Sometimes it may be desirable to create a PCB footprint from an existing PCB, perhaps when a part is salvaged from a board or detailed dimensions aren't available. Measuring the pattern on an existing board using a digital caliper is possible but is tedious and is subject to errors. This method is adapted from an article on Hack-A-Day: Reverse engineering salvaged part footprints. I show a method to accomplish this technique using Infanview, a image manipulation program I highly recommend.
The first step is to take a high-resolution picture showing the footprint in question. This may be done using the macro setting on a camera or using a microscope camera depending on the size of the footprint. If the board is bare, a standard flatbed scanner may be used to create a great image. When taking the picture, the camera should be parallel with the board to prevent a skewed image. For this article, I'll be using a scanned image of my PCD demo board. The resolution of this picture is about 1200 pixels by 1200 pixels.
The first step is to open the picture in Irfanview. It will look something like the picture below. The background might be black instead of white depending on how the program is configured. Once the board picture is loaded, the paint dialog will be opened.
Next, the measurement tool is selected from the paint menu.
With an image from a scanner, the scaling is contained in the image file, so no calibration should be required. The demo PCB board is 100 mm x 100 mm, so the calibration can be quickly verified. With the measurement tool selected, move the cursor to one end of the dimension to be measured, press the left mouse button and drag to the other end point. A dialog box shows the dimension. The magenta line shows the dimension I measured: 99.66 mm or 3.92" - very close to the actual size of 100 mm.
The measurements are correct even when the imagine is zoomed in. Here's a simple check - zoomed in, the dimensions of the 207/15 resistor package were checked. The measurement on the image is 15.3 mm between the holes.
If the board image was taken with a camera, the scaling will have to be set. Right clicking the measurement tool icon will open the scaling dialog box. Physically measure some dimension on the board and adjust the scale factor until the correct size is shown. The scaling process is a bit of educated trial and error.
This is a great way to determine dimensions on an undocumented board and can yield reasonably accurate results which can be used to quickly reverse engineer PCB footprints. Hope you find it useful.