
(4) Resin with special structure is used. For example, epoxy resin containing phenyl alkyl is used in the resin combination.
In order to improve the adhesion of copper foil and polyimide film, the surface treatment technology of copper foil and the pressing and forming technology of CCL were developed, and the flexible CCL with good adhesion between copper foil and polyimide film of insulating layer was made. A new type of halogen-free line graphic protection coating material was developed on the premise of keeping the softness of the coating material and ensuring its flame retardancy. By using these two new coating materials, the flexible printed circuit board is completely halogen-free.