Usually what we call a multi-layer PCB circuit board is formed by laminating a core board and a prepreg. base material. The prepreg constitutes the so-called wetting layer, which acts to bond the core board. Although it also has a certain initial thickness, its thickness will change during the pressing process.

The outermost layer of the multi-layer circuit board is the solder mask layer, which is what we often call "green oil". Of course, it can also be matte black or other colors. The thickness of the solder mask is generally not easy to determine accurately. The area without copper foil on the surface is slightly thicker than the area with copper foil, but due to the insufficient thickness of the copper foil, the copper foil is still relatively prominent. When we use it, you can feel it when your finger touches the surface of the printed board.

Usually the two outermost dielectric layers of the multilayer PCB circuit board are wetting layers, and a separate copper foil layer is used as the outer copper foil on the outside of the two layers. The original thickness specifications of the outer layer copper foil and inner layer copper foil are generally 0.5OZ, 1OZ, 2OZ (1OZ is about 35um or 1.4mil), but after a series of surface treatments, the final thickness of the outer layer copper foil will generally increase. About 1OZ. The inner layer copper foil is the copper cladding on both sides of the core board. Its final thickness is very small compared to the original thickness, but due to etching, it is generally reduced by a few microns.