gEDA-user: IPC standard SMT footprints (0603, 0402 vs. RESC0603N etc.)

[Colin D Bennett <colin@xxxxxxxxxxx>]

I discovered that the pcb footprint library contains a number of 0603
SMT footprints.  The primary ones I am concerned with are:

- 0603
- RESC0603L / RESC0603N / RESC0603M

and likewise for 0402.  I understand that the IPC standard defines
three footprints for these SMT parts, called Least, Nominal, and Most,
which are for high-density, medium-density, and low-density designs,
respectively.  But this leaves me with some big questions:

(1) Why is RESC0603L/N/M much smaller than '0603'?
(2) Why is there no similarly named set of RESC0805L/N/M for 0805 size?
(3) Why does RESC1608M nearly match the '0603' footprint?
    Is there an imperial/metric naming confusion happening?  I thought
    0603 was a standard name for this size.

Thanks.

Below is a message I found in the archives that also asks about the
multiple footprints but I don't see that it was satisfactorily answered
since 0603 and RESC0603M are so different in size... why?


Subject: Re: gEDA-user: Is there a directory of footprints for PCB?
From: "John Luciani" <jluciani@xxxxxxxxx>
Date: Sun, 26 Oct 2008 16:16:24 -0400

> On Sun, Oct 26, 2008 at 3:25 PM, Kipton Moravec <kip@xxxxxxxxxx>
> wrote:
> 
> > newlib ->  not_vetted_ingo -> smt0603.ele
> > pcblib -> ~geda -> 0603 Standard SMT resistor, capacitor, etc [0603]
> > pcblib -> ~geda -> CAPC0603L, Standard SMT resistor, capacitor, etc
> > [CAPC0603L]
> > pcblib -> ~geda -> CAPC0603M, Standard SMT resistor, capacitor, etc
> > [CAPC0603M]
> > pcblib -> ~geda -> CAPC0603N, Standard SMT resistor, capacitor, etc
> > [CAPC0603N]
> > pcblib -> ~geda -> RESC0603L, Standard SMT resistor, capacitor, etc
> > [RESC0603L]
> > pcblib -> ~geda -> RESC0603M, Standard SMT resistor, capacitor, etc
> > [RESC0603M]
> > pcblib -> ~geda -> RESC0603N, Standard SMT resistor, capacitor, etc
> > [RESC0603N]
> > pcblib-newlib -> geda -> 0603.fp
> > pcblib-newlib -> geda -> CAPC0603L.fp
> > pcblib-newlib -> geda -> CAPC0603M.fp
> > pcblib-newlib -> geda -> CAPC0603N.fp
> > pcblib-newlib -> geda -> RES0603L.fp
> > pcblib-newlib -> geda -> RESC0603M.fp
> > pcblib-newlib -> geda -> RESC0603N.fp
> >
> > There are huge differences in the size of these footprints!
> >
> > For example, the whole CAPC0603L footprint fits in the space
> > between the pads of smt0603.ele
> >
> > First of all why so many? When would you use one versus another?
> 
> The "L" suffix is for Least material, "N" for nominal and "M" for
> most. This is from IPC-7351. You choose between the sizes based on
> your manufacturing process and design goals. For hand assembly I would
> use either N or M.


Regards,
Colin


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