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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.


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The design schematic is analyzed through simulation using a suitably defined test stimulus, and the operation of the design is verified. If the design does not meet the required specification, then either the design must be modified, or in extreme cases, the design specification must be changed.


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When the design schematic is complete, the PCB layout is created, taking into account layout directives (set by the particular design project) and the manufacturing process design rules.


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On successful completion of the layout, it undergoes analysis by (i) resimulating the schematic design to account for the track parasitic components (usually the parasitic capacitance is used), and (ii) using specially designed signal integrity tools to confirm that the circuit design on the PCB will function correctly. If not, the design layout, schematic, or specification will require modification.


When all steps to layout have been completed, the design is ready for submission for manufacture.


1.2 EMC on the Printed Circuit Board


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?


It should not surprise circuit designers that the layout of the PCB can have a significant effect on the EMC performance of a system, usually more so than the actual choice of components. Consequently, PCB layout is one of the most critical areas of consideration for design to meet EMC regulations.


The fact that there are so many different PCB designs in existence is a testimony to the low cost of producing a PCB, but relaying a complete PCB because of poor layout design causes significant increases in costs not present in the actual material price of the board. Relaying a PCB will create a delay in time to market, hence lost sales revenue. New PCB layouts or changes usually entail new solder masks, reprogramming component placement machines, rewriting the production instructions, etc., hence cost may not be present in the final product part cost, but in the development and production overhead.


Although a significant factor in overall EMC performance, the recommendations for minimising the effect of PCB layout on EMC are general good PCB design practices. The cost of implementing these recommendations is solely in the time taken to ensure that these good design practices are implemented, vigilance and experience are the two main requirements, not necessarily new design software or extensive retraining.


Printed circuit boards (PCBs) are by far the most common method of assembling modern electronic circuits. They comprise a sandwich of one or more insulating layers and one or more copper layers which contain the signal traces and the powers and grounds; the design of the layout of PCBs can be as demanding as the design of the electrical circuit.


Most modern systems consist of multilayer boards of anywhere up to eight layers (or sometimes even more). Traditionally, components were mounted on the top layer in holes which extended through all layers. These are referred to as "through-hole" components. More recently, with the near universal adoption of surface mount components, you commonly find components mounted on both the top and the bottom layers.


The design of the PCB can be as important as the circuit design to the overall performance of the final system. We shall discuss in this chapter the partitioning of the circuitry, the problem of interconnecting traces, parasitic components, grounding schemes, and decoupling. All of these are important in the success of a total design.


PCB effects that are harmful to precision circuit performance include leakage resistances, IR voltage drops in trace foils, vias, and ground planes, the influence of stray capacitance, and dielectric absorption (DA). In addition, the tendency of PCBs to absorb atmospheric moisture (hygroscopicity) means that changes in humidity often cause the contributions of some parasitic effects to vary from day to day.


In general, PCB effects can be divided into two broad categories—those that most noticeably affect the static or DC operation of the circuit, and those that most noticeably affect dynamic or AC circuit operation, especially at high frequencies.


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.


Some other aspects of grounding that must be managed include the control of spurious ground and signal return voltages that can degrade performance. These voltages can be due to external signal coupling, common currents, or simply excessive IR drops in ground conductors. Proper conductor routing and sizing, as well as differential signal-handling and ground isolation techniques enable control of such parasitic voltages.


One final area of grounding to be discussed is grounding appropriate for a mixed-signal, analog/digital environment. Indeed, the single issue of quality grounding can influence the entire layout philosophy of a high performance mixed-signal PCB design—as it well should.


Function of OrCAD PCB Editor in the printed circuit board design process


PCB Editor is used to design the PCB by generating a digital description of the board layers for photoplotters and CNC machines, which are used to manufacture the boards. Separate layers are used for routing copper traces on the top, bottom, and all inner layers; drill hole sizes and locations; soldermasks; silk screens; solder paste; part placement; and board dimensions. These layers are not all portrayed identically in PCB Editor. Some of the layers are shown from a positive perspective, meaning what you see with the software is what is placed onto the board, while other layers are shown from a negative perspective, meaning what you see with the software is what is removed from the board. The layers represented in the positive view are the board outline, routed copper, silk screens, solder paste, and assembly information. The layers represented in the negative view are drill holes and soldermasks. Copper plane layers are handled in a special way, as described next.


Fig. 1.17 shows routed layers (top and bottom and an inner, for example) that PCB Editor shows in the positive perspective. The background is black and the traces and pads on each layer are a different color to make it easier to keep track of visually. The drill holes are not shown because, as mentioned already, the drilling process is a distinct step performed at a specific time during the manufacturing process.


PCBs usually contain epoxy resin, fiberglass, copper, nickel, iron, aluminum and a certain amount of precious metals such as gold and silver; those materials and metals along with electronic parts are attached to the board by a solder containing lead and tin. The main material composition of PCBs was determined and is shown in Table 13.1. From the table, the composition of metals, ceramic and plastics could reach 40%, 30% and 30%, respectively. Further, the concentrations of precious metals in waste PCBs are richer than in natural ores, which makes their recycling important from both economic and environmental perspectives. Table 13.2 shows the average content and value ratio of different metals in PCBs. One can see that Au, Cu, Pd and Ag account for nearly all of the economic material value in waste PCBs. Therefore, PCB recycling focuses on recovering these metals above all else.


For the technology and engineering of very complex boards, the United States, the United Kingdom, Germany and France still have a competitive advantage. There is every reason to believe that the advantage will soon be lost to Asia. Asia produces three-fourths of the world's PCBs, with over 1000 manufacturers in China alone. The PCB industry, like the larger electronics industry, has always had a global component. Only in the past four years, however, has the US manufacturing base faced a serious decline. In 2003, the United States produced 15% of the world's PCBs, trailing Japan, the largest producer at 29%, and China, the second largest at 17%. Taiwan was the fourth largest producer at 13%. Europe produced only 10%, and South Korea 8%. No American company is now among the top ten manufacturers of PCBs. China has overtaken Japan as the leader in PCB production and is forecast to produce $10.6 billion worth of PCBs, accounting for 25% of the world total (LaDou, 2006).
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