PCB Design Tutorial.pdf

PCB Design

Tutorial

by David L. Jones

Revision B - June 29th 2008

Freely distributable for educational and personal use.

PCB Design Tutorial by David L. Jones

Introduction............................................................................................................................................... 3

The Old Days......................................................................................................................................... 3

PCB Packages ...................................................................................................................................... 3

Standards.............................................................................................................................................. 3

The Schematic....................................................................................................................................... 4

Imperial and Metric................................................................................................................................. 4

Working to Grids .................................................................................................................................... 5

Working from the top .............................................................................................................................. 6

Tracks................................................................................................................................................... 6

Pads ..................................................................................................................................................... 7

Vias ...................................................................................................................................................... 8

Polygons ............................................................................................................................................... 8

Clearances ............................................................................................................................................ 8

Component Placement & Design ................................................................................................................ 9

Basic Routing ...................................................................................................................................... 11

Finishing Touches ................................................................................................................................ 13

Single Sided Design ............................................................................................................................. 13

Double Sided Design ............................................................................................................................ 14

Other Layers ........................................................................................................................................... 14

Silkscreen ........................................................................................................................................... 14

Solder Mask ........................................................................................................................................ 14

Mechanical Layer................................................................................................................................. 15

Keepout .............................................................................................................................................. 15

Layer Alignment ................................................................................................................................... 15

Netlists ............................................................................................................................................... 15

Rats Nest............................................................................................................................................ 16

Design Rule Checking .......................................................................................................................... 16

Forward and Back Annotation................................................................................................................ 17

Multi layer Design ................................................................................................................................ 17

Power Planes ...................................................................................................................................... 18

Good Grounding................................................................................................................................... 19

Good Bypassing .................................................................................................................................. 19

High Frequency Design Techniques ....................................................................................................... 19

Double Sided Loading........................................................................................................................... 20

Auto Routing........................................................................................................................................ 20

Auto Placement ................................................................................................................................... 21

Design For Manufacturing......................................................................................................................... 21

Panelization......................................................................................................................................... 21

Tooling Strips....................................................................................................................................... 21

Fiducial Marks ..................................................................................................................................... 22

Thermal Relief...................................................................................................................................... 22

Soldering............................................................................................................................................. 22

Basic PCB Manufacturing...................................................................................................................... 23

Surface Finishies ................................................................................................................................. 24

Electrical Testing ................................................................................................................................. 24

Signature............................................................................................................................................. 24

Submitting your design for manufacturing................................................................................................ 25

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PCB Design Tutorial by David L. Jones

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.

Lets have a go shall we...

The Old Days

Back in the pre-computer CAD days, PCBs were designed and laid out by hand using adhesive tapes and pads

on clear drafting film. Many hours were spent slouched over a fluorescent light box, cutting, placing, ripping up,

and routing tracks by hand. Bishop Graphics, Letraset, and even Dalo pens will be names that evoke fond, or not

so fond memories. Those days are well and truly gone, with computer based PCB design having replaced this

method completely in both hobbyist and professional electronics. Computer based CAD programs allow the

utmost in flexibility in board design and editing over the traditional techniques. What used to take hours can now

be done in seconds.

PCB Packages

There are many PCB design packages available on the market, a few of which are freeware, shareware, or

limited component full versions. Professionals use the expensive high end Windows based packages such as

Protel, Mentor, or Orcad. Hobbyists use Eagle's Cadsoft program which has great functionality and is low cost

compared to the more expensive listed above. Eagle also has a free version of their Cadsoft software which is

only limited by the size of the board that you can create. Be cautious of some of the so called "free" software as

these force you to use the specific manufacturer of the software and do not allow you to shop your design

around to get the best pricing. Some to avoid are PCB123, ExpressPCB, PCB Pool and PCB Artist.

This article does not focus on the use of any one package, so the information can be applied to almost any PCB

package available. There is however, one distinct exception. Using a PCB only package, which does not have

schematic capability, greatly limits what you can do with the package in the professional sense. Many of the

more advanced techniques to be described later require access to a compatible schematic editor program. This

will be explained when required.

Standards

There are industry standards for almost every aspect of PCB design. These standards are controlled by the

former Institute for Interconnecting and Packaging Electronic Circuits, who are now known simply as the IPC

(www.ipc.org). There is an IPC standard for every aspect of PCB design, manufacture, testing, and anything else

that you could ever need. The major document that covers PCB design is IPC-2221, “Generic Standard on

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PCB Design Tutorial by David L. Jones

Printed Board Design”. This standard superseded the old IPC-D-275 standard (also Military Std 275) which has

been used for the last half century.

Local countries also have their own various standards for many aspects of PCB design and manufacture, but by

and large the IPC standards are the accepted industry standard around the world.

Printed Circuit Boards are also known (some would say, more correctly known) as Printed Wiring Boards, or

simply Printed Boards. But we will settle on the more common term PCB for this article.

