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Introduction to PCB Design
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.
There are many PCB design packages available on the market, a few of which are freeware, shareware, or limited component full versions. Protel is the defacto industry standard package in Australia. Professionals use the expensive high end Windows based packages such as 99SE and DXP. Hobbyists use the excellent freeware DOS based Protel AutoTrax program, which was, once upon a time, the high-end package of choice in Australia. Confusingly, there is now another Windows based package also called AutoTrax EDA. This is in no way related to the Protel software. 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.
Before you even begin to layout 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 also they are useful for people reviewing the design.
Your schematic really should be drawn with the PCB design in mind.
Grounding is fundamental to the operation of many circuits. Good or bad grounding techniques can make or break your design. There are several grounding techniques which are always good practices to incorporate into any design.
· Use copper, and lots of it. The more copper you have in your ground path, the lower the impedance. This is highly desirable for many electrical reasons. Use polygon fills and planes where possible.
· Always dedicate one of your planes to ground on multi-layer boards. Make it the layer closest to the top layer.
· Run separate ground paths for critical parts of your circuit, back to the main filter capacitor(s). This is known as “star” grounding, because the ground tracks all run out from a central point, often looking like a star. In fact, try and do this as matter of course, even if your components aren’t critical. Separate ground lines keep current and noise from one component from affecting other components.
· If using a ground plane, utilize “split” plane techniques to give effective star grounding.
· “Stitch” required points straight through to your ground plane, don’t use any more track length than you need.
· Use multiple vias to decrease your trace impedance to ground.
Active components and points in your circuit that draw significant switching current should always be “bypassed”. This is to “smooth” out your power rail going to a particular device. “Bypassing” is using a capacitor across your power rails as physically and electrically close to the desired component or point in your circuit as possible. A typical bypass capacitor value is 100nF, although other values such as 1uF, 10nF and 1nF are often used to bypass different frequencies. You can even have two or three different value capacitors in parallel. When bypassing, you cannot replace multiple capacitors with one single capacitor, it defeats the entire purpose of bypassing! It is not uncommon for a large design to have hundreds of bypass capacitors.
As a general rule, you should use at least one bypass capacitor per IC or other switching component if possible. Common values of bypass capacitors are 100nF for general purpose use, 10nF or 1nF for higher frequencies, and 1uF or 10uF for low frequencies.
Special low Equivalent Series Resistance (ESR) capacitors are sometimes used on critical designs such as switch mode power supplies.
Basic PCB Manufacture
A PCB usually consists of a blank fiberglass substrate (“the board”), which is usually 1.6mm thick. Other common thicknesses are 0.8mm and 2.4mm. There is many types PCB substrate material, but by far the most common is a standard woven epoxy glass material known as FR4. This material has standard known properties, typical values of which are shown in the accompanying table. The most often used parameter is probably the dielectric constant. This figure is important for calculating high-speed transmission line parameters and other effects. An FR4 PCB is made up of glass and resin. Glass has a dielectric constant of approximately 6, and the resin has a dielectric constant of approximately 3. So an FR4 PCB can typically have a figure ranging from under 4, to almost 5. If you need an exact figure you will have to consult with your PCB manufacturer.
Typical FR4 Properties:
Dielectric Constant 3.9 to 4.8
Dielectric Breakdown 39kV/mm
Water Absorption <1.3%
Dissipation Factor 0.022
Thermal Expansion 16-19ppm/degC