Especially in my early years working in the electronics sector, I used to often wonder why the top layer of a PCB was green when inspecting a circuit board.
Depending on who you question, the response will vary, but there is one point on which everyone can agree: wearing a welding helmet facilitates inspection, protects drivers, and lessens eye strain during manual assembly.
PCB solder masks come in a variety of varieties, each with a unique composition, application method, and pricing.
The capabilities of the manufacturer and the inspection/assembly process must be taken into consideration when choosing the appropriate type and thickness of solder mask for a board. The four categories of solder masks for pcbs are as follows:
Mask for liquid epoxy solder
Top and bottom layer masks, Liquid Photosensitive Solder Masks (LPSM), Dry Film Solder Masks (DFSM),
What does a PCB solder mask do?
A solder mask is used to shield metal components from oxidation and stop electrical bridges from forming between solder pads on a circuit board. Especially if solder bath or reflow processes are used, this phase is crucial in pcb solder mask construction.
Although the mask layer does not provide complete control, it does give some control over where the molten solder points to land on the board when using these techniques.
As I previously thought that a solder mask was an entire layer of solder placed on the board, the term “solder mask” better captures the idea, in my opinion.
Various PCB solder mask types
A polymer layer are types of pcb vias which is put on the metal rails of a printed circuit board to provide the basis of each solder mask layer. The appropriate solder mask for a PCB will depend on the board’s use and budget. Solder masks come in a variety of forms.
The most straightforward method is to screen-print liquid epoxy onto the wires. It is comparable to using an airbrush with a template or stencil to paint. Almost any color can be used to apply the solder mask.
When it comes to solder masks, screen printing a liquid epoxy onto the Board is the most straightforward approach. This is the least expensive and most widely used choice. A
mesh substrate is employed in this procedure to enable the usage of ink-blocking patterns. The liquid epoxy is combined with the solder mask stain to cure the desired color.
Solder Mask Liquid Photosensitive (LPSM)
A dry film or liquid mask photolithography procedure, similar to that used for photoresist exposure in semiconductor manufacturing, is utilized with more complex solder masks.
LPSM masks can be sprayed onto the surface, which is frequently a less expensive application method, or they can be screen printed like epoxy.
The most sophisticated (and accurate) way is to define the mounting holes, pads, and vias using lithography.
Using the Gerber files, a photolithography mask that corresponds to the intended solder mask is produced in this method.
After that, the panelized board is meticulously cleaned to remove any dust that might have gotten inside the tough solder mask. The liquid LPSM thoroughly coats both sides of the panels.
One thing you’ll note about the LPSM method is that the photolithography mask’s black portions designate the locations where you want to expose the leads, while the solder mask’s clear portions define the areas of the board you want to cover.
Following the LPSM application, the plates are dried in an oven before going through a UV developer. After precisely aligning the photolithography mask on the dry plate, ultraviolet light is used to illuminate the plate.
The exposed regions of the LPSM material are cured using ultraviolet radiation, while the unexposed areas are rinsed with a solvent, leaving a firm layer of solder mask.
Solder Mask with Dry Film (DFSM)
DFSM solder mask is applied using a procedure similar to that of LPSM. In a process similar to photolithography, both types of PCB masks are exposed, but instead of a liquid, a dry film is placed in sheets using a vacuum lamination method.
The step of vacuum laminating the board forces the exposed solder mask sheet to cling to it and eliminates air bubbles. Following exposure, the solder mask’s unaffected regions are dissolved with a solvent, and the remaining layer is then thermally cured.
Masks for the top and bottom layers
Top and bottom solder masks are two varieties that are frequently referenced in various tutorials on PCB solder mask types.
Here, they merely refer to whether the solder mask is placed on the board’s top or bottom sides; no specific manufacturing method or material is mentioned.
Curing and surface finishing are the final steps.
The plates must be cleaned to remove dust before using the methods described above. They will next go through the final phases of hardening and curing. Without UV exposure, the liquid epoxy solder mask cures thermally. LPSM and DFSM films are cured by exposure to UV rays during the photolithography process. These films undergo heat treatment to cure and harden them after exposure.
Whatever solder masking technique is employed, the finished solder mask will still leave some copper on the board exposed. To stop rust, these exposed surfaces should be coated with a surface finish.
Hot air solder leveling (HASL) is the most typical surface finish, but electroless nickel gold plating (ENIG) and electroless nickel/palladium plating are also popular (ENEPIG). With the paste mask, additional holes may need to be made in the mask layer.
The paste mask is manufactured in a variety of methods based on the various manufacturing processes and is used to attach pads and other components to the printed circuit board.