Given below is the process for application of solder mask layer onto a PCB:
Step 1: Board cleaning
The board is cleaned to remove dirt and other contaminants and then the board surface is dried.
Step 2: Solder mask ink coating
Next, the board is loaded into a vertical coater for solder mask ink coating. Coating thickness is decided by factors such as the reliability required for the PCB and the domain in which it will be used. The solder mask thickness will vary when it comes to being added on different sections of the circuit board such as the traces, the copper foil, or the substrate. This mask layer thickness will depend on the equipment capability and the PCB manufacturing capabilities.
Step 3: Pre-hardening
This phase varies from total hardening as pre-hardening looks toward making the coating relatively solid on the board. This facilitates the removal of the unwanted coating which in turn can be easily removed from the PCB in the developing stage.
Step 4: Imaging and hardening
Imaging is done using a photo film which is laser plotted to define the solder mask area. This film is aligned to the panel that is already solder ink coated and tack dried. During this imaging process, the film aligned to the panel is subjected to UV exposure. Upon receiving the UV light, the opaque area allows the UV light to transfer through the film, and thereby the ink below the opaque gets polymerized (hardened).
In the case of LDI imaging, the photo films are not required as the UV laser will directly harden the portions on the board which need to retain the solder mask ink.
Step 5: Developing
The circuit board is then dipped in the developer to clean away the unwanted solder mask to ensure the required copper foil is accurately exposed.
Step 6: Final hardening and cleaning
Final hardening is necessary to make the solder mask ink available when mounted on the PCB surface. Next, the boards that have been covered with solder mask need to be cleaned before further processes such as applying the surface finish.