kicad-developers team mailing list archive

[Lorenzo Marcantonio <l.marcantonio@xxxxxxxxxxxx>]

On Thu, Jan 15, 2015 at 04:44:38PM -0500, Carl Poirier wrote:
> Because that's what IPC-7351B says about can components. I don't think they
> include the THT cans in this standard, but Lorenzo can confirm that one.
> (Or anyone else with access to the documents)
> 
> On Thu, Jan 15, 2015 at 4:32 PM, nnn <nnn4000@xxxxx> wrote:
> 
> >  I don't understand the exception for aluminum capacitors. Does it apply
> > to both THT and SMD? Why clearance for THT capacitors should be greater
> > than clearance for other THT?

First thing: 7351 is only for SMD; THT are covered by 7251 (draft freely
available). AFAIK in 7251 you only have to decide if pinning is radial
or axial. However both the LP designer and the slides on the net are
based on the unpublished 7351C and the work-in-progress 7070 standard.

If you look here
http://www.ipc.org/CommitteeDetail.aspx?Committee=5-21A, the Hausherr
guy is one of the leader so I'm expecting that some of the information
and stuff from LP designer is a preview for the incoming standard. In
other words, Mentor/PCB Libraries will always be ahead on the
implementation since they actually ratify the standards :P

Second thing: the IPC tables are done by *experiment*, i.e. they
soldered an hellish number of board with different spacing and
statistically derived the optimal spacing/values/whatever. It's one of
the main reason for having them paying for the standard AFAIK.

Also, the rule for canned capacitors applies to canned (2 pin) crystal too.

The reason for the spacing isn't explained, but I think is due to the
balance/form factor of these parts (in fact, there is a specific
table for the taller parts).

If you look at the construction for the L-ribbon leaded pins in
a typical canned capacitor, the body *doesn't* touch the board. It's all
suspended above the thin and reasonably tall pins, so there is
considerable leverage available for the cap (a common fault is the cap
tearing off the pads...). Also the pins are not flats but slightly
'tented', unlike wound coils (no idea why). See here:
http://s3-blogs.mentor.com/tom-hausherr/files/2010/10/under_bodyoutward_l_lead.png

I think that the biggest courtyard is to allow for a slight movement of
the parts while unsoldered (paste is tacky, but it's not glue). Also if
the capacitor is tilted (other common failure during inspection) you
need a bigger space for rework it into position...

-- 
Lorenzo Marcantonio
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