Most photomasks are called a liquid photoimiageable mask and are designed as an artwork image like all the trace layers using the PCB design software.
The mask is tough, it withstands soldering without damage and when placed over metal traces keeps any solder from sticking to those places.So to get solder in the right places, openings are placed in the soldermask artwork where no soldermask is applied. Typically these are solder pads for thru-hole components, solder pads for surface mount components, mechanical mounting pads where a ground connection has to be made, contacts for mounted contacts that directly contact board etch pads for continuity, etc. Solder mask is left between pads and over all the exposed traces to prevent solder shorts from solder bridges and splashes. Usually the soldermask opening will be about 3 mils (.003″) larger than the solder pad that is exposed.They also provide protection against shorts in operation by metallic junk.

The students who have removed the computer know that the memory stick has a gold finger, as shown below:


The golden finger here is to opening, plug and play.There is also a very common function of opening the opening, which is to increase the thickness of the copper foil in the later period, which is convenient for excessive current, which is more common in the power board and the motor control board.

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