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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.
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.
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.
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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