Now that you've placed your first pad, lets position it!
You need to deselect the 'Smd' tool or you'll keep adding new pads every time you click somewhere.
Go back to the left-hand toolbar, and click an action like 'move' (highlighted in yellow below) that doesn't create something new:
Meet Your Other Eagle BFF: The Properties Dialogue Box!
The best way to precisely position your pads is to manually enter their exact X + Y position (relative to center/home/zero) in the properties dialogue box.
You can also tweek the length and width of the pad and a bunch of other important settings here, but we'll cover some of that later.
Now that you've deselected the Smd tool (by selecting 'move' or something similar), move your mouse over your pad, right-click on it
' from the popup menu:
You should see a dialogue box similar to the following:
The properties dialogue box is extremely important, and you'll find yourself using this again and again throughout the part and PCB design process, so get acquainted with it now!
You can see that we have a textbox labelled 'Position:'. This is where we can precisely place our pad exactly where it needs to be, using simple co-ordinates relative to the center/home/zero position.
Which brings us to our next logical question ... "How do we know exactly where to position our pads?!?"
Determine Pad Position Precisely
Going back to the datasheet we mentioned earlier, we can see that there are four 0.6 x 0.6 mm pads in our 2.0 x 1.6 mm rectangular footprint.
This is where you always need to do a bit of math, and calculate positions and sizes based on the data present in the datasheet.
Manufacturers will rarely display every possible value in the datasheet, but there is almost always enough information to infer the numbers you need with a bit of simple division, addition and subtraction.
The first pad that we'll place is the one in the top left-hand corner, which is marked as 'pin 1' in the datasheet:
It will save time to start with pin one and then place the pads in the right numeric order, since Eagle will auto-increment the names to follow that same order
You can infer the exact center position of pad one by looking at the numbers available in the two illustrations below:
Looking at the recommended footprint in the image above (the image to the right), we can see that there is exactly 0.5mm distance between the right-hand edge of pad one (the top left pad) and the central '0' position on the X axis. There is also 0.35mm distance between the bottom edge of pad one and the '0' position on the Y axis.
Calculate the Pad's X Position
Since we know that pads in eagle as placed relative their center point, it's easy to calculate the exact 'X' position for this pad:
- Find the distance from the center of our part (the zero position indicated by the dash-dot lines) to the right-hand edge of our pad (0.5mm)
- Calculate the distance from the right-hand edge of our pad to it's center position, which is the pad width / 2 (0.6mm / 2 = 0.3mm)
- Add them together (0.5mm space + 0.3mm to the center of our pad), and we know that our pad should be centered at -0.8mm on the X axis.
- Now enter '-0.8' in the first textbox beside 'Position' in our properties dialogue:
NOTE: We enter a 'negative' value on the X axis since we want to position the pad 0.8mm before the zero or home position of our part ... entering a positive value would position it 0.8mm after the home position on the X axis
Calculate the Pad's Y Position
Similarly, we can calculate the center Y position for the pad by taking the 0.35mm from the zero position to the bottom edge of our pad, plus 1/2 the pad height, meaning 0.35mm space + 0.3mm to pad center, = 0.65mm on the Y axis
Enter this value in the second box beside 'Position:' to set the Y location:
In this case, Y is a positive value since we want to place it 'above' the home or zero position of our part. To place it below the zero position on the Y axis we would enter -0.65mm. Everything is always positioned relative the center or home position!
Now click 'OK' and the pad should be centered at -0.8mm to the left and 0.65 mm to the top relative to the 'home' position for our package, mirroring the position of pad one in our datasheet:
You might need to zoom out a bit to see your pad, using either the scroll wheel of the 'zoom' icons in the top toolbar
That's it! You've now accurately sized, placed and positioned your first pad, and from here on in it's just a lot of rinse and repeat!