Re: gEDA-user: pcb SMT footprints

[Steve Meier <smeier@xxxxxxxxxxxxxxxxxxx>]

The IPC-SM-782A also claims the 603 chip resistor dimensions can be
worst case.

Length (max) 1.7 mm
Width  (max) .95 mm
gap (min) .7 mm

This imples the length of the terminal could be as big as .5mm

Given this the width of the Land Pattern seems reasonable but the over
all length seems excesive. 

I think the reason that they use is that they like to have room for the
solder to stick out past the device (solder toe) by ~0.5mm this would
add an extra 1mm to the overall length. The solder heal is the extra pad
under the terminal that allows solder to stick further under the device
then its actual terminal.

Ok the spec declares the pad length (terminal) to be 1.1mm

So. 

Terminal + Solder Toe + Solder Heal = 0.5mm + 0.5mm + 0.25mm = 1.25mm

Implying an allowance of -.15 mm for worst case slop????

Hmmm... lets look at the device specs again...

Length (min) 1.5mm

So for the nominal 1.6 mm terminal length and gap of ~.9mm 

Terminal + Solder Toe + Solder Heal = 0.35mm + 0.5mm + 0.25mm = 1.1mm

Implying an allowance of 0mm for positioning slop.

On the sides they like the pads larger by no smaller then 0.01mm adding
0.02mm for perfect placement. So this pattern allows .03mm side to side
slop in positioning


Steve Meier


On Thu, 2005-11-17 at 10:06 -0800, Steve Meier wrote:
> IPC-SM-782A (section 8.1) defines the dimensions of a 603 chip resistor
> land pattern to be
> 
> Length 2.80 mm
> Width  1.00 mm
> gap    0.60 mm
> 
> The IPC standard has been criticized before for having large foot print
> sizes.
> 
> Steve Meier
> 
> 
> 
> 
> 
> On Thu, 2005-11-17 at 08:33 -0800, Larry Doolittle wrote:
> > Stephen Meier wrote:
> > > Here is a set of foot prints that I have been working on.
> > 
> > That 0603 footprint is much larger than any other I've seen.
> > Is that on purpose?
> > 
> > Meier  IPC-RLPN-132A  1608 [0603]
> >    length      110
> >    width        39
> >    terminal     43
> >    gap          24
> > 
> > Dan McMahill wrote:
> > > For example an 0603 capacitor is CAPC2012L, CAPC2012N, and CAPC2012M.
> > > L=least, N=nominal, M=most. So if you want to be agressive with small
> > > pads use the "L", and "M" for larger pads. The 2012 is from the metric
> > > dimensions.
> > 
> > Hmmm.  An 0603 part has nominal dimensions 60 mils x 30 mils,
> > or in metric, 1.6 mm x 0.8 mm.  Where did "2012" come from?
> > I guess it's a typo for "1608", and Dan mixed up this conversation
> > with a discussion of 0805 parts.
> > 
> > Dan, can you send me (or point me to) the CAPC1608* footprints,
> > so I can compare them with the others?
> > 
> >       - Larry
> > 
> > P.S.  Sorry about the sorting of this message in the thread.
> > At least I got it in the right thread!
>