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I've noticed while using Altium that SOIC ICs have 3 different SOIC PCB layout to select from, they are named NSOBX_L, NSOBX_N and NSOBX_M (being X de number of pins). The difference I've noticed is that the 'L' variant has smaller body size, and all of them have the same distance between pads but pad length varies through all of them. I know that when deciding which one to use the wise thing to do is checking if they fit the manufacture's recommended layout for the specific IC but I'd like to know anyway.

EDIT: I've checked the PCB layout and confirmed that lead pitch is 1.27 for all of them. From the wikipedia link that Andy left in the comments I can tell that MSOIC is excluded, there's a chance that the 'N' SOIC variant stands for Narrow SOIC but still there are the other two left.

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In Altium Designer, the L, M, N designators refer to the pad and footprint size. I believe these are from IPC-7351, but you need to pay for it.

  • Least is 10% smaller than N; use for a really tight corner
  • N is Nominal size
  • Most is 10% larger than N; relaxed, good for hand assembly

All are compliant with the package size, so you can use any of them when laying out and the component will still fit.

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    \$\begingroup\$ Ok, that also explains why 'N' is always the layout by default \$\endgroup\$
    – Bizcochito
    Jun 8 '18 at 11:16

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