The number of layers of the board represents several independent wiring layers. Usually the number of layers is even, and contains two layers on the outermost side. Most of the motherboards are 4 to 8 layers, but it can be technically able to achieve nearly 100 layers of PCB boards. Most of the large supercomputers use quite many layers of motherboards, but because these computers can already replace the cluster of many ordinary computers, the super -multi -layer board has gradually been gradually not used. Because each layer in PCB is closely combined, it is generally not easy to see the actual number, but if you carefully observe the motherboard, you may see.

　　The automatic detection technology of the circuit board is applied with the introduction of surface installation technology, and the packaging density of the circuit board is rapidly increased. Therefore, the automatic detection of the circuit board is not only basic, but also economically even for the low -density and general number of circuit boards. In complex circuit board testing, two common methods are needle bed testing and dual probe or flying needle testing method.