1. IPC-SM-782 3.6.1.4Grid-Based Component Placement SMT component placement and orientation is generally more difficult than THT printed boards for two reasons: higher component densities, and the ability to put components on both sides of the board. For THT designs, the component leads are on 2.54 mm [0.100 in] centers and, assuming 1.3 mm [0.065 in] lands, the spacing between lands would be 1.2 mm.

2. IPC-SM-782 3.6.1.4Grid-Based Component Placement In high density SMT designs, however, the spacing between lands is often less, down to 0.63 mm [0.025 in] and smaller. Grid based component placement (0.100 inch grid is standard with THT) is complicated by the large variety of land sizes associated with the SMT component packages now becoming available. Most SMT designs being done today have abandoned the 2.54 mm [0.100 in] grid based placement rules of the THT boards. This ultimately results in components being randomly placed, and vias being even more randomly placed across the board.

3. IPC-SM-782 3.6.1.4Grid-Based Component Placement Two problems created by random component placement are a loss of uniform grid based test node accessibility and a loss of logical, predictable routing channels on all layers (possibly driving layer counts). In addition the accepted international grid identified in IEC publication IEC 97 states that for new designs the grid should be 0.5 mm, with a further subdivision being 0.05 mm.

4. IPC-SM-782 3.6.1.4Grid-Based Component Placement One solution to the problem is to build CAD libraries with all component lands connected to vias on 0.05 mm centers (or greater, based on design) to be used for testing, routing, and rework ports. Then when doing the component placement on the CAD system, simply place the components so that there is a minimum space of 0.5 mm between lands, then snap the vias of the component being placed out to the next 1.0 mm grid point.

5. IPC-SM-782 3.6.1.4Grid-Based Component Placement With this procedure, all of the components should have between 0.4 mm and 0.6 mm (or an average of 0.5 mm) spacing between the lands. From the assembly point of view, it is easier to process a PCB which has the component centroids on a 1.0 mm grid, with approximately equal spacing between all of the lands across the board in both directions.