The Schematic

Before you even begin to lay out your PCB, you MUST have a complete and accurate schematic diagram. Many

people jump straight into the PCB design with nothing more than the circuit in their head, or the schematic

drawn on loose post-it notes with no pin numbers and no order. This just isn’t good enough, if you don’t have an

accurate schematic then your PCB will most likely end up a mess, and take you twice as long as it should.

“Garbage-in, garbage-out” is an often used quote, and it can apply equally well to PCB design. A PCB design is

a manufactured version of your schematic, so it is natural for the PCB design to be influenced by the original

schematic. If your schematic is neat, logical and clearly laid out, then it really does make your PCB design job a

lot easier. Good practice will have signals flowing from inputs at the left to outputs on the right. With electrically

important sections drawn correctly, the way the designer would like them to be laid out on the PCB. Like putting

bypass capacitors next to the component they are meant for. Little notes on the schematic that aid in the layout

are very useful. For instance, “this pin requires a guard track to signal ground”, makes it clear to the person

laying out the board what precautions must be taken. Even if it is you who designed the circuit and drew the

schematic, notes not only remind yourself when it comes to laying out the board, but they are useful for people

revi ewing the design.

Your schematic really should be drawn with the PCB design in mind.

It is outside the scope of this article to go into details on good schematic design, as it would require a complete

article in its own right.

Imperial and Metric

The first thing to know about PCB design is what measurement units are used and their common terminologies,

as they can be awfully confusing!

As any long time PCB designer will tell you, you should always use imperial units (i.e. inches) when designing

PCBs. This isn’t just for the sake of nostalgia, although that is a major reason! The majority of electronic

components were (and still are) manufactured with imperial pin spacing. So this is no time to get stubborn and

refuse to use anything but metric units, metric will make laying out of your board a lot harder and a lot messier. If

you are young enough to have been raised in the metric age then you had better start learning what inches are

all about and how to convert them.

An old saying for PCB design is “thou shall use thous”. A tad confusing until you know what a “thou” is.

A “thou” is 1/1000th of an inch, and is universally used and recognised by PCB designers and manufacturers

everywhere. So start practicing speaking in terms of “10 thou spacing” and “25 thou grid”, you’ll sound like a

professional in no time!

Now that you understand what a thou is, we’ll throw another spanner in the works with the term “mil” (or “mils”). 1

“mil” is the same as 1 thou, and is NOT to be confused with the millimeter (mm), which is often spoken the

same as “mil”. The term “mil” comes from 1 thou being equal to 1 mili inch. As a general rule avoid the use of

“mil” and stick to “thou”, it’s less confusing when trying to explain PCB dimensions to those metricated non-PCB

people.

Some PCB designers will tell you not to use metric millimeters for ANYTHING to do with a PCB design. In the

practical world though, you’ll have to use both imperial inches (thous) and the metric millimeter (mm). So which

units do you use for what? As a general rule, use thous for tracks, pads, spacings and grids, which are most of

your basic “design and layout” requirements. Only use mm for “mechanical and manufacturing” type

requirements like hole sizes and board dimensions.

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PCB Design Tutorial by David L. Jones

You will find that many PCB manufacturers will follow these basic guidelines also, for when they ask you to

provide details for a quote to manufacture your board. Most manufacturers use metric size drills, so specifying

imperial size holes really is counterproductive and can be prone to errors.

Just to confuse the issue even further, there are many components (new surface mount parts are an example)

which have metric pin spacing and dimensions. So you’ll often have to design some component footprints using

metric grids and pads. Many component datasheets will also have metric dimensions even though the spacing

are designed to an imperial grid. If you see a “weird” metric dimension like 1.27mm in a component, you can be

pretty sure it actually has a nice round imperial equivalent. In this case 1.27mm is 50 thou.

Yes, PCB design can be confusing!

So whatever it is you have to do in PCB design you’ll need to become an expert at imperial to metric conversion,

and vice-versa. To make your life easier though, all the major PCB drafting packages have a single “hot key” to

convert between imperial and metric units instantly (“Q” on Protel for instance). It will help you greatly if you

memorise a few key conversions, like 100 thou (0.1 inch) = 2.54mm , and 200 thou (0.2 inch) = 5.08mm etc

Values of 100 thou and above are very often expressed in inches instead of thous. So 0.2 inch is more

commonly used than 200thou.

1 inch is also commonly known as 1 “pitch”. So it is common to hear the phrase “0.1 inch pitch”, or more simply

“0.1 pitch” with the inches units being assumed. This is often used for pin spacing on components.

100 thou is a basic “reference point” for all aspects of PCB design, and a vast array of common component lead

spacing are multiples or fractions of this basic unit. 50 and 200 thou are the most common.

Along with the rest of the world, the IPC standards have all been metricated, and only occasionally refer to

imperial units. This hasn’t really converted the PCB industry though. Old habits die hard, and imperial still reigns

supreme in many areas of practical usage.

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 through-

hole 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.

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