What materials are PCB circuit boards mainly assembled from?

A white screen layer is applied on top of the solder mask layer. Screen printing adds letters, numbers and symbols to the PCB board for easy assembly and to indicate a better understanding of the board by humans. We often use screen labels to indicate the function of each pin or LED.
   
PCBs have alternating layers of different materials that are laminated together with heat and adhesive so that the result is a single object.   

FR4 substrate   

The substrate or substrate is usually fiberglass. Historically, the most common sign for this fiberglass was "FR4". This sturdy core gives the PCB board rigidity and thickness. There are also flexible PCB boards built on flexible high-temperature plastic.   
Cheaper PCB boards and perforated boards (shown above) will be made using other materials, such as epoxy or phenolic resins, which lack the durability of FR4 but are much cheaper. When you solder it, you'll know you're using this type of PCB board - they have a very unpleasant, unpleasant odor. These types of substrates are also commonly found in low-end consumer electronics. Phenolic resins have a lower thermal decomposition temperature, which can cause them to layer, smoke, and burn when soldering irons remain on the board for too long.   

copper   

The next layer is a thin copper foil, laminated to the board with heat and adhesive. On a normal double-sided PCB board, copper is applied to both sides of the substrate. In lower cost electronics, a PCB board may have copper on only one side. When we refer to a double-sided or double-layered circuit board, we are referring to the number of copper layers. This can be 1 layer or as many as 16 layers or more.  
PCB board exposed copper, no solder mask or screen printing.   
Copper thickness can vary and is measured by weight in ounces per square foot. The vast majority of PCBs have 1 ounce of copper per square foot, but some PCB boards that handle very high power may use 2 or 3 ounces of copper. Each square ounce converts to about 35 microns or 1.4 thousandths of an inch of copper.   

solder mask 

The layer on top of the copper foil is called the solder mask. This layer gives the PCB a green color (or, in SparkFun, red). It overlies the copper layer, insulating the copper traces from accidental contact with other metals, solder, or conductive sites. This layer helps the user solder to the correct location and prevents solder jumpers.   

silk screen   

A white screen layer is applied on top of the solder mask layer. Screen printing adds letters, numbers and symbols to the PCB board for easy assembly and to indicate a better understanding of the board by humans. We often use screen labels to indicate the function of each pin or LED.   
Screen printing is most commonly white, but any ink color can be used. Black, gray, red, and even yellow screen printing colors are widely available; however, it is not common to see multiple colors on a single board.
 
Many circuit boards are expensive in terms of craftsmanship, as well as material selection. Kunshan Shixue Electronic Circuit Board - Expert in the production of multi-layer PCBs, PCB circuit boards, aluminum substrates and high-conductivity aluminum substrates.