Solder mask paste mask layer PCB EMS

What is the difference between solder mask and paste mask layer?

Solder mask and paste mask are similar terms and both are closely related to the PCB assembly. So now we would like to help and explain - what is the difference between them?

Solder mask is a layer of solder mask on a PCB that does not cover the pads. All other areas of the board should be covered with solder ink. It can be divided into top and bottom layers. The small circles or squares in the gerber file are just the solder mask holes. A few blog posts ago we mentioned that solder mask is usually green.

The solder paste mask layer is usually simply called a stencil. It's used at the beginning of SMT assembly to cover the pads with tin paste. Usually the cutouts in the solder mask are smaller or the same size as the actual pads on the board.

How does the paste application process look like? There is a template on the PCB (aligned with the pads), then the machine applies paste on it. The paste is applied for a reason - in SMT process it is due to the tin, which hardens later under the heat, that particular elements stick to the PCB.