I'll make an attempt at addressing your question...

Most likely it was based upon the need to get the lead into the hole. That's
a little different process than landing a surface mount part on the top of a
circuit pattern.

The axial parts are delivered with the leads straight and parallel to the
body of the part... the insertion machine cuts the leads and puts the 90 deg
bend into the leads of the part at the time of insertion and the alignment
of the insertion head with the cut and clinch heads below the board is
critical... this insertion head has guides on either side of it that slide
the leads down from the cutting heads into the holes in the board. This
process keeps control of the lead wires all through the process and the key
here is not missing the hole and crashing the part into the board... This is
true of radial parts as well, they are trimmed from the tape, and inserted
with guides that put the leads into the holes... the body can vary but the
lead spacing must not... 

The tolerances are based upon the ability of the machine to hit the hole(s)
accurately. That's why we always use larger holes and standard lead spacing
increments when design footprints for Thru Hole devices. These are machine
dependant, and if you are using a Universal or Panasert or some other type
of insertion machine it will have documented guides on the machine's
capabilities and tolerances that will affect how you design your footprints.


Pin one is the standard location for the origin for a thru hole part... or
any polarized 2 lead part would use the polarization as pin one to make it
consistent with that scheme. 

Since surface mount parts are picked up and placed by the center of mass or
physical center of the part we use the center of the part for the origin...
it just makes sense.

I believe those are the reasons for either method, but I'm always interested
to hear other opinions or experiences...  

I will have to ask our auto-insertion programmer here if things have changed
for her part of the operation. I don't know if the user interfaces for
programming have changed over the years... I have a feeling that they have
not but I will let you know what she says... 


Best regards,


Bill Brooks 
PCB Design Engineer, C.I.D.+
Tel: (760)597-1500 Fax: (760)597-1510
Datron World Communications, Inc.
Vista, California


-----Original Message-----
From: Kevin L. Seaman [mailto:[log in to unmask]] 
Sent: Monday, February 12, 2007 10:24 AM
To: [log in to unmask]
Subject: [DC] Thru-Hole Center Point Location

PCB Designers,

What were/are the motivations for putting the center point (0,0) of a
thru-hole footprint at pin #1 instead of the body center like an SMD?

Do the original reasons still exist today?

Thank you,

Kevin L. Seaman
OrCAD CIS & Allegro PCB Library Liaison
Sr. Staff PCB Layout Engineer
Broadcom Corp. Irvine, CA
(949) 926-5656 (ext. 65656)

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