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PCB Layout Program

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 (PWB),and etched wiring board. Populating the board with electronic components forms a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).

PCBs 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.

History

The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap can be more efficient.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.

In recent years, the use of surface mount parts has gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

Physical composition

Most PCBs are composed of between one and twenty-four conductive layers separated and supported by layers of insulating material (substrates) laminated (glued with heat, pressure & sometimes vacuum) together.

The most common substrate for single-layer PCBs is FR-2. The most common substrate for multi-layer PCBs is FR-4. Other substrates include power electronic substrate and Kapton (used to make flexible electronics). The conductive layers are almost invariably made of copper, which sometimes is gold-coated.

Layers may be connected together through drilled holes called vias. To form an electrical connection, the holes are either electroplated or small rivets are inserted. Even though they may not form electrical connections to all layers, these holes are typically drilled completely through the PC board. The exception are high-density PCBs, which may have blind vias (which are visible only on one surface), or buried vias (which are visible on neither).

Booby Holden is a melt!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Safety Certification (US)

Safety Standard UL 796 covers component safety requirements for printed wiring boards boards for use as components in devices or appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.

The boards may use organic or inorganic base materials in a single or multilayer, rigid or flexible form. Circuitry construction may include etched, die stamped, precut, flush press, additive, and plated conductor techniques. Printed-component parts may be used.

The suitability of the pattern parameters, temperature and maximum solder limits shall be determined in accordance with the applicable end-product construction and requirements.

"Cordwood" construction

Cordwood construction can give large space-saving advantages and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems). In 'cordwood' construction, two leaded components are mounted axially between two parallel planes. Instead of soldering the components, they were connected to other components by thin nickel tapes welded at right angles onto the component leads. To avoid shorting together of different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards would allow component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel leaded components had to be used to allow the interconnecting welds to be made. Some versions of cordwood construction used single sided PCBs as the interconnection method (as pictured). This meant that normal leaded components could be used.

Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction now appears to have fallen into disuse, probably because high packing densities can be more easily achieved using surface mount techniques and integrated circuits.

Multiwire boards

Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting matrix (often plastic resin). It was used during the 1980s and 1990s. (Augat Inc., U.S. Patent 4,648,180)

Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed to forget completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no complex algorithms to use even for high density designs), reduced cross talk (an electrical phenomenon appearing where a current in one wire generates another current in another conductor, that is highly amplified when wires are parallel - this nearly never happens in Multiwire), but costs too high to compete with cheaper PCB technologies when large quantities are needed.

Surface-mount technology

Surface-mount technology was developed in the 1960s, gained momentum in Japan in the early 1980s and became widely used globally by the mid 1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be directly soldered to the surface of the PCB. Components became much smaller and component placement on both sides of the board became far more common with surface-mounting than through-hole mounting, allowing much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor cost and greatly increasing production rates. SMDs can be one-quarter to one-tenth the size and weight, and passive components can be one-half to one-quarter the cost of through-hole parts. Integrated circuits (where the chip itself is the most expensive part) are often priced the same regardless of package type however. As of 2006, some wire-ended components, such as small signal switch diodes (philips 1N4148 for instance), are actually significantly cheaper than corresponding SMD versions.

References

Coombs, Clyde F., Jr. (Ed.) (1995). Printed Circuits Handbook, Fourth Edition, McGraw-Hill ISBN 0-07-012754-9.

External links

Design guidelines

Digital System Design Guidelines by Tony Goodloe
PWB/PCB Design – Analog, RF & EMC Considerations in Printed Wiring Board Design
High Frequency Laminate Information by Chet Guiles

