Solder mask, as the name suggests, is a “mask” that is used around Pads and Vias on a Printed Circuit board to aide in getting better solderability and reduce solder balling, short-circuiting and bridging of adjacent or nearby solder joints. The mask has openings where the Solder needs to be on the copper, while all other places the mask is closed to prevent any solder from accumulating and forming balls or bridges.
A special paint (Solder Resist) is used through this Solder Mask, between two solder joints / pads / vias to not let solder bridge over and form joints where it’s not needed. Solder Masks are generally a “negative” output film, meaning that it usually covers the whole board, and only the openings (Vias, pads, thermal vias, special areas, etc.) that need solder, are exposed.
Application of Solder Mask
The Solder mask is a negative film, that is used to apply Solder Resist paint on the Printed Circuit Board. The Solder Resist paint comes in quite a few colors (Green, Black, Red, Yellow, Blue, etc.) with Green being the most common (and sometimes cheaper) option.
When the PCB goes through the Reflow Oven process of soldering, and the molten Solder wave passes over the PCB and the component legs, there is a very high chance of Solder bridging and joining. The Solder-Resist paint (applied through the Solder Mask) prevents this from happening.
Solder Masks help control soldering defects and Soldering machine / process related issues. The air-gaps and spacings need to be maintained or minimized around the features (Vias, Pads, etc.) to ensure good solderability and minimum defect.
While some engineers don’t use a Solder-mask -OR- generally make a big opening in the solder mask for components like QFP packages, this generally results in solder bridging between few adjacent pins of the package, which is sometimes very hard to find and see by naked eye, and thus resulting in a non-working product.
Components like BGA or QFN specially on a High density Printed Circuit Board, require carefully designed Solder masks with attention to detail on covering all the features between Pads / Vias to reduce soldering defects.
Solder mask thickness or Solder resist thickness also plays a crucial role here. If the thickness is beyond 0.04mm, the Solder-past application becomes tricky. Such boards require low profile Photo-sensitive Solder Masks.
Solder Mask Types
Solder masks are generally available as both Liquid wet photo-imageable coatings as well as Dry film-based transfer coating. The Dry film-based Solder masks and resists generally range in thickness form 0.07mm – 0.1mm. However, due to less companies making these now a days, and more research going into the wet process giving better and more precise feature sizes and gaps has made the wet process a more suitable choice.
Also there is wastage in Dry Film-based masks while in Wet process, no matter which size of the PCB, it can be easily coated with the Photo-sensitive solder resist liquid and processed with exposing under the mask film and then developing. Low profile Liquid Photo-sensitive Solder masks are well suited for most Surface Mount applications, giving precise feature sizes and gaps under acceptable tolerances.