18-09-2012, 04:03 PM
PCB Design Tutorial
[attachment=33984]
Introduction
You've designed your circuit, perhaps even bread boarded a working prototype, and now it's time to turn it into a
nice Printed Circuit Board (PCB) design. For some designers, the PCB design will be a natural and easy
extension of the design process. But for many others the process of designing and laying out a PCB can be a
very daunting task. There are even very experienced circuit designers who know very little about PCB design,
and as such leave it up to the "expert" specialist PCB designers. Many companies even have their own
dedicated PCB design departments. This is not surprising, considering that it often takes a great deal of
knowledge and talent to position hundreds of components and thousands of tracks into an intricate (some say
artistic) design that meets a whole host of physical and electrical requirements. Proper PCB design is very often
an integral part of a design. In many designs (high speed digital, low level analog and RF to name a few) the
PCB layout may make or break the operation and electrical performance of the design. It must be remembered
that PCB traces have resistance, inductance, and capacitance, just like your circuit does.
This article is presented to hopefully take some of the mystery out of PCB design. It gives some advice and
“rules of thumb” on how to design and lay out your PCBs in a professional manner. It is, however, quite difficult to
try and “teach” PCB design. There are many basic rules and good practices to follow, but apart from that PCB
design is a highly creative and individual process. It is like trying to teach someone how to paint a picture.
Everyone will have their own unique style, while some people may have no creative flair at all!
Indeed, many PCB designers like to think of PCB layouts as works of art, to be admired for their beauty and
elegance. “If it looks good, it’ll work good.” is an old catch phrase.
Working to Grids
The second major rule of PCB design, and the one most often missed by beginners, is to lay out your board on a
fixed grid. This is called a “snap grid”, as your cursor, components and tracks will “snap” into fixed grid positions.
Not just any size grid mind you, but a fairly coarse one. 100 thou is a standard placement grid for very basic
through hole work, with 50 thou being a standard for general tracking work, like running tracks between throughhole
pads. For even finer work you may use a 25 thou snap grid or even lower. Many designers will argue over
the merits of a 20 thou grid vs a 25 thou grid for instance. In practice, 25 thou is often more useful as it allows
you to go exactly half way between 50 thou spaced pads.
Why is a coarse snap grid so important? It’s important because it will keep your components neat and
symmetrical; aesthetically pleasing if you may. It’s not just for aesthetics though - it makes future editing,
dragging, movement and alignment of your tracks, components and blocks of components easier as your layout
grows in size and complexity.
A bad and amateurish PCB design is instantly recognisable, as many of the tracks will not line up exactly in the
center of pads. Little bits of tracks will be “tacked” on to fill in gaps etc. This is the result of not using a snap grid
effectively.
Good PCB layout practice would involve you starting out with a coarse grid like 50 thou and using a progressively
finer snap grid if your design becomes “tight” on space. Drop to 25 thou and 10 thou for finer routing and
placement when needed. This will do 99% of boards. Make sure the finer grid you choose is a nice even division
of your standard 100 thou. This means 50, 25, 20, 10, or 5 thou. Don’t use anything else, you’ll regret it.
A good PCB package will have hotkeys or programmable macro keys to help you switch between different snap
grid sizes instantly, as you will need to do this often.
There are two types of grids in a PCB drafting package, a snap grid as discussed, and a “visible” grid. The visible
grid is an optional on-screen grid of solid or dashed lines, or dots. This is displayed as a background behind your
design and helps you greatly in lining up components and tracks. You can have the snap grid and visible grid set
to different units (metric or imperial), and this is often very helpful. Many designers prefer a 100 thou visible grid
and rarely vary from that.
[attachment=33984]
Introduction
You've designed your circuit, perhaps even bread boarded a working prototype, and now it's time to turn it into a
nice Printed Circuit Board (PCB) design. For some designers, the PCB design will be a natural and easy
extension of the design process. But for many others the process of designing and laying out a PCB can be a
very daunting task. There are even very experienced circuit designers who know very little about PCB design,
and as such leave it up to the "expert" specialist PCB designers. Many companies even have their own
dedicated PCB design departments. This is not surprising, considering that it often takes a great deal of
knowledge and talent to position hundreds of components and thousands of tracks into an intricate (some say
artistic) design that meets a whole host of physical and electrical requirements. Proper PCB design is very often
an integral part of a design. In many designs (high speed digital, low level analog and RF to name a few) the
PCB layout may make or break the operation and electrical performance of the design. It must be remembered
that PCB traces have resistance, inductance, and capacitance, just like your circuit does.
This article is presented to hopefully take some of the mystery out of PCB design. It gives some advice and
“rules of thumb” on how to design and lay out your PCBs in a professional manner. It is, however, quite difficult to
try and “teach” PCB design. There are many basic rules and good practices to follow, but apart from that PCB
design is a highly creative and individual process. It is like trying to teach someone how to paint a picture.
Everyone will have their own unique style, while some people may have no creative flair at all!
Indeed, many PCB designers like to think of PCB layouts as works of art, to be admired for their beauty and
elegance. “If it looks good, it’ll work good.” is an old catch phrase.
Working to Grids
The second major rule of PCB design, and the one most often missed by beginners, is to lay out your board on a
fixed grid. This is called a “snap grid”, as your cursor, components and tracks will “snap” into fixed grid positions.
Not just any size grid mind you, but a fairly coarse one. 100 thou is a standard placement grid for very basic
through hole work, with 50 thou being a standard for general tracking work, like running tracks between throughhole
pads. For even finer work you may use a 25 thou snap grid or even lower. Many designers will argue over
the merits of a 20 thou grid vs a 25 thou grid for instance. In practice, 25 thou is often more useful as it allows
you to go exactly half way between 50 thou spaced pads.
Why is a coarse snap grid so important? It’s important because it will keep your components neat and
symmetrical; aesthetically pleasing if you may. It’s not just for aesthetics though - it makes future editing,
dragging, movement and alignment of your tracks, components and blocks of components easier as your layout
grows in size and complexity.
A bad and amateurish PCB design is instantly recognisable, as many of the tracks will not line up exactly in the
center of pads. Little bits of tracks will be “tacked” on to fill in gaps etc. This is the result of not using a snap grid
effectively.
Good PCB layout practice would involve you starting out with a coarse grid like 50 thou and using a progressively
finer snap grid if your design becomes “tight” on space. Drop to 25 thou and 10 thou for finer routing and
placement when needed. This will do 99% of boards. Make sure the finer grid you choose is a nice even division
of your standard 100 thou. This means 50, 25, 20, 10, or 5 thou. Don’t use anything else, you’ll regret it.
A good PCB package will have hotkeys or programmable macro keys to help you switch between different snap
grid sizes instantly, as you will need to do this often.
There are two types of grids in a PCB drafting package, a snap grid as discussed, and a “visible” grid. The visible
grid is an optional on-screen grid of solid or dashed lines, or dots. This is displayed as a background behind your
design and helps you greatly in lining up components and tracks. You can have the snap grid and visible grid set
to different units (metric or imperial), and this is often very helpful. Many designers prefer a 100 thou visible grid
and rarely vary from that.