[sdiy] SMT PCB layout question

[Tony Clark]

On Thu, Feb 24, 2011 at 10:44 AM, Justin Owen <juzowen at googlemail.com> wrote:
> Hey all,
>
> I'm laying out my first PCB for SMT parts. Please forgive the simple question and the (probably) very obvious answer.
>
> On a PTH board I can run traces to a component's pad on the top or the bottom of a board - or I can use a via.
>
> On a SMT board the pad is only on one surface (i.e. not through hole).
>
> I can use a through hole via to get a trace from bottom of board to top of board - but is there any other way to connect a trace on the bottom of a board to an SMT component on the top of a board?
>
> I've done a PTH version of this same board - but now I'm doing an SMT version I'm finding that much of the space I gained from going to smaller components is being taken up by through-hole vias.

Hi Justin,

You will have to make use of vias to make connections from top to
bottom, no real way around that.  When I am designing boards, I try to
minimize the use of vias as much as possible, so often the package
selection and the placement of SMT parts becomes vital to keeping the
traces on one side.  I have some suggestions, but since I consider PCB
design to be as much of an art as a science, it will really be up to
your ability to visualize and think three dimensionally to solve your
particular PCB puzzle.

Here are a couple of things you can do to minimize vias:

Look for connection points to thru-hole parts and utilize the
thru-hole part as the via (usually no PCB is 100% SMT).  What I mean
by that is that if you have two SMT parts that are connected in a
given distance - but the only way to make the optimal (shortest)
connection is through a trace running on the bottom side of the board
through the use of two vias - but both parts connect to a thru-hole
part (like a connector), try to run the trace for one SMT part to the
thru-hole part on the top layer, then run the trace for the second SMT
part to the thru-hole part on the bottom layer using a single via.

Another way to eliminate some vias is to use larger SMT parts and
utilize the voids between the pads to run traces.  It might seem
counter-intuitive to use a larger part to help eliminate a hole, but
this can actually help alleviate a lot of routing problems.  Most of
the time even larger SMT parts are still a lot smaller than their
equivalent thru-hole counterparts.

Yet another trick that I have done is to incorporate the via under the
pad of the SMT part.  This really only works for passive parts which
have a large enough pad area to accommodate the hole.  Basically you
are making a thru-hole footprint that will accommodate the SMT part.
This is okay if you are hand assembling boards, but I would not advise
doing this for boards that are going to be automated as the hole will
literally suck up all the solder paste used during reflow soldering
assembly and you will end up with a weak solder connection.

And finally, one way you can minimize vias is to put SMT parts on both
sides of the board.  If you have a densely populated board, sometimes
there just isn't enough room for all of the trace connections to exist
on one side, or the amount of vias becomes problematic, then you might
consider moving some parts from the top layer to the bottom layer.
Again, this is okay if you are hand assembling boards or if you are
willing to fork out the extra dough for the automated process of a
double sided SMT board.  Even if you are having the boards automated,
you could opt to have one side automated, then hand assemble the
bottom side.  I would only do this as a last resort option.

Hope some of that helps.

Tony

-- 

Tony Clark
Great Lakes Modular
www.greatlakesmodular.com
Design, Engineering, and Manufacturing Services