Plated Hole Polygons

Vias
Vias connect the tracks from one side of your board to another, by way of a hole in your board. On all but cheap
home made and low end commercial prototypes, vias are made with electrically plated holes, called Plated
Through Holes (PTH). Plated through holes allow electrical connection between different layers on your board.
What is the difference between a via and a pad? Practically speaking there is no real difference, they are both
just electrically plated holes. But there are differences when it comes to PCB design packages. Pads and Vias
are, and should be, treated differently. You can globally edit them separately, and do some more advanced
things to be discussed later. So don’t use a pad in place of a via, and vice-versa.
Holes in vias are usually a fair bit smaller than component pads, with 0.5-0.7mm being typical.
Using a via to connect two layers is commonly called “stitching”, as you are effectively electrically stitching both
layers together, like threading a needle back and forth through material. Throw the term stitching a few times into
a conversation and you’ll really sound like a PCB professional!

Polygons

“Polygons” are available on many PCB packages. A polygon automatically fills in (or “floods”) a desired area with
copper, which “flows” around other pads and tracks. They are very useful for laying down ground planes. Make
sure you place polygons after you have placed all of your tacks and pads.
Polygon can either be “solid” fills of copper, or “hatched”

Clearances
Electrical clearances are an important requirement for all boards. Too tight a clearance between tracks and pads
may lead to “hairline” shorts and other etching problems during the manufacturing process. These can be very
hard to fault find once your board is assembled. Once again, don’t “push the limits” of your manufacturer unless
you have to, stay above their recommended minimum spacing if at all possible.
At least 15 thou is a good clearance limit for basic through hole designs, with 10 thou or 8 thou being used for
more dense surface mount layouts. If you go below this, it’s a good idea to consult with your PCB manufacturer
first.
For 240V mains on PCB’s there are various legal requirements, and you’ll need to consult the relevant standards
if you are doing this sort of work. As a rule of thumb, an absolute minimum of 8mm (315 thou) spacing should be
allowed between 240V tracks and isolated signal tracks. Good design practice would dictate that you would have
much larger clearances than this anyway.
For non-mains voltages, the IPC standard has a set of tables that define the clearance required for various
voltages. A simplified table is shown here. The clearance will vary depending on whether the tracks are on an
internal layers or the external surface. They also vary with the operational height of the board above sea level

Component Placement & Design
An old saying is that PCB design is 90% placement and 10% routing. Whilst the actual figures are of no
importance, the concept that component placement is by far the most important aspect of laying out a board
certainly holds true. Good component placement will make your layout job easier and give the best electrical
performance. Bad component placement can turn your routing job into a nightmare and give poor electrical
performance. It may even make your board unmanufacturable. So there is a lot to think about when placing
components!
Every designer will have their own method of placing components, and if you gave the same circuit (no matter
how simple) to 100 different experienced designers you’d get a 100 different PCB layouts every time. So there is
no absolute right way to place your components. But there are quite a few basic rules which will help ease your
routing, give you the best electrical performance, and simplify large and complex designs.
At this point it is a good idea to give you an idea of the basic steps required to go about laying out a complete board:
Ø Set your snap grid, visible grid, and default track/pad sizes.
Ø Throw down all the components onto the board.
Ø Divide and place your components into functional “building blocks” where possible.
Ø Identify layout critical tracks on your circuit and route them first.
Ø Place and route each building block separately, off the board.
Ø Move completed building blocks into position on your main board.
Ø Route the remaining signal and power connections between blocks.
Ø Do a general “tidy up” of the board.
Ø Do a Design Rule Check.
Ø Get someone to check it
This is by no means a be-all and end-all check list, it’s highly variable depending on many factors. But it is a
good general guide to producing a professional first-class layout.