Printed Circuit Boards
OrCAD PCB Editor is based on Allegro PCB Editor, so this book will be useful to new Allegro printed circuit boards Editor users as well. Allegro PCB Editor is a powerful, full-featured design tool. While OrCAD PCB Editor has inherited many of those features, including a common file format, it does not possess all of the capabilities available to the Allegro PCB tiers, such as Allegro High-Speed Option, Analog/RF Option, FPGA System Planner, Design Planning, and Miniaturization Option. Consequently most of the basic tools and features are described here, but only a few of the more-advanced tools are covered, as outlined later.
PC board traces must be sized appropriately (both in width and thickness, or copper weight10) to carry the current that you need without excessive temperature rise. A rule of thumb is that a 10-mil-wide, 1-ounce PC board trace can carry in excess of 500 mA with a 20 °C temperature rise above ambient. PC board copper weight vs. trace thickness is shown in Table 15.5. An estimate of the current-carrying capability for 20 °C temperature rise of PC board traces is shown in Figure 15.12. The fusing current (Figure 15.13) for PC board traces is significantly higher.
OK – So What’s a Printed Circuit Board?
I’ve just mentioned a printed circuit board, but what exactly is a printed circuit board? Well, look inside any modern electronics appliance (television, computer, mobile phone, etc.) or even many electrical appliances (washing machine, iron, kettle, etc.) and you’ll see a printed circuit board – often known by the multilayer PCB.
A printed circuit board is a thin baseboard (about 1.5 mm) of insulating material such as resin-bonded paper or fiberglass, with an even thinner layer of copper (about 0.2 mm) on one or both surfaces. (If copper is only on one surface it’s then known as single-sided printed circuit board; if copper is on both surfaces it’s known as double-sided printed circuit board.) The copper on the surface of a printed circuit board has been printed as a circuit (yes, OK, that’s why it’s called printed circuit board – geddit?), so that components on the printed circuit board can be soldered to the copper, and thus be connected to other components similarly soldered. Photo 12.1 shows a fairly modern printed circuit board to show you what they look like. The printed circuit board shown is quite a complex one, with hundreds of components – from a computer actually – but the printed circuit board in a washing machine, say, may only hold a handful of components. Photo 12.2 shows how the copper on a printed circuit board comprises a pattern of copper – sometimes called the copper track – rather than a solid layer. This pattern or track is the key to making connections between components.
PCB design begins with an insulating base and adds metal tracks for electrical interconnect and the placement of suitable electronic components to define and create an electronic circuit that performs a required set of functions.
The term printed isn’t exactly an accurate description of how the copper on the surface of a printed circuit board is formed. In fact, all printed circuit boards start life with a complete layer of copper on one or both sides of the insulating board. Then, unwanted copper is removed from the board, leaving the wanted copper pattern behind. Typically, this copper removal is usually – though not always – done by etching the copper away using strong chemicals.
Figure 12.1 shows a cross-section of a simple printed circuit board. In it you can see the insulating board, the copper track, and the holes for component leads. Components fit to the printed circuit quite easily. Their leads are inserted through the board holes, and are then soldered to the copper track. Figure 12.2 shows how this works. In terms of the amateur enthusiast in electronics, simple (and relatively inexpensive) hand-tools are all that are required in this soldering process – we’ll look at these, and how to use them, later.
Initially, a design specification (document) is written that identifies the required functionality of the thick copper PCB. From this, the designer creates the circuit design, which is entered into the PCB design tools.
Almost every printed circuit board (PCB) is different and completely application specific. Even within similar products the PCB can be different, for example open two PCs from different manufacturers, with the same processor, clock speed, keyboard interface, etc., the actual PCB layout will be different. This diversity means that every high tg PCB has a unique level of EMC performance, so what can possibly be done to ensure that this is within certain limits?
Another very broad area of high frequency PCB design is the topic of grounding. Grounding is a problem area in itself for all analog and mixed-signal designs, and it can be said that simply implementing a PCB-based circuit does not change the fact that proper techniques are required. Fortunately, certain principles of quality grounding, namely the use of ground planes, are intrinsic to the PCB environment. This factor is one of the more significant advantages to PCB-based analog designs, and appreciable discussion in this section is focused on this issue.