On 12:45 AM 26/02/2003, RAD said:
Hi all,

I'm newbie in Protel, and i need to make a new PCB with mixed components (SMD and convetional) with imperial & metric units.
Can anyone help me?? What is the best way to draw this?

Thanks in advance!!

I routinely design boards with random combinations of footprints design in metric and older imperial dimensions. When I design the footprint I design the part in the appropriate grid. the Q key switches between metric and imperial.

P99SE has problems with rounding errors that can be irritating but very rarely would they be more that that. They are not likely to cause a board manufacturing fault. We are talking about a few thousandths of a mm. It is very irritating though to see a pad that was at an offset of 1 mm suddenly change to 0.99998 mm or whatever, when you toggle the grid units about.

P99SE (and DXP :-) ) have an imperial fundamental unit, metric support is by simple conversion. It seems that the conversion in P99SE is not a rounding conversion but a truncating conversion. DXP has attempted to address this issue, but not in the manner in which many of us would have preferred.

So if you are worried about truncation errors simply design your metric-based footprints in imperial - do the conversion yourself and just preserve enough decimal places so you feel comfortable about the pad locations - 0.01 mil is ample precision for all current technology and 0.1 mil is probably heaps.

The reference point for surface mount components is traditionally the centroid rather than pin 1, so this can also cause pads to be off grid once placed. This is not a big worry. It does cause some problems for the autorouter (even though it is supposed to be gridless), but for manual routing there is no issue if you use the electrical snap grid feature.

With the electrical snap on (Tools-Preferences I think, Shift-E toggles it on and off anyway), when you are placing a track the system will jump to any nearby pad or track end. Then, assuming you have either 90 or 45 degree placement mode active, the first track from the pad will stay off grid and so keep it all looking neat and tidy.

The track placement modes can be cycled through with the SPACE and Shift-SPACE keys while placing a track.

Mixed metric/imperial is very common these days, I have done many boards like this and not had any issues from the truncation error in P99SE, though maybe I am no longer aware of the workarounds if any. One issue that arises is that boards that passed a DRC can suddenly begin to fail a DRC due to minute movements of entities due to truncation. These errors are of the order of a poofteenth but enough to confound the rules system is a pair of entities were spaced exactly at the design rule limit in mm.

If you set your design rules in mils and design the board in mils (not necessarily the components) then you should not have too many issues. Many people are successfully designing boards without even these constraints but if you want to avoid such issues until you are more experienced taking a conservative path makes sense.

Bye for now,
Ian Wilson

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