Electronic – Why do some PCBs have the courtyard in the silkscreen layer

The IPC-7351 standard defines a coordinate system origin for surface mount parts, as well as a standard orientation. There is a summary of the orientations on the last page of this document: http://landpatterns.ipc.org/IPC-7351BNamingConvention.pdf [Update: archived link], and illustrations in the full standard.

For most surface mount parts:

• The origin is at the centroid of the part's body
• The standard orientation is with pin 1 at upper left, except:
  • two-leaded components, where it's to the left
  • components where pin 1 is in the middle of an edge, where it's at top center

The reason for the origin being at the centroid of the part body is that pick and place machines use a suction cup to pick up the part at that point. Orientation on the reel can vary, so you have to tell the machine which way they are rotated when you load the reel.

It's called silkscreen because this is the name of a type of printing: screen printing.

The traditional screen printing process is like this: Inside the frame is a sheet of fine woven material (the silk) which is porous enough that you can push paint through it using a squeegee. A mask is placed between the silk screen and the paper. It's all pressed together, then the squeegee is wiped across the screen. Paint is pushed through the mask onto the paper, forming the image. Really, the only job of the screen is to protect the mask from the squeegee.

A similar process is used when printing the white paint on PCBs. However, instead of a separate mask, the screen is covered with a photosensitive layer, which is developed to produce the required mask as part of the screen.

Of course, for PCB manufacture, this process is done by machine:

If you look closely at a PCB, you can see that the silk screen image is actually quite rough, and it's apparent that the image was pushed through a net.