Arrgh - PCB footprint pad dimensions!

L

Leon Heller

Guest
Why is it that the manufacturers' footprints for SM devices nearly always
give the dimensions between the edges of the pads? I thought it was standard
engineering practice to give the dimensions between centres, which is what
is required by the PCB package I use, so I have to do a few calculations
every time I create a new SM footprint.

Leon
--
Leon Heller, G1HSM
http://www.geocities.com/leon_heller
 
"Leon Heller" <leon_heller@hotmail.com> wrote in message
news:41555a41$0$20253$cc9e4d1f@news-text.dial.pipex.com...
Why is it that the manufacturers' footprints for SM devices nearly always
give the dimensions between the edges of the pads? I thought it was
standard engineering practice to give the dimensions between centres,
which is what is required by the PCB package I use, so I have to do a few
calculations every time I create a new SM footprint.
Click to expand...
Further to the above, I've just seen a footprint which has *one* dimension
between the *centre* of a central pad and the edge of another with *all* the
others between the edges!!!

Leon
 
they do that so you can learn how to use a calculator!

"Leon Heller" <leon_heller@hotmail.com> wrote in message
news:41555ffe$0$20250$cc9e4d1f@news-text.dial.pipex.com...
"Leon Heller" <leon_heller@hotmail.com> wrote in message
news:41555a41$0$20253$cc9e4d1f@news-text.dial.pipex.com...
Why is it that the manufacturers' footprints for SM devices nearly
always
give the dimensions between the edges of the pads? I thought it was
standard engineering practice to give the dimensions between centres,
which is what is required by the PCB package I use, so I have to do a
few
calculations every time I create a new SM footprint.

Further to the above, I've just seen a footprint which has *one* dimension
between the *centre* of a central pad and the edge of another with *all*
the
others between the edges!!!

Leon
Click to expand...
 
On Sat, 25 Sep 2004 12:45:05 +0100, Leon Heller wrote:

Why is it that the manufacturers' footprints for SM devices nearly always
give the dimensions between the edges of the pads? I thought it was standard
engineering practice to give the dimensions between centres, which is what
is required by the PCB package I use, so I have to do a few calculations
every time I create a new SM footprint.

Leon
Click to expand...
One of my pet peeves as well. So far I haven't screwed up the offset
calculations on a board, but it's only a matter of time...

Bob
 
On Mon, 27 Sep 2004 15:35:27 GMT, the renowned Bob Stephens
<stephensyomamadigital@earthlink.net> wrote:

On Sat, 25 Sep 2004 12:45:05 +0100, Leon Heller wrote:

Why is it that the manufacturers' footprints for SM devices nearly always
give the dimensions between the edges of the pads? I thought it was standard
engineering practice to give the dimensions between centres, which is what
is required by the PCB package I use, so I have to do a few calculations
every time I create a new SM footprint.

Leon

One of my pet peeves as well. So far I haven't screwed up the offset
calculations on a board, but it's only a matter of time...

Bob
Click to expand...
I always try to print it out 1:1 and lay the part overtop with a new
footprint. If there's no samples in hand, that's when problems (this
and other kinds) tend to occur.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Using my CAD software, when creating Patterns like this - I will usually
also place the Package Outline information on an Assembly Layer -
superimposing it onto the Pad Layout. I will also create little square pads
on the Assembly Layer - that way... if something looks 'odd' it will usually
jump out at me, and I can investigate further, correcting if necessary.

I had a Pattern recently - for a SIM Card - that dimensioned to the plastic
edges... and the Pad edges. That was a challenge. I think mechanicers like
to pose interesting challenges when it comes to dimensioning parts.

Regards,

James Jackson
Oztronics



"Spehro Pefhany" <speffSNIP@interlogDOTyou.knowwhat> wrote in message
news:pmdgl09bt4ctbkcb9ip07nv6h45ddlfmvq@4ax.com...
On Mon, 27 Sep 2004 15:35:27 GMT, the renowned Bob Stephens
stephensyomamadigital@earthlink.net> wrote:

On Sat, 25 Sep 2004 12:45:05 +0100, Leon Heller wrote:

Why is it that the manufacturers' footprints for SM devices nearly
always
give the dimensions between the edges of the pads? I thought it was
standard
engineering practice to give the dimensions between centres, which is
what
is required by the PCB package I use, so I have to do a few
calculations
every time I create a new SM footprint.

Leon

One of my pet peeves as well. So far I haven't screwed up the offset
calculations on a board, but it's only a matter of time...

Bob

I always try to print it out 1:1 and lay the part overtop with a new
footprint. If there's no samples in hand, that's when problems (this
and other kinds) tend to occur.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com
Click to expand...
 
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