Autorouter math- space for tracks between pads

Harry Sieber wrote:

I am trying to autoroute a fairly simple board with V5.11.0 after a

long period of not using Eagle. I want to coerce the autorouter into

putting 8 mil tracks between the pins of a 2 mm through hole

connector (87758-3416, from con-molex.lbr). I have reduced the

connector

pad size in the library part from .050 to .045 in. diameter to allow

more room. The math says: pads on 2mm centers=.07874 inch

center-to-center.

With .045 pads that should allow .03374 free space between pads. With

an 8 mil track there should be almost 13 mils space between the

track and pads. However I have to set the minimum space value to 6

mils (DRC->Clearance) for the router to run tracks between the pads.

This is too fine for the board prototype house (and good practice)

and I don't see why the default value of 8 mils would not work when

there is 12.87 mils of space- if I set this value to 8 mils I get DRC

clearance violations. Placement and routing grids are set at .001.

Any suggestions would be appreciated! Thanks

Harry

Looking at the library, and not changing the pad diameter, it appears that

it should all work for you.

BUT!...Measure the pad diameters and space between them on the board.

Its likely the DRC settings have increased the size of the pads on the board

and thus narrowed the gap. You reduced the pad diameter in the library but

the diameter they end up as on the board is related to a percentage of the

drill diameter.

The library, for all intents and purposes, can really only force pads larger

than what the DRC will calculate.

The restring as measured in the library is 7mil but likley your DRC setting

is 10mil so they increases to 10mil. While in the library if you make the

pad diameter AUTO it will be depicted with a size determined by the DRC

settings (based on the drill diameter) Making the minimum setting 9mil in

the DRC enables an 8mil wire to pass though with 8mil clearances. Be aware

though that all pads will be reduced to 9mil restrings if the maths

calculates a size smaller than 9mil initially.

HTH

Warren

--

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On Mon, 24 Sep 2012 13:05:25 +0000, Harry Sieber wrote:

I am trying to autoroute a fairly simple board with V5.11.0 after a long

period of not using Eagle. I want to coerce the autorouter into putting

8 mil tracks between the pins of a 2 mm through hole connector

(87758-3416, from con-molex.lbr). I have reduced the connector pad size

in the library part from .050 to .045 in. diameter to allow more room.

The math says: pads on 2mm centers=.07874 inch center-to-center.

With .045 pads that should allow .03374 free space between pads. With an

8 mil track there should be almost 13 mils space between the track and

pads. However I have to set the minimum space value to 6 mils

(DRC->Clearance) for the router to run tracks between the pads.

This is too fine for the board prototype house (and good practice) and I

don't see why the default value of 8 mils would not work when there is

12.87 mils of space- if I set this value to 8 mils I get DRC clearance

violations. Placement and routing grids are set at .001. Any suggestions

would be appreciated! Thanks Harry

I've had similar problems.

Place everything on a metric grid (use control-click to pick parts up by

their centers, so when you put them down they'll be on-grid).  Then route

on a metric grid.

It's a pain in the behind if you've already got everything placed on an

inch grid, but if you have a preponderance of metric "problem parts" then

that's the way to go.

I couldn't tell you what to do if you had a mix of metric and English

fine-pitch parts, though.

--

Tim Wescott

Control system and signal processing consulting

www.wescottdesign.com