What is PCB? How to make a circuit board? - PCB FAQ - Circuit Board Schematic
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What does it take to build Printed Circuit Boards?
In electronics, printed circuit boards, or PCBs, are used to
mechanically support and electrically connect electronic components using
conductive pathways, or traces, etched from copper sheets laminated onto a
non-conductive substrate. Alternative names are printed wiring board or PWB or
etched wiring board.
Printed circuit boards are rugged, inexpensive, and can be highly
reliable. They require much more layout effort and higher initial cost than
either wire-wrapped or point-to-point constructed circuits, but are much
cheaper, faster, and consistent in high volume production.
The two major stages for building a
printed circuit
board are
- Printed Circuit Board Design
- Printed Circuit Board Manufacturing ( PCB
Fabrication )
Printed Circuit Board Design
Usually an electronics or electrical engineer designs the circuit, and a layout
specialist designs the Printed circuit board. The designer must obey numerous
PCB layout
guidelines to design a PCB that functions correctly, yet is inexpensive to
manufacture.
The Circuit Design
The Circuit Diagram, also called the Schematic or Logic
Diagram, maps out the electronics and connections in the most readily readable
form. The designer needs to do background work while producing the Circuit
diagram, researching specifications of components, interaction between
components (especially timing and loading) physical packages, and arrangement
of connector pinouts. The circuit will often start on paper and finish in
Computer Aided Design (CAD) format. The finished circuit diagram, supported by
notes if required, is the main reference document for the design.
Electronic Design Automation (EDA)
PCB designers often use electronic design automation to
produce a layout. The EDA program stores design information, facilitates
editing the design, and can also automate repetitive design tasks.
The first stage is converting the circuit schematic into a net list. The net
list is conceptually a list of component pins and the circuit nodes, or nets
that each pin connects to. Often the schematic capture EDA program, operated by
a circuit design engineer, is responsible for netlist generation, and the
netlist is imported into the PCB layout program.
The next step is to decide the position of each device. The easy way to do this
is to specify a grid of lettered rows and numbered columns where the devices
should go. The computer then assigns pin 1 of each device in the bill of
materials to a grid location. Typically, the operator may assist the automated
placement routine by specifying rooms, or specific regions of the circuit boards, where
certain groups of components should be placed. For example, the parts
associated with a power supply subcircuit might be assigned to a region near
the power input connector. In other cases devices may be manually placed,
either to optimize the electrical performance of the circuit, or to place
components such as knobs, switches, and connectors as required by the
mechanical design of the system.
The computer then explodes the device list into a complete pin list for the
circuit board by using templates from a library of footprints associated with each type
of device. Each footprint is a map of a device's pins, usually with a
recommended pad and drill hole layout for each device. The library allows the
footprint to be drawn only once, and then shared by all devices of that type.
In some systems, high-current pads are identified in the device library, and
the associated nets are flagged for attention by the PCB designer. High current
runs require wider traces, and the designer or circuit design engineer usually
decides the width.
The computer program then merges the netlist (sorted by pin name) with the pin
list (sorted by pin name), transferring the physical coordinates of the pin
list to the netlist. The netlist is then resorted, by net name.
Some systems can optimize the design by swapping the positions of parts and
logic gates to reduce the length of copper runs. Some systems also
automatically discover power pins in the devices, and generate runs or vias to
the nearest power plane or conductor.
The programs then try to route each net in the signal-pin list, finding some
sequence of connections in the available layers. Often layers are assigned to
power and ground, with one layer to vertical, and another to horizontal wires.
The power layers shield the circuits from noise.
The routing problem is equivalent to the traveling salesman problem, and is
therefore NP complete, and therefore not amenable to a perfect solution. One
practical routing algorithm is to pick the pin farthest from the center of the
circuit boards, then use a greedy algorithm to select the next-nearest pin with the same
signal name.
After automated routing, usually there is a list of nets that must be manually
routed.
Once routed, the system may have a series of strategy subroutines to reduce the
production cost of the PCB. For example, one routine might remove unneeded vias
(each via is a drill hole, and costs money to make). Another might round edges
of conductor runs, and widen or move runs apart to maintain safe spacing.
