What is Dry Film Solder Mask (DFSM)
Dry Film Solder Mask (DFSM) is a permanent material utilized in the PCB industry to provide circuit protection on the outer layer of various types of printed circuit boards, including flexible, rigid-flex, and IC substrates. It is a water-based or solvent-based material that is typically supplied in rolls with thicknesses ranging from 75μm to 100μm.
DFSM safeguards the printed circuit board from oxidation and prevents the formation of solder bridges between closely spaced solder pads. Additionally, it offers electrical insulation, enabling the placement of higher voltage traces in close proximity to one another.
DFSM is applied using a hot roll lamination process during PCB manufacturing. This involves using a laminator machine to apply the dry film mask to the board surface under controlled heat and pressure. The pressure is carefully adjusted to ensure proper distribution of the mask without any air inclusion between the traces.
After lamination, the DFSM undergoes an exposure process, which can be carried out using non-UV or UV exposure methods, depending on the specific type of dry film mask used. The film is then cooled to room temperature before exposure. The amount of exposure energy received by the film affects its resolution, adhesion, and level of ionic contamination. Following exposure, a recommended hold time of 15 to 30 minutes is advised before proceeding to the development stage.
Development of the unexposed areas of DFSM is accomplished using a mild alkaline solution in a conveyorized spray developing unit. This is followed by thorough rinsing with mains water and DI water, and finally, a turbine dry.
The final step in the process is the final cure, which involves a two-step process of high-intensity UV exposure followed by thermal curing. This final cure is crucial in achieving optimal physical, chemical, electrical, environmental, and end-user assembly soldering performance properties of the DFSM.
DFSM differs from liquid photoimageable (LPI) solder mask in that it is a dry material and does not flow into through holes in the boards, preventing unqualified copper plating. However, it is worth mentioning that DFSM is generally more expensive than LPI solder mask.