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I'm making a footprint for a part, and I'm unsure how the length of an edge is defined.

I've circled in red the two dimensions that are confusing me.

For example, does the 0.1 indicate the length of the straight part, from the middle of the radius, or something else.

enter image description here

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    \$\begingroup\$ Presumably it'd be the length of the flat edge. Honestly, what I do when I make a footprint from a spec like this, I just very closely examine the final result and the example... Does it "look right"? i.e. the proportions are all correct? Your eye is a pretty good tool in this regard. If something is way off, you won't need to make measurements to know it. You'll be able to see it. \$\endgroup\$
    – Kyle B
    Commented Apr 27, 2022 at 22:53

3 Answers 3

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The drawing is to scale. Measure it.

enter image description here

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  • \$\begingroup\$ I did my best an it appears to be the straight part of the line, but I'd still like to understand the dimensioning if I come across it again. \$\endgroup\$ Commented Apr 27, 2022 at 23:18
  • \$\begingroup\$ the dimensioning is unclear. \$\endgroup\$ Commented May 2, 2022 at 5:22
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If in doubt, bring an image of the drawing in to a 2D CAD tool and properly scale it.
You'll notice that the drawing has an error in it - the 0.75 dimension is really 0.075.

enter image description here

The superimposed green lines are 0.075mm & 0.100 mm in length. You'll see that the dimensions are for the straight section. All radii are 0.05mm.

This drawing does not conform to proper drawing standards, thus, when you come across a poorly drawn drawing, you'll need to break rules and scale the drawing to figure it out. There are a surprising number of poorly drawn drawings for footprints as well as mechanical drawings for connectors that are missing critical dimensions.

I find that it is easier to draft up the footprint in a proper 2D CAD tool and import the drawing as a DXF to the PCB CAD tool. The imported drawing can either be used to create the actual pads or as a template.

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The length of the edge is not necessary. All you need is the dimensions of the "bounding rectangle" for the pads - which are the same for every pad - and the radius of the arcs - again, same everywhere, it looks like. The rest follows from symmetry.

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