Designing PCBs: SMD Footprints
Nate 
Choosing a Part
As new ICs come out, the PCB layout softwares of the world will not have that specific part within their component libraries. For example, Eagle may have a given footprint (SOIC-8 or QFN-24) but I wouldn’t trust it. I have lost so much money on PCBs that had the wrong footprint that I don’t use the built-in libraries, Eagle or other. Sometimes I’ll use a friend’s library, but we all make mistakes (I actually owe David Mellis a beer for messing up one of his boards because he relied on a footprint of ours). Don’t trust anyone or anyone’s library without a one-to-one print and a thorough review of the datasheet and pin mapping.
The FT230X from FTDI looks like an interesting new IC. Let’s do a breakout board for it. Before we can start laying out a board, we’ll need to create a brand new footprint (aka 'package' in Eagle), schematic symbol, and device for it before we can begin to layout a PCB.
In other words:
or using Eagle's language
Now that we've chosen a part, let's make a footprint for it!