In this tutorial, I am going to provide some basic rules and guidelines to design effective PCB on CAD tools for beginners. The beginner must know about measurement units and common technologies used in PCB design. Without knowing these rules, the beginners might confuse always to start with PCB design.
There are two types of measurement units are used in PCB design on CAD tools.
1. Imperial inches (thou)
2. Metric millimeter (mm)
1 thou is also referred as 1 mil which is equal to 1/1000th of an inch. The term ‘mil’ indicates one milli -inch.
1 thou or 1 mil= 1/1000th of an inch
Practically, both these measurement units are used in PCB designing. Generally, thou’s are used for basic ‘design and layout’ requirements like track width, pad size, spacing’s and grids dimensions, etc.
Where metric millimeter is used for ‘mechanical and manufacturing’ type requirements like board dimensions and hole sizes. Most of the manufacturers produce metric size drills, so it is difficult to get imperial size drills in the market. Most of the surface mount components have metric pin spacing and dimensions. While designing such surface mount component footprints, the designers have to use metric grid and pads.
We can convert imperial inches to metric millimeter and vice versa,
Basic design rules for design and layout components in CAD tools
1. Working with grid:
Setting up the grid size is the major part to start your design. First of all you have to layout your board on a fixed grid and this is called snap grid. This grid makes your components, cursor and tracks will snap into fixed grid positions. For a “through hole work” PCB’s 100 thou is a standard placement grid and 50 thou is used for general tracking work like running tracks between through hole tracks.
2. Working with tracks: There is no recommended standard for track sizes. As general rule of thumb, the bigger the track width have lower DC resistances, lower inductance, can be easier and cheaper for the manufacturer to etch, inspect and rework. For beginners, the following are the recommended track width in different scenarios.
a. 25 thou for signal tracks
b. 50 thou for power and ground tracks
c. 10 to 15 thou for tracks going between IC and component pads.
3. Working with pads:
There is an important parameter known as the pad/hole ratio. A simple rule of thumb in PCB design, the pad/hole ratio should be at least 1.8. Or the pad size should be at least 0.5mm larger than the hole size.
4. Working with vias:
Vias connect the tracks from one side of your board to another called electrical stitching. Holes in vias are usually a fair bit smaller the component pads, with 0.5-0.7mm.
5. Working with polygons:
A polygon automatically fills in a desired area with copper, which flows around the other pads and tracks. They are very useful for laying down ground planes. One important rules about polygon placement is make sure you place polygons after you have placed all of your tracks and pads.
For basic through designs, at least 15 thou is a good clearance. With 10 thou or 8 thou being used for more complex and denser surface mount layouts. For 240v mains on PCB’s a minimum of 8mm (315 thou) spacing should be allowed between 24v tracks and isolated signal tracks.
By using these basic design rules you can start your PCB layout with less errors.