Standards and Specifications

Others

@@ Line 25: / Line 25: @@
 Layers may be connected together through drilled holes called [[via (electronics)|vias]]. To form an electrical connection, the holes are either electroplated or small rivets are inserted. Even though they may not form electrical connections to all layers, these holes are typically drilled completely through the PC board.  The exception are high-density PCBs, which may have ''blind vias'' (which are visible only on one surface), or ''buried vias'' (which are visible on neither).
+Booby Holden is a melt!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
-==Manufacturing==
-[[Image:PCB design and realisation smt and through hole.png|thumb|250px|A PCB (left) as a design on a computer and (right) realised as a board and populated with components. The board is double sided, with through-hole plating, green solder resist, white solder paste, and white silkscreen printing. Both surface mount and through-hole components have been used.]]
-===Patterning (etching) ===
-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 (eg. by etching), leaving only the desired copper traces. A few PCBs are made by ''adding'' traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple [[electroplating]] steps.
-There are three common "subtractive" methods (methods that remove copper) used for the production of printed circuit boards:
-# '''[[Silk screen|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 circuit]]s.
-# '''[[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 CAM, or [[computer-aided manufacturing]] software. Laser-printed transparencies are typically employed for ''phototools''; however, direct laser imaging techniques are being employed to replace phototools for high-resolution requirements.
-# '''[[PCB milling]]''' uses a two or three-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|Gerber]] file format.
-"Additive" processes also exist. The most common is the "semi-additive process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces.
-The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (vias) in the circuit board.
-===Lamination===
-Some PCBs have trace layers inside the PCB and are called ''multi-layer'' PCBs. These are formed by bonding together separately etched thin boards.
-===Drilling===
-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 file]]s". 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 [[laser]]s. 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 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 boards with 2 or more layers, are plated with copper to form ''plated-through holes'' that electrically connect the conducting layers of the PCB.  For multilayer boards, those with 4 layers or more, drilling typically produces a ''smear'' comprised of the bonding agent in the laminate system.  Before the holes can be plated through, this ''smear'' must be removed by a chemical ''de-smear'' process, or by ''plasma-etch''.
-===Exposed conductor plating and coating===
-The pads and lands to which components will be mounted are typically plated, because bare copper oxidizes quickly, and therefore is not readily solderable. Traditionally, any exposed copper was plated with [[solder]]. This solder was 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.  Other platings used are OSP (organic surface protectant), immersion silver, electroless nickel with immersion gold coating (ENIG), and direct gold.  [[Edge connector]]s, placed along one edge of some boards, are often [[gold plated]].
-===Solder resist===
-Areas that should not be soldered to may be covered with a polymer ''solder resist'' (''solder mask'') coating. The solder resist prevents solder from bridging between conductors and thereby creating short circuits.  Solder resist also provides some protection from the environment.
-===Screen printing===
-Line art and text may be printed onto the outer surfaces of a PCB by [[Screen-printing|screen printing]]. When space permits, the screen print text can indicate component designators, switch setting requirements, test points, and other features helpful in assembling, testing, and servicing the circuit board.
-Screen print is also known as the ''silk screen'', or, in one sided PCBs, the ''red print''.
-===Test===
-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 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.
-===Populating===
-After the PCB is completed, electronic components must be attached to form a functional ''printed circuit assembly'', or PCA. In ''through-hole'' construction, component leads may be inserted in holes and electrically and mechanically fixed to the board with a molten metal solder, while 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 PCA because some required components are available only in surface-mount packages, while others are available only in through-hole packages.
-Again, [[JEDEC]] guidelines for PCB component placement, soldering, and inspection are commonly used to maintain [[quality control]] in this stage of PCB manufacturing.
-After the board is populated, the populated board may be tested with an [[in circuit test|in-circuit test]] system. To facilitate this test, PCBs may be designed with extra pads to make temporary connections. Sometimes these pads must be isolated with resistors. The in-circuit test may also exercise [[boundary scan]] test features of some components. In-circuit test systems may also be used to program nonvolatile memory components on the board.
-In boundary scan testing, test circuits integrated into various ICs on the board form temporary connections between the pcb traces to test that the ICs are mounted correctly. Boundary scan testing requires that all the ICs to be tested use a standard test configuration procedure, the most common one being the Joint Test Action Group ([[JTAG]]) standard.
-===Protection and packaging===
-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.
-Many assembled PCBs are [[Electrostatic discharge|static]] sensitive, and therefore must be placed in [[antistatic bag]]s during transport. When handling these boards, the user must be [[Ground (electricity)|earthed]]; failure to do this might transmit an accumulated static charge through the board, damaging or destroying it. Even bare boards are sometimes static sensitive. Traces have gotten so fine that it's quite possible to blow an etch off the board (or change its characteristics) with a static charge. This is especially true on non-traditional PCBs such as [[Multi-Chip Module|MCMs]] and [[microwave]] PCBs.
 ==Safety Certification (US)==