Hi Mathew,

IPC-782-A is meant to be very much only a guide - it was generated by about 200
companies sending volunteers to the IPC sub-committee, and as a result gives
dimensions, pad sizes and specifications which will virtually guarantee
manufacturability for any assembly process.

That's the rub - 782A has a poor reputation for being something it isn't, and it
was never meant to be a definitive document because of the broad range of
requirements designers face. Some designers complain bitterly that its
suggestions are very broad and no help at all in tight boards. I think most
designers expect it to give minimum sizes so that they know what they can get
away with, but that particular issue depends on your assembly process and your
board fab shop.

In your case, use it (782A)  as a guide. Talk first with your assembly people
about solder paste deposition tolerances, component placement accuracy and
tolerance and reflow/wave soldering requirements. Talk then with your board
fabricator about types of solder mask, solder mask fabrication tolerances and
track widths/spaces and tolerances. Then sit down and plan your board before you
start - how much room you have for placement and tracking will be your major
decision. Then work out your pad sizes, track sizes and placement clearances to
get to where you want to go.

Any one of us could give you lots of specific recommendations - they'll all be a
little different. For example, I do mobile phone layouts and I bet my pads are
smaller than most, but that's only because they have to be to get everything to
fit.

If you're looking for something more definite, Jim Blankenhorn at SMT Plus has a
series of manuals with defined sizes that you might find useful. I've spoken
with a number of designers who use his manual (and libraries now too, I think)
and they say they've never had trouble. Might be worth your while to chase that
up.

If I can help you some more, email me directly at  [log in to unmask]

Cheers

Andy Kowalewski           (972) 550 6365



____________________Reply Separator____________________
Subject:    [DC] IPC-SM-720A pad dimensions etc?
Author: Matthew Lamkin <[log in to unmask]>
Date:       3/10/00 3:55 PM

Hi there, can someone clearup/clarify some little problems that I have
in understanding the dimensioning of SMT pads from this std etc.

What I'm trying to do is re-create the footprints that I use with
Cadstar3, to be what the IPC-SM-782A specifies them to be, as we have
recently got new
pick & place machines that can use these pads instead of the extremely
massive ones that we had to use with the old machines.

First off, the 0805 resistor's:
Subsection 8.1 page 3, says that the grid placement courtyard is
measured in 0.5mm x 0.5mm squares, and that the
0805 uses a 4x8 courtyard.
This equates to 4mm x 2mm.
Therefor this leaves a placement gap between the pads of 0.4mm at the
ends & 0.25mm along the length.

However any other books that I have, say that I should leave a minimum
of 1mm gap between the pads, which means
that I should be creating a placement outline 0.5mm from the edge of the
pads.

Can I reduce this placement gap to that in the standard now? or am I
interpreting it wrongly?

Next 1206 resistors:
The same subsection says that the grid placement courtyard is to be 4x10
(2mm x 5mm).
This leaves 0.3mm at the ends & 0.1mm along the length.
Surely this is wrong, as it is smaller than the 0805, can someone
clarify this for me.

Next problem is that I have a 1n4002 diode in a package called D1F from
Flint electronics.
Looking through the catalogue shows that DO214 is only a gnats kneecap
different.

This is given a placement courtyard of 8x16 (8mm x 4mm) which leaves
0.6mm at the ends & 0.8mm along the length.

This package is higher than the resistors, so how do I work out the
placement courtyard distance? I know that there can be a difference in
spacing depending upon the height to allow for correct heat transfer,
but how do I work it out?
I also have similar problems with SMT relays, as I need to know how far
away I need to place smaller components, based upon its height.

Next problem is Soldermask clearance.
I'm aware that solder mask is generally to be 0.2mm (0.008") larger than
the pads.
Does this mean that for a square/rectangular pad, would this be 0.1mm
(0.004") each side of the pad?

I know that there has to be some allowance for misalignment of the
board/mask, however I think that I have previously had
this 0.008" bigger each side, giving me little room for running tracks
near the SMT pads.
However now that I need to make a new SMT board, the size of a box of
matches with loads of components on I can see
that I have to reduce my component dimensions accordingly.


Many Thanks - Matthew Lamkin.