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What are the differences between solder mask and paste mask?

Many beginners may be confused about Solder Mask Layer and Paste Mask Layer, because they two do have some similarities. In this article we will introduce the differences between solder mask  and paste mask, so that the beginners can understand better.

Solder mask is the solder mask layer, except the pad (surface pads mount pads, plug-in pads, vias) on PCB boards, the other areas should be covered with solder resist ink. These pads are exposed, so as to avoid tin while wave soldering. It is also usually called solder mask layer or green layer.

Solder mask layer can be divided into Top Layers and Bottom Layers. The small circles or small square circles in gerber file are the holes on solder mask for exposed pads, generally larger than the pad.

Paste mask layer is generally called stencil in the industry. Generally the hollow shape size is same with SMD pads, or slightly smaller. This stencil is used in the SMD automatic assembly soldering process to coat the SMD pads with tin paste. In the surface mount device (SMD) welding process, first cover the stencil to the circuit board (aligned with the corresponding pads), and then apply the paste, remove the excess paste with a blade, next take away the stencil, so that the solder pads are applied with paste. Afterwards attach components to the solder paste to (by hand or assembly machine), and finally through the reflow soldering machine to complete the SMD assembly. Usually the sizes on the stencil will be smaller than the actual welding pads on the circuit board. Paste mask layer can also divided into Top Paste and Bottom Paste.

Paste Mask

Paste Mask

Here are some differences between solder mask and paste mask:

1) The openings on solder mask layer have no solder mask ink, but the openings on paste mask layer have paste.

2) The solder mask layer is one part on the circuit board, but the paste mask layer is not. Paste mask layer is just for the stencil.

3) Solder mask is used for applying solder mask ink, but paste mask is used for applying paste.

4) Solder mask is applied when making PCB board, but paste mask is used when assembling.

5) Solder mask has many available colors, but paste mask is usually gray.

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