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Basic Routing

Posted on August 30, 2012 by koosha

Now
it’s time for some basic routing rules. Routing is also known as
“tracking”.
Routing is the process of laying down tracks to connect
components on your board. An electrical connection between two or more pads is
known as a “net”.

Keep nets as short as possible. The longer your total track length, the
greater it’s resistance, capacitance and inductance. All of which can be
undesirable factors.

Tracks should only have angles of 45 degrees. Avoid the use of right
angles, and under no circumstances use an angle greater than 90
degrees.

This is important to give a professional and neat appearance to your board.
PCB packages will have a mode to enforce 45 degree movements, make use of
it.

There should never be a need to turn it off. Contrary to popular belief,
sharp right angle corners on tracks don’t produce measurable EMI or other
problems. The reasons to avoid right angles are much simpler – it just doesn’t
look good, and it may have some manufacturing implications.

Forget nice rounded track corners, they are harder and slower to place and
have no real advantage. Stick to 45 degree increments. Rounded track bends
belong to the pre-CAD taped artwork era.

“Snake” your tracks around the board, don’t just go “point to point”.
Point to point tracking may look more efficient to a beginner at first, but
there are a few reasons you shouldn’t use it. The first is that it’s ugly,
always an important factor in PCB design! The second is that it is not very
space efficient when you want to run more tracks on other layers.

Enable your Electrical grid, which is sometimes referred to as a “snap to
center” or “snap to nearest” option. Let the software find the centers of pads
and ends of tracks automatically for you. This is great for when you have pads
and tracks which aren’t lined up to your current snap grid. If you don’t have
these options enabled then you may have to keep reducing your snap grid until
you find one that fits. Far more trouble than it’s worth. There is almost
never a reason to have these options disabled.

Always take your track to the center of the pad, don’t make your track and
pad “just touch”. There are few reasons for this. The first is that it’s
sloppy and unprofessional. The second is that your program may not think that
the track is making electrical connection to the pad. Proper use of a snap
grid and electrical grid will avoid problems here.

Use a single track, not multiple tracks tacked together end to end. It may
make no difference to the look of your final board, but it can be a pain for
future editing. Often you’ll have to extend a track a bit. In this case it’s
best to delete the old one and place a new one. It may take a few extra
seconds, but it’s worth it. People looking at your finished board may not
know, but YOU’LL know! It’s the little touches like this that set good PCB
designers apart.

Make sure your tracks go right through the exact center of pads and
components, and not off to one side. Use of the correct snap grid will ensure
that you get this right every time. If your track doesn’t go through the exact
center then you are using the wrong snap grid. Why do you need to do this? It
makes your board neater and more symmetrical, and it gives you the most
clearance.

Only take one track between 100 thou pads unless absolutely necessary.
Only on large and very dense designs should you consider two tracks between
pads. Three tracks between pads is not unheard of, but we are talking
seriously fine tolerances here.

For high currents, use multiple vias when going between layers. This will
reduce your track impedance and improve the reliability. This is a general
rule whenever you need to decrease the impedance of your track or power
plane.

Don’t “drag” tracks to angles other than 45 degrees

“Neck down” between pads where possible. Eg, a 10 thou track through two
60 thou pads gives a generous 15 thou clearance between track and pad.

If your power and ground tracks are deemed to be critical, then lay them
down first. Also, make your power tracks as BIG as possible.

Keep power and ground tracks running in close proximity to each other if
possible, don’t send them in opposite directions around the board. This lowers
the loop inductance of your power system, and allows for effective
bypassing.

Keep things symmetrical. Symmetry in tracking and component placement is
really nice from a professional aesthetics point of view.

Don’t leave any unconnected copper fills (also called “dead copper”),
ground them or take them out. If you are laying out a non-plated through
double sided board, then there are some additional things to watch out for.
Non-plate through holes require you to solder a link through the board on both
the top and bottom layer.

Do not place vias under components. Once the component is soldered in
place you won’t be able to access the joint to solder a feed through. The
solder joint for the feed through can also interfere with the compnent.

Try and use through hole component legs to connect top tracks to bottom
tracks. This minimises the number of vias. Remember that each via adds two
solder joints to your board. The more solder joints you have, the less
reliable your board becomes. Not to mention that that it takes a lot longer to
assemble.

This entry was posted in design by koosha. Bookmark the permalink.

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