3.3. Padstack design

3.3.1. Avoid overlaps

Avoid using overlapping padstacks, especially for constructing complex pads.

For example for mesh paste pattern over a center pad (e.g. in QFN) use a single center pad padstack and use plain rectangles (not padstacks) on the paste layer. The rectangle objects could be associated with the terminal by setting their term attribute (heavy terminal concept).

In such a center pad if vias are also required, make sure the via does not have copper, paste and mask on the same layer as the centerpad rectangle is on (top layer normally).

gEDA/PCB used to have many limitations which forced the user to use multiple pad objects; these problems have been resolved in pcb-rnd for many years by now. If a custom shaped padstack is required, just use a polygon shape If complex paste or mask patterns are required, don't do them as padstacks but simply draw them as layer objects (both paste and mask layers are editable).

3.3.2. Rely on padstack rotation

Many footprints have the same pads on two or four sides, e.g. SOIC or QFN. Draw the padstack only once and use the same padstack for each side with the appropriate n*90 rotation. Use the least number of different padstacks possible.

3.3.3. Padstack origin

Most padstacks will be circular, square-like or rectangular. In the former two cases it's best to have the padstack origin in the center of the padstack. In the rectangular case, the origin shall be in the center along the shorter edge; along the longer edge pick a meaningful internal point on the pad (e.g. center, toe or heel).

This rule is to help the GUI to snap while routing traces.

3.3.4. Terminal IDs

All terminals should have a terminal ID; when the datasheet has pin/pad numbering, that shall be used. When the datasheet does not have an ID for a terminal that is expected to be connected ever, assign a non-colliding terminal ID:

• for example connector's chassis terminals could be called 0 or chassis
• if there are multiple terminals for the same purpose
  • if they are equivalent (e.g. DSUB connector chassis) name them the same; that assumes internal connection and it will be enough to connect one
  • if they each need to be connected, e.g. multiple ground pins/tabs, have different names for them and d not use intconn; this way the schematics will need to connect them all
  • if the difference between the terminals is significant and there is internal connection the user will rely on, use different terminal IDs and intconn; for example a 0 ohm nonetlist jumper resistor would have terminal IDs 1 and 2 and intconn in between