What is SMT stencil and its usage?

A thin sheet material (metal) with a circuit land pattern cut into the material. The most common material is brass and stainless steel.

In surface mount assembly, the stencil is the gateway to accurate and repeatable solder paste deposition. As solder paste is printed through the stencil apertures, it forms deposits that hold the components in place and, when reflowed, secure them to the substrate, generally PCB. The stencil design - its composition and thickness, the size and shape of its apertures - ultimately determines the size, shape and positioning of the deposits, which are crucial to ensuring a high-yield assembly process.