What is Circuit Design
3. Process Overview
The silkscreen layer is used for assembly, identification, and component identification for service/maintenance.
The silkscreen layer sits on the outermost layer of a PCB and provides components reference designators as well as pin 1 or anode/cathode identification for polarized parts.
Plenty of companies still apply the silkscreen layer using silkscreen processes, complete with screen, squeegee and pattern templates.
Image of silkscreen process from http://www.gwent.org/gem_thick_film.html
But silkscreen printers that utilize ink-jet technology are gaining a larger foothold in the industry. These devices print directly onto PCBs in the same manner inkjet printers print directly onto a piece of paper.
Image of silkscreen printer from http://www.zhengyekeji.com/
Here's an example of the quality from one of these machines:
Image of silkscreen print quality from http://www.shengyekeji.com/
"The Silkscreen layer is not always aligned as it should be on top of a PCB. And if your silkscreen print ends up on top of a pad, it will disrupt the solder process. So don't squeeze things in too close to lands."