PCB Solder Mask
The printed circuit board uses copper as the main conductive path. Since copper has the high probability of reacting with the atmosphere and producing oxides, the conductivity reduces and the PCB performance will be destroyed. Also during the soldering process, the liquid solder may flow and form a solder bridge commonly referred to as two closed traces and lead to short circuit interconnection. To protect the copper from oxidation and to avoid soldering bridges, a thin coating is applied to the traces commonly referred to as solder mask.
There are few types when choosing solder mask. Depending upon the need and cost, appropriate solder mask is selected.
Epoxy is a basic and cheap solder mask which is mainly used in low precision and prototyping PCBs. Since epoxy is semi solid in nature, it is usually applied through the silkscreen printing.
DFSMs are like dry film photo resists used in copper etching. Prepared PCBs are vacuumed pressed with the sheet of DFSM. The solder pad is eliminated by covering the corresponding area with an anti-UV developer. The settings were then exposed to UV light to cure the DFSM. After a few minutes of exposure, the solder mask is transferred from the sheet to the PCB. The photoresist is then removed with a solvent.
This is a liquid solder mask much like an ink which can be spray painted or immersed on a marked laminate. The solder cutouts are done by placing a temporary ink marks which is removed after the solder mask application. The applied PCB are exposed to UV light to dry out the ink. The dried PCBs are cleaned and re-inserted in to the exposure for a final cure.