RFC: Adding IPC high density chip footprints

[evanshultz]

See pointhi/kicad-footprint-generator#421.

IPC 7351B land pattern naming shows one of three suffix letters are to be placed at the end of the footprint name (except for BGAs) to indicate the density. They are:

• M: Most Material Condition (Level A)
• N: Nominal Material Condition (Level B)
• L: Least Material Condition (Level C)

The library is currently built with Level B, medium density footprints, but this isn't indicated in the footprint name. In many applications, such as reducing inductance to vias in high frequency (high dV/dt) circuits, the high-density footprints are valuable. They can substantially improve Z/f when considering power distribution networks at high frequencies.

The footprint framework already supports making various densities, so my question here is about naming of footprints. My first thought is to keep the existing footprint names and add _L to the end. It may conform best to IPC is the existing footprints end in _N, but IMO that's unnecessary as the nominal density can be considered the default condition and users would normally select it and consider the one with a suffix as some variation. Users who knew about IPC density levels and wanted more dense footprint are probably capable, especially with the improved descriptions we would need for our footprints, to find these footprints and use them. I wouldn't be surprised if there's a better scheme but keeping some relationship to IPC's recommendation when building footprints to their standard seems prudent to me.

Thoughts? Rene suggested adding _Density[M|N|L] in the conversation linked above. What else can we consider and then boil it down to a good solution?

To hopefully avoid having the conversation I want to initiate from being derailed again, please note that I am only talking about chip footprints. Capacitors. Resistors. Not DFNs. Not QFNs. Not other packages types. Only chip parts. Not other package types. And only the naming of the footprints to indicate density. And only adding the most dense footprints to chip packages. That's it. Nothing more is in scope of my initial question.

The doc mentioned above is widely available online, for example at http://ohm.bu.edu/~pbohn/__Engineering_Reference/pcb_layout/pcbmatrix/IPC-7x51%20&%20PCBM%20Land%20Pattern%20Naming%20Convention.pdf.

[poeschlr]

My next project (after the current ones are done) will be to get the generators into a state where we can add this functionality in a nice way.