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8 References
9 External links
9.Do It Yourself (DIY) guides
9.Copper foil and prepreg are typically laminated together with epoxy resin.PCB (left) as a design on a computer and (right) realised as a board and populated with components.Patterning (etching)
The vast majority of printed circuit boards are made by bonding 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.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.Subsequent etching removes the unwanted copper.Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.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.NCD) files or "Excellon files".The drill file describes the location and size of each drilled hole.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 and no outer layers.For multilayer boards, those with 4 layers or more, drilling typically produces a smear comprised of the bonding agent in the laminate system.Exposed conductor plating and coating
The places 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.RoHS directive in the EU, which restricts the use of lead.Other platings used are OSP (organic surface protectant), immersion silver, immersion tin, electroless nickel with immersion gold coating (ENIG), and direct gold.Edge connectors, placed along one edge of some boards, are often gold plated.The solder resist prevents solder from bridging between conductors and thereby creating short circuits.This is because a company may order large numbers of PCB's from a PCB specialty house for a product that it manufactures, but then it conducts final assembly of the boards only when it runs out of its existing stock of fully assembled boards.The tin plating process, protects the copper traces and also helps in soldering the boards when the need arises.Screen printing
Line art and text may be printed onto the outer surfaces of a PCB by screen printing.Lately some digital printing solutions have been developed to substitute the traditional screen printing process.This technology allows printing variable data onto the PCB, including serialization and barcode information for traceability purposes.Bed of nails tester, a fixture or a Rigid needle adapter is used to make contact with copper lands or holes on one or both sides of the board to facilitate testing.An "open" is between two points that should be connected and are not.In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.There are a variety of soldering techniques used to attach components to a PCB.Some parts are impossible to solder by hand, such as Ball Grid Array (BGA) packages.JEDEC guidelines for PCB component placement, soldering, and inspection are commonly used to maintain quality control in this stage of PCB manufacturing.When boards fail the test, technicians may desolder and replace failed components.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.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.PCBs such as MCMs and microwave PCBs.Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat deflection, and direct support of live electrical parts.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.Some versions of cordwood construction used single sided PCBs as the interconnection method (as pictured).Multiwire), but the cost is too high to compete with cheaper PCB technologies when large quantities are needed.Japan in the early 1980s and became widely used globally by the mid 1990s.Surface mounting lends itself well to a high degree of automation, reducing labour cost and greatly increasing production and quality rates.Integrated circuits (where the chip itself is the most expensive part) are often priced the same regardless of package type however.Do It Yourself (DIY) guides
Making PCBs with a laser printer and photo paper, no UV machine needed anymore.All text is available under the terms of the GNU Free Documentation License.Polychlorinated biphenyls (PCBs) are a class of organic compounds with 1 to 10 chlorine atoms attached to biphenyl which is a molecule composed of two benzene rings each containing six carbon atoms.PCBs are classified as persistent organic pollutants which bioaccumulate in animals.They are formed by electrophilic chlorination of biphenyl with chlorine gas.Commercial PCBs are mixtures of 50 or more PCB congeners.Aroclor 1260 is a sticky yellowish resin).Other physical and chemical properties vary widely across the class.Viton, polyethylene, polyvinyl acetate (PVA), polytetrafluoroethylene (PTFE), butyl rubber, nitrile rubber, and Neoprene.Environmental and metabolic degradation generally proceeds quite slowly relative to most other compounds.History
PCBs were commercially produced as complex mixtures containing multiple isomers at different degrees of chlorination.Manufacturing levels increased in response to the electrical industry's need for a "safer" cooling and insulating fluid for industrial transformers and capacitors.The public, legal, and scientific concerns about PCBs arose from research indicating they were likely carcinogens having the potential to adversely impact the environment and therefore undesirable as commercial products.The only North American producer, Monsanto, marketed PCBs under the trade name Aroclor from 1930 to 1977.These were sold under trade names followed by a 4 digit number.PCB mixtures have been used for a variety of applications, including dielectric fluids for capacitors and transformers, heat transfer fluids, hydraulic fluids, lubricating and cutting oils, and as additives in pesticides, paints, carbonless copy ("NCR") paper, adhesives, sealants, plastics, reactive flame retardants, and as a fixative for microscopy.They were also used in surgical implants.Manufacture peaked in the 1960s, by which time the electrical industry had lobbied the U.In 1966, they were determined by Swedish chemist Dr.Soren Jensen to be an environmental contaminant, and it was Dr.Arochlors (chlorinated biphenyls) were known toxins in 1947.They also produce a serious and disfiguring dermatitis."Their commercial utility was based largely on their chemical stability, including low flammability, and desirable physical properties, including electrical insulating properties.Their chemical and physical stability has also been responsible for their continuing persistence in the environment, and the lingering interest decades after regulations were imposed to control environmental contamination.However, they continued to be allowed in "totally enclosed uses" such as transformers and capacitors, which, however, tended to explode on a fairly regular basis.Stone of the New York State Department of Environmental Conservation who first published his findings in the early 1970s that PCBs were leaking from transformers and had contaminated the soil at the bottom of utility poles.Concern over the toxicity and persistence (chemical stability) of PCBs in the environment led the United States Congress to ban their domestic production in 1977, although some use continues in closed systems such as capacitors and transformers.The use of PCBs in "closed" uses include:
capacitors
insulating fluids in transformers
vacuum pump fluids
hydraulic fluids
Closed uses of PCBs were also banned in the UK from 1981, when nearly all UK PCB synthesis ceased.Estimates have put the total global production of PCBs on the order of 1.The United States was the single largest producer with over 600,000 tons produced between 1930 and 1977.Please help improve this article by adding reliable references.PCBs into the Hudson River