As electronic equipment becomes further miniaturized and multifunctional, higher density has been requested also for the circuit boards used in the electronic equipment.
For making the density of the circuit boards higher, the circuit boards are generally multilayered. The multilayer circuit board is typically obtained by laminating an electrical insulating layer (A2) on an inner layer board consisting of an electrical insulating layer (A1) and a conductor circuit (B1) formed on the surface thereof, forming a conductor circuit (B2) on the electrical insulating layer (A2), and further alternately laminating a number of electrical insulating layers and conductor circuits as necessary.
Formation of multilayered high-density wiring causes generation of heat in the board itself and the electronic devices themselves during operation. Flame retardants are generally formulated in the electrical insulating layers to prevent the ignition due to the heat generation.
Further, used boards are often disposed of by incineration. To the electrical insulating layers, for example, materials in which halogen-based flame retardants are formulated as disclosed in Japanese Patent Laid-Open No. 2-255848 have been used, thereby imparting flame retardancy. The materials in which halogen-based flame retardants are formulated generate halogen-based hazardous substances on burning. As problems of global environmental damage and warming have been closed up, use of the halogen-based flame retardants has been restricted. Thus, there is a need for a material having flame retardancy that is comparable or superior to conventional materials without generating halogen-based hazardous substances.
As halogen-free flame retardants, phosphorus-based flame retardants such as a salt of basic nitrogen-containing compound with phosphoric acid are known. Generally, the salt of basic nitrogen-containing compounds and phosphorus are obtained by reacting the basic nitrogen-containing compounds with phosphoric acid in the presence of a condensation agent and then baking the product. The obtained salt are needle-like or fiber-like particles. However, such phosphorus-based flame retardants are generally poor in flame retardancy compared with halogen-based flame retardants and have difficulty in adequately enhancing the flame retardancy of the electrical insulating layers for multilayer circuit boards.
In addition, use of flame retardants in particulate form may cause variations in electrical properties (dielectric constant) of the multilayer circuit boards due to moisture absorption.