1. A flexible circuit board comprising:a substrate;
a first signal line layer formed on a first side of the substrate;
a second signal line layer formed on a second side of the substrate opposite to the first side; and
two insulating layers, a first one of the insulating layers formed over the first signal line layer, and a second one of the insulating layers formed over the second signal line layer;wherein:the substrate comprises a base layer and a bonding layer formed on each of opposite sides of the base layer;
each of the two bonding layers is formed by coating a bonding solution and drying the bonding solution;
the bonding solution is composed of an adhesive and a solvent;
a viscosity of the adhesive is 5000 millipascal-seconds; and
the adhesive is composed of a precursor having a mass fraction of 35%-65%, carboxyl modified polyphenylene oxide having a mass fraction of 10%-15%, bisphenol-F-epoxy resin having a mass fraction of 10%-15%, a silane coupling agent having a mass fraction of 0%-1.5%, silica filler having a mass fraction of 5%-20%, and a flame retardant filler having a mass fraction of 10%-20%.