[sdiy] SMD layout tips

[Ingo Debus]

Am 10.09.2012 um 12:38 schrieb ChristianH:

> Now I stick to the usual inch grid (50 or 25 mils), drawing a trace from
> the metric TQFP pads and let it snap to the nearest inch grid position. 

Yes, me too. Normally I use 0.15875 mm (or 0.00625 inch). Sometimes it helps to have an alternate coarser grid, like 0.635 mm / 0.025 inch.

I strongly recommend against both a metric and inch-ic grid for PCB traces. If you have to do some rework on the layout later, you never know which grid the traces are in.
Sometimes it's necessary to have the board outlines or component placement in a metric grid, or even completely out of any grid, but usually not for the traces.


> What really p***es me off is that some of Eagle's SMD library packages
> with e.g. 16 pins are defined with their local origin at the exact
> center - meaning that all the pads are off grid when the component is
> placed on a 50 mil grid position. And there aren't even millimeters
> involved...

The reason is probably to get the coordinates for a pick-and-place machine right. There's an Eagle ULP that generates these coordinates ("run mount"), but it only gives you the coordinates of the local origins. If these aren't at the centers of the components, the pick-and-place machine would place them incorrectly.

Ingo