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

[Gabriel Paubert <paubert@xxxxxxx>]

On Wed, May 25, 2011 at 11:10:13AM -0700, Colin D Bennett wrote:
> On Wed, 25 May 2011 16:56:46 +0200
> Gabriel Paubert <paubert@xxxxxxx> wrote:
> 
> > On Wed, May 25, 2011 at 07:11:49AM -0700, Colin D Bennett wrote:
> > > On Wed, 25 May 2011 06:41:26 -0700
> > > Colin D Bennett <colin@xxxxxxxxxxx> wrote:
> > > 
> > > > (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.
> > > 
> > > I think I've answered some of this for myself... from Wikipedia's
> > > âSurface-mount technologyâ article:
> > > 
> > >     01005 (0402 metric) : 0.016" à 0.008" (0.4 mm à 0.2 mm)
> > >     0201 (0603 metric) : 0.024" à 0.012" (0.6 mm à 0.3 mm)
> > >     0402 (1005 metric) : 0.04" à 0.02" (1.0 mm à 0.5 mm)
> > >     0603 (1608 metric) : 0.063" à 0.031" (1.6 mm à 0.8 mm)
> > >     0805 (2013 metric) : 0.08" à 0.05" (2.0 mm à 1.25 mm)
> > >     1206 (3216 metric) : 0.126" à 0.063" (3.2 mm à 1.6 mm)
> > > 
> > > So the source of the confusion over footprints is that '0603' is a
> > > valid name for both a metric and an imperial size.. both
> > > different?  Whoever decided to name the packages that was must have
> > > been on crack.
> > 
> > Indeed. I found a set of recommended footprints that distinguishes
> > resistors from capacitors: 
> > 
> > http://www.ibselectronics.com/pdf/pa/walsin/smt_notes.pdf
> > 
> > I have used it quite successfully, including for long edge
> > capacitors (0612 or 1632 metric) and 4 resistors in a single package 
> > (4x0603 or 4x1608 metric). I also took into account the
> > recommendations on page 10 when ordering the stencil, shrinking a bit
> > the solder paste apertures generated by PCB.
> 
> Thanks for the note.  It is a curious coincidence that I was actually
> reading this very PDF at the moment I read your message!  I had just
> found it on Google.
> 
> Have you found the Walsin-recommended footprints for SMD chip resistors
> and capacitors to be useful, and an improvement over other footprints
> such as the pcb default "0603", "RESC1608N", etc.?

Useful, yes. Actually I found this document a long time ago and have
been using exclusively these footprints for my designs so I cannot
compare with the others. They work very well for my circuits. 

I had to interpolate them in a project in which I used high frequency 
chokes from the Murata LQW04A series in a weird 03015 (metric 0804) package.

The other interesting part of the document was, as I mentioned, the 
specification for the stencil apertures.

> My fairly uniformed understanding is that the IPC standard specifies
> the one set of footprints for use both resistors and capacitors.
> (e.g., 0603-least, 0603-nominal, 0603-most.)  But is differentiation
> between device types useful?  At least the package height differs (MLCC
> is square along the X axis, while chip resistors are flatter, having a
> height H much less than the Y dimension), but I don't know how this
> would affect placement and soldering.

Resistors have metallization basically on the underside, while capacitors
terminals wrap around the device. They really need the extra length for 
properly wetting the large and high end contact. My guess is that you would 
not see a measurable difference when installing a resistor on a capacitor 
footprint, but the other way around would be extremely sensitive to small 
placement error before reflow (with risk of tombstoning). This guess
is based on a limited experience. 

This said, most capacitors look to have a square profile, but resonance 
frequencies observed on microstrip line change between mounting the 
layers parallel or perpendicular to the ground plane. The problem is 
that, for most capacitors, it is very hard to determine the orientation 
of the layers.

By the way, for chokes like Murata LQW18 series, I use the resistor footprint.

	Regards,
	Gabriel


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