Another strategy might adjust large copper areas so that they form nets, or
large blank areas may get unconnected "checks" of copper. The nets and checks
reduce pollution by extending the life of the etching bath, and speed
production by evening-out the copper concentration in the etching bath.
Some systems provide design rule checking to validate the design for electrical
connectivity and clearance, rules for circuit board manufacture, assembly and test,
heat flow and other errors.
The silk-screen, solder mask, and solder paste stencil(s) are often designed as
auxiliary layers.
Finally, the copper layers are then converted to Gerber files, a format of
numerical control file for a photoplotter. Historically, an additional aperture
file was required to link each numerically designated aperture referred to in
the Gerber file with an actual shape to be plotted. Newer Gerber files embed
the aperture information in the Gerber file itself. The hole locations are
encoded in drill files. The drill files may be sorted to minimize drill-head
movement time, and bit changes.
Most PCBs are composed of between one and sixteen (or even
more) conductive layers separated and supported by layers of insulating
material (substrates) laminated (glued) together. Layers may be connected
together through drilled holes called vias. Either the holes are electroplated
or small rivets are inserted. High-density PCBs may have blind vias, which are
visible only on one surface, or buried vias, which are visible on neither.
Low-end consumer grade PCB substrates frequently are made of
paper impregnated with phenolic resin, sometimes branded "Pertinax". They carry
designations such as XXXP, XXXPC, and FR-2. The material is inexpensive, easy
to machine by drilling, shearing and cold punching, and causes less tool wear
than glass fiber reinforced substrates. The letters "FR" in the designation
indicate Flame Resistance.
High-end consumer and industrial circuit board substrates
are typically made of a material designated FR-4. This consists of a woven
fiberglass mat impregnated with a flame resistant epoxy resin. It can be
drilled, punched and sheared, but due to its abrasive glass content requires
tools made of tungsten carbide for high volume production. Due to the
fiberglass reinforcement, it exhibits about five times higher flexural strength
and resistance to cracking than paper-phenolic types, albeit at higher cost.
PCBs for high power radio frequency (RF) work use plastics
with low dielectric constant (permittivity) and dissipation factor, such as
Rogers® 4000, Rogers® Duroid, DuPont® Teflon® (types GT and GX), polyimide,
polystyrene and cross-linked polystyrene. They typically have poorer mechanical
properties, but this is considered an acceptable engineering tradeoff in view
of their superior electrical performance.
PCBs designed for use in vacuum or in zero gravity, as in
spacecraft, being unable to rely on convection cooling, often have thick copper
or aluminum cores to dissipate heat from electrical components.
Not all circuit boards use rigid core materials. Some are
designed to be very or slightly flexible, using DuPont's® Kapton® polyimide
film, and others. This class of boards, sometimes called flex circuits, or
rigid-flex circuits, respectively, are difficult to create but have many
applications. Sometimes they are flexible to save space (PCBs inside cameras
and hearing aids are almost always made of flex circuits so they can be folded
up to fit into the limited available space). Sometimes, the flexible part of
the circuit board is actually being used as a cable or moving connection to
another board or device. One example of the latter application is the cable
connected to the carriage in an inkjet printer. Power electronic applications
require low-thermal resisivity substrates, with thick copper track to carry
high currents. The main technologies are ceramic-based substrates (Direct
Bonded Copper) and metal-based substrates (Insulated Metal Substrate).
The vast majority of “printed circuit boards” are made by
adhering a layer of copper over the entire substrate, sometimes on both sides,
(creating a “blank PCB”) then removing unwanted copper after applying a
temporary mask (e.g. by etching in ferric chloride), leaving only the desired
copper traces. A few PCBs are made by adding traces to the bare substrate
usually by a complex process of multiple electroplating.
There are three common methods used for the production of printed circuit
boards:
- Silk screen printing uses etch-resistant inks to protect the copper foil.
Subsequent etching removes the unwanted copper. Alternatively, the ink may be
conductive, printed on a blank (non-conductive) board. The latter technique is
also used in the manufacture of hybrid circuits.
- Photoengraving uses a photomask and chemical etching to
remove the copper foil from the substrate. The photomask is usually prepared
with a photoplotter from data produced by a technician using computer-aided PCB
design software. Laser-printed transparencies are sometimes employed for
low-resolution photoplots.
- PCB Milling uses a 2 or 3 axis mechanical milling system
to mill away the copper foil from the substrate. A PCB milling machine
(referred to as a 'PCB Prototyper') operates in a similar way to a plotter,
receiving commands from the host software that control the position of the
milling head in the x, y, and (if relevant) z axis. Data to drive the
Prototyper is extracted from files generated in PCB design software and stored
in HPGL or Gerber file format.
Some PCBs have trace layers inside the PCB and are called
multi-layer PCBs. These are formed by bonding together (using high pressure in
a press) separately etched thin boards.
Holes, or vias, through a PCB are typically drilled with
tiny drill bits made of solid tungsten carbide. The drilling is performed by
automated drilling machines with placement controlled by a drill tape or drill
file. These computer-generated files are also called numerically controlled
drill (NCD) files or "Excellon files". The drill file describes the location
and size of each drilled hole.
When very small vias are required, drilling with mechanical bits is costly
because of high rates of wear and breakage. In this case, the vias may be
evaporated by lasers. Laser-drilled vias typically have an inferior surface
finish inside the hole. These holes are called micro vias.
It is also possible with controlled-depth drilling, laser drilling, or by
pre-drilling the individual sheets of the PCB before lamination, to produce
holes that connect only some of the copper layers, rather than passing through
the entire circuit board. These holes are called blind vias when they connect an
internal copper layer to an outer layer, or buried vias when they connect two
or more internal copper layers.
The walls of the holes, for circuit boards with 2 or more layers, are plated with
copper to form plated-through holes that electrically connect the conducting
layers of the PCB.
The pads and lands to which components will be mounted are
typically plated, because the bare copper is not readily solderable.
Traditionally, any exposed copper was plated with solder. This solder was
traditionally a tin-lead alloy, however new solder compounds are now used to
achieve compliance with the RoHS directive in the EU, which restricts the use
of lead. Edge connectors, made on the sides of some circuit boards, are often gold
plated. Gold plating is also sometimes applied on the whole boards.
Areas that should not be soldered to may be covered with a
polymer solder resist coating. The solder resist prevents short circuits
between nearby component leads.
Line art and text may be printed onto the outer surfaces of
a PCB by silk screening. When space permits the silk screen text can indicate
component designators, switch setting requirements, test points, and other
features helpful in assembling, testing, and servicing circuit boards. In
one sided PCBs, silk screen is also known as the 'red print'.
In through-hole construction, component leads may be
inserted in holes and electrically and mechanically fixed to circuit board with a
molten metal solder.In surface-mount construction, the components are simply
soldered to pads or lands on the outer surfaces of the PCB. Often through-hole
and surface-mount construction must be combined in a single PCB because some
required components are available only in surface-mount packages, while others
are available only in through-hole packages.
Unpopulated boards may be subjected to a bare-board test
where each circuit connection as defined in a netlist is verified as correct on
the finished board. For high-volume production, a Bed of nails tester or
fixture is used to make contact with copper lands or holes on one or both sides
of the board to facilitate testing. A computer will instruct the electrical
test unit to send a small amount of current through each contact point on the
bed-of-nails as required, and verify that such current can be seen on the other
appropriate contact points. For small- or medium-volume circuit boards, flying-probe
testers use moving test heads to make contact with the copper lands or holes to
verify the electrical connectivity of the board under test.
PCBs intended for extreme environments often have a
conformal coat, which is applied by dipping or spraying after the components
have been soldered. The coat prevents corrosion and leakage currents or
shorting due to condensation. The earliest conformal coats were wax. Modern
conformal coats are usually dips of dilute solutions of silicone rubber,
polyurethane, acrylic, or epoxy. Some are engineering plastics sputtered onto
the PCB in a vacuum chamber. Mass-production PCBs have small pads for automated
test equipment to make temporary connections. Sometimes the pads must be
isolated with resistors.
Part of the information provided from
http://en.wikipedia.org/wiki/Printed_circuit_board