SOIC, PLCC surface mount pads & spacing

[Fred Martin]

I'm doing some PCB layout with a couple of SMT components that I
haven't used before. Does anyone know (I figure this is an
off-the-top-of-your-head question for many):

* preferred size of SMT pad for SOIC, PLCC?
* inter-row pad spacing for a 28-pin SOIC? Measuring the device, it
seems to be about 7/16", but that doesn't seem right. How about a
narrow 14- or 16-pin SOIC?

Post or reply is fine. Thanks much.

-Fred
--
Fred Martin | fr...@media.mit.edu | (617) 253-5108 | 20 Ames St. Rm. E15-320
Epistemology and Learning Group, MIT Media Lab | Cambridge, MA 02139 USA

[Juha Kuusama]

In <1994May11....@news.media.mit.edu> fr...@media.mit.edu (Fred Martin) writes:
> ...

> * preferred size of SMT pad for SOIC, PLCC?

This is a more involved question than you actually think it is. The
simple answer for one-off designs to be assembled by hand tools is
"anything the device fits on". For automatic assembly you first decide
the soldering method (reflow or wave). If wave, you also decide the
direction of the wave (consult the factory!).

I don't even try to describe the various patterns for wave soldering:
there are "solder thief" patterns to shape the wave around the component
and the first pads into the wave are likely of different sizes form the rest.
Even for reflow soldering the recommended pad sizes are different for
different packages. There might be dummy pads and traces. There might
be limitations or recommendations in the orientation of components,
like distance between big and small components, orientation of the
conponents regards to the wave etc. Consult your board and IC manufacturer
for guidelines. (Hint: Philips has good guideline book which is very easy
to read and use. Free for big enough customers, no idea for the small
customer price :-) ).
--
Juha Kuusama
Sample Rate Systems Oy
e-mail: Juha.K...@mail.sci.fi

disclaimer: I'm posting from a machine called sci.fi. I don't really need one.