Missing top solder mask complaint when trying to order PCB? (PCBWay)
Soldermask, also called solder resist, is the coating that you see on most professional PCBs, most commonly in green. It is used to keep solder off of copper that you don't want solder on (such as the tracks themselves). Generally it is a sheet with holes cut out slightly larger than the pads they encompass. This allows the assembler to solder to the copper pads (the solder mask has an opening around these pads) but the solder will not wick down the trace.



Eagle calls its top soldermask layer tStop and its bottom soldermask layer bStop. In order for the manufacturer to fabricate these boards, they need to have both. This means that when you export the Gerber files, you need to make sure the tStop and bStop files are included and are sent to the manufacturer.