9 LAND PATTERN ORIGIN

9.1CAD Library Part Origins

The land pattern origin is the component “Center of Gravity” so in most cases it’s the Center of the library part. An example of where it’s not the center is the DPAK or TO-252 component.

The land pattern origin is the zero point in which all the features of the land pattern are derived. The origin is also the center of rotation. The point in which the land pattern rotates around is the Land Pattern Origin.

Before SMT parts came into existence, the common land pattern origin for through hole parts was Pin 1. The main reason for this was a convenience to the PCB designer to keep the component pins on a common grid to make the routing phase of the PCB design layout easy.

When the PCB designer provided X Y coordinate data to the assembly shop, the engineers at the assembly shop had to invent calculators for the various through hole devices that relocated the part origin from pin 1 to the center of the part.

Today with SMT and Through Hole mixed technology and with the need to streamline and automate the assembly process, all land origins should be located in the part center. The only exception to this rule would be components or connectors that absolutely require hand assembly. Then the PCB designer can select the land pattern origin to whatever makes their job easier.

A good feature to add to the CAD library land pattern is a cross hair placed on a documentation layer so that it is visible to the PCB designer. This is a part placement aid. When the PCB Designer selects a component to move it, most CAD tools will make the part origin jump to wherever the cursor is. This is very annoying when you’re fine-tuning a part placement and your goal is just to tweak the placement. If you can visually see the cross hair on the origin, you can select the part at the cross hair location and the part will not jump.

9.2Sample Pictures of Land Pattern Origins