Multilayer PCB Prototype Material Composition
The last of those layers, substrate, is made of fiberglass and is also known as FR4, with the FR letters standing for "fire retardant." This substrate layer provides a solid foundation for PCBs, though the thickness can vary according to the uses of a given board.
A cheaper range of boards also exist on the market that don't utilize the same aforementioned multilayer PCB prototype materials, but instead consist of phenolics or epoxies. Due to the thermal sensitivity of these boards, they tend to lose their lamination easily. These cheaper boards are often easy to identify by the smell they give off when being soldered.
PCB second layer is copper, which is laminated onto the substrate with a mixture of heat and adhesive. The copper layer is thin, and on some boards, there are two such layers - one above and one below the substrate. PCBs prototype with only one layer of copper tend to be used for cheaper electronics devices.
The massively-used copper clad laminate (CCL) can be classified into different categories according to different classification standards including reinforcing material, used resin adhesive, flammability, CCL performance. The brief classification of CCL is shown in the following table.
|Paper base class||PF resin (XPC, FR1, FR2)|
Epoxy resin (FE-3)
|Glass fiber cloth base class||Epoxy resin (FR4, FR5)|
|Composite epoxy material (CEM)||/|
|Lamination multilayer base class||/|
|Special material base class||BT, PI, PPO, MS|
|Flammability||Flame-proof type||UL94-VO, UL94-V1|
|CCL with ordinary performance||/|
|CCL with low dielectric constant||/|
|CCL with high heat resistance||/|
|CCL with low coefficient of thermal expansion||/|
Above the green soldermask is the silkscreen layer, which adds letters and numerical indicators that make a PCB readable to tech programmers. This, in turn, makes it easier for electronics assemblers to place each PCB in the proper place and in the right direction on each component. The silkscreen layer is usually white, though colors such as red, yellow, gray and black are also sometimes used.