3 A Good Layout Starts With Good Part Libraries
Before you draw a schematic with a CAD tool, you need to have libraries with the symbols and footprints of the components, you will use for your project. Many people try to find one on the internet and even ask in some forum for a certain library. That is sure time consuming and you never know, what you will get. Usually, the symbols of those libraries are not the greatest and the footprint might not even be approved. I would recommend to make your own libraries.
Figure 2: A schematic diagram symbol (for the NE555) in the library editor
It is well spent time to think about the schematic diagram symbol. Maybe it is good to use the same symbol like in the datasheet. Maybe it is not. It is recommended to group the pins, which will usually be connected (by wire or other components) or which belong together, like data bus pins. Creating a symbol, that resembled the pinout of the physical IC is not good. The “direction” of a pin should be correct. That can be in, out, pas(sive), power, etc. This might help to unveil failure.
It is important to stay in grid for the pins of symbols. Usually, that grid is 100mil/2.54mm. So, do not switch off "snap to grid" (or however it is called in your library editor).
Figure 3: A DIP-08 case in the library editor
Many standard parts like DIP-08, 0805 or PLCC44 have decent footprints in the standard libraries of your CAD software, which you can copy and paste into your own libraries. It is quite practical, to have separate libraries for every manufacturer. This way, you can find those parts easily, when you want to use them in other projects or you can look up the library name in your schematic to find out the manufacturer later.
Look for footprint recommendations in the data sheets. Especially for through hole parts you have to have a suitable drill diameter for your pads. If there is none, calculate the diagonal of the pin (Pytagoras‘ a²+b²=c² is your friend) and add 0.2mm to get a suitable drill diameter.
Eagle can calculate the outer diameter of the pads automatically. It can of course be set manually. The copper around the drill is called an "annular ring" and it should not be narrower than 10 mil for pads and 8 mil for vias.
For surface mount parts, don’t make the pads too big. Just consider the tolerances in the data sheet for maximum with and length of the pads. The best is, to copy and paste from the standard libraries.
Figure 4: The footprint of a screw terminal connector
Draw the shape of the parts properly. You want the parts placed beside each other to fit into their place. If your footprint is too small, you might have conflicts. Also, the orientation should be obvious. The person, who has to assemble the boards later will ask you lots of questions, if that is not the case. Maybe you are this person and you cannot remember, what you have thought of, when drawing the footprints. A good footprint is useful and prevents trouble.
Depending on your ability when handling 3D data, there are 3D models of many parts (especially connectors) available from the manufacturers of the parts. I usually download these 3D models, made a DXF file from the top view and imported it into my (Eagle) libraries. This might sound a bit advanced (indeed, it is), so you can keep it in mind and try it, when you feel more comfortable with creating library parts. It definietly looks impressive.
Figure 5: Square versus round pads
Do not use oval or square pads! They take more space than required and you rather use it for your layout. Let’s imagine, you have a connector with two rows. With round pads, you can easily get two traces through the space between two diagonally adjacent pins (Figure 5). With square pads, you only get one through there. A square pad for marking Pin 1 is quite handy, though.
Story time!!! Back in the past millenium, a colleague (who was the square pad advocate), had designed an adaptor board for a high voltage cable tester. I think, those high voltage test were carried out with 1000 volts. As you might know, electrical fields are highest on acute angles (that is where lightning strikes). The "pointy" 90° angles was good enough for some beautiful sparkover. No, square pads are not really great!