Introduction to PCB
Printed Circuit Boards (PCBs) are the most common way of assembling modern electronic circuits. With the increasing use of modern electronics and increasingly growing complexity of electronic circuits, the design of the layout of PCBs is as demanding as the design of electronic circuits. Many companies set up 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 (artistic) design that meets a whole set 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 is also important to notice that PCB traces have resistance, inductance, and capacitance, just like your circuit does.
This article shall serve as an introductory document for beginners. There are several good practices and rules to follow. There are hundreds of CAD software packages available in the market; open source, freeware, shareware, and expensive full version.
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 is 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 standards is IPC-2221, “Generic Standard on 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 This standard superseded the old IPC-D-275 standard (also Military Std 275) which has been used for the last half-century.
Introduction to PCB Technology
Printed circuit boards are mechanical structures that support components and provide electrical conduction paths between circuits. Simple PCB consists of a layer of a non-conductive substrate, which is coated with conductive copper laminate on both sides. This copper layer is etched away to leave desired copper traces, which provides conduction paths between circuit components.
A substrate is a base element of PCB, typically an insulator material. This is also known as laminate. Variety of materials is used as a substrate. It is important to have the right mechanical properties of PCB that suit your application. Substrates of different grades are available. These grades primarily describe flammability, high-temperature stability and moisture absorption of the board.
|FR-1||Paper/phenolic: room temperature punchable, poor moisture resistance.|
|FR-2||Paper/phenolic: suitable for single-sided PCB consumer equipment, good moisture resistance.|
|FR-3||Paper/epoxy: designed for the balance of good mechanical and electrical characteristics.|
|FR-4||Glass cloth/epoxy: excellent mechanical and electrical properties.|
|FR-5||Glass cloth/epoxy: excellent mechanical and electrical properties.|
|G10||Glass cloth/epoxy: high insulation resistance, the highest bond strength of glass laminates, high humidity resistance.|
|G11||Glass cloth/epoxy: high flexural strength retention at high temperature, extreme resistance to solvents.|
FR–4 is commonly used in industrial-quality equipment, while FR–2 is used in high-volume consumer applications. These two board materials appear to be industry standards. Deviating from these standards can limit the number of raw board material suppliers and PCB houses that can fabricate the board because their tooling is already set up for these materials. Nevertheless, there are applications in which one of the other grades may make sense. It may even be necessary to consider Teflon or even ceramic board substrate for very high-frequency applications. One thing can be counted on, however: the more exotic the board substrate, the more expensive it will be.
Several thickness options are available. Most commonly used thicknesses are 0.062” and 0.031”.
Moisture absorption of the material is one of the important aspects while selecting a board material. Just about every desirable performance characteristic of the board will be negatively impacted by moisture. This includes the surface resistance of the board, dielectric leakage, high-voltage breakdown and arcing, and mechanical stability. Also, pay attention to the operating temperature. High operating temperatures can occur in unexpected places, such as in proximity to large digital ICs that are switching at high speeds. The PCB and circuit characteristics may vary with the temperature.
Fig.: PCB Core
A core is formed by the substrate and conducting layers of copper. Typically consists of two sheets of copper laminated over both sides of the substrate. There are two main methods used for copper lamination, rolling and electroplating. Many copper thickness options are also available. Thicknesses of conductors are specified in terms of its weights in ounces per square foot.
The term “weight” refers to the weight in ounces per square foot.
2oz = 70 um = 0.0024”
1oz = 35 um = 0.0012”
0.5oz = 17.5 um = 0.0006”
0.25oz = 8.75 um = 0.0003”
For most applications, 1-ounce copper is sufficient. If the circuit consumes a lot of power, 2-ounce may be better.