PCB solder resist layer and help the difference between welding layer

In the PCB production process, what is the solder resist? What is a run-off? And what is the difference between the two? In general, the compound layer is mainly to prevent circuit board copper foil directly exposed in the air, have the effect of protection, and help stencil do welding layer is used for, in the tin can accurately put solder paste in need on the welding of the SMT solder. PCB solder resist layer and run-off resistance welding plate is the difference between resistance welding layer, is refers to the PCB board to a part of the green oil. Actually this resistance welding layer using the negative output, so in the form of the resistance welding layer is mapped to the board, not on the green oil resistance welding, is showing the copper sheet. Usually in order to increase the thickness of copper, resistance welding layer on the line to the green oil, then add tin to achieve the effect of increasing the thickness of the copper wire. Help welding layer is for the use of machinery patch, corresponds to the SMT components of bonding pad, usually using a steel plate in SMT processing, corresponds to the components to PCB punching bonding pad, and then steel plate up and solder paste, PCB under steel, solder paste, leakage is just every welding plate with solder, so usually is not greater than the actual welding welding layer plate size. Dongguan circuit board factory, circuit board to help welding and resistance welding layers on the difference between two layers are tin solder, and not on a tin, a green oil, but: 1, help the welding layer used in SMT packaging. 2, by default, there is no resistance welding layer area should be green oil; 3, resistance welding layer is on the whole piece of resistance welding green oil window, the purpose is to allow the welding;