Learn Board Layout b ...

How do you go about designing your own land pattern?  Usually, I'd recommend that you start by reading IPC J-STD-001 to gain an understanding of the inspection solder joints specifications since those criteria guide everything else.  But J-STD-001 is locked behind a paywall, and I believe that "fair-use" copyright laws only allow me to use a very small part of the copyrighted work.

So instead, we'll start with a brief introduction and then jump into the process.

JSTD-001 specifies the acceptable assembly criteria solder-joints, from through-hole parts to BGAs.

A look at Gull-Wing Land Patterns from J-STD-001

"The gull-wing land-pattern is the one I most often hear discussed amongst engineers because of the apparent disagreement between the allowable toe-overhang and common-sense."

You will see that J-STD-001 lays out all mechanical dimensions in painstaking detail.  Every part of the joint is defined and every dimension specified in a separate table.  

This image, taken from IPC-J-STD-001 Figure 7-7 shows the various dimensions and definitions of the mechanical connection.

Dimension B is what usually gains the most attention since there is no requirement that the toe of the lead stays on the pad except to prevent short circuits with adjacent nets!

This table accompanies figure 7.7

Many engineers complain about the toe-overhang detail because on its face, it really is absurd to imagine a part protruding very far off of a land pattern.  But really, it's a bit of a blessing.  It allows designers to make a land-pattern that is the exact size of the part, which in turn keeps the courtyard small.  If IPC specified even a 0% overhang, and the part wasn't perfectly centered on a pad, then assembly houses yield would go down and the cost to manufacture each project would increase.

Land Pad Expansion

For proper electrical, thermal, and mechanical bonding to occur between part leads/pads and land pads of side-terminated components, the land pad usually has to be larger than the part pad.  This allows the surface tension of the solder metal to both align the part and form a fillet at the joint.

Filets both increase the reliability of an assembly and aide the inspection process, as they indicated a properly wetted solder joint.

The land-pad size, the size of the metallization on the part, and a few other factors can be used to mathematically determine the optimal pad dimensions based on J-STD-001.  Here's a mathematical model for through-hole parts from 1975 for interested readers.  Fortunately, you don't need to invest any amount of time understanding the mathematical derivations.  You just need to make sure your assembled part will meet J-STD-001 criteria.

Land Pattern Accuracy

Unless you are dealing with very fine-pitched parts, the size of the land pad doesn't matter nearly as much as mathematical equations lead you to believe.

If it did, we'd have 39 different land patterns for 0805 multi-layer ceramic chip capacitors, since they come in thicknesses of {0.33, 0.50, 0.55, 0.56, 0.60, 0.70, 0.71, 0.74, 0.75, 0.76, 0.80, 0.85, 0.87, 0.88, 0.89, 0.90, 0.91, 0.94, 0.95, 0.98, 1.00, 1.02, 1.05, 1.10, 1.14, 1.15, 1.17, 1.20, 1.25, 1.27, 1.30, 1.35, 1.37, 1.40, 1.45, 1.50, 1.52, 1.55, 1.80 } millimeters.  No working engineer has time or interest in keeping track of that.  And would you really want a redesign if your purchasing department had to replace a 0.90 mm thick capacitor with a 1.10 mm thick capacitor?  

So IPC generated some conservative one-size-fits-all values for the IPC-7351B specification, and, for the most part, everyone is fine with that.

The land-pad dimensions do start to become an issue when using the density level C - Minimum (Least) Land Protrusion, since the courtyard contours both the package and the land pads.  For high-density designs, every fraction of a millimeter counts.  You can then dust off the research papers and your calculator, or use a tool such as the one at https://pcblibraries.com (https://www.pcblibraries.com/account/user/memberdownloads.asp) to generate your new parts for you.

"IPC-7351 has been effectively demoted from 'standard' to 'recommendation', and IPC-7351C has been in committee purgatory for several years.  Tom Hausherr's team, at PCBLibraries.com, is working independently of the IPC7351 committee to create the new defacto standard.  A free version of his software allows you to make custom parts on a one-by-one basis.  The newest version of the software is set to release in May/June of 2020.  A $99/year subscription should allow you to upload your manufacturer's part numbers, and then download a parts library to any EDA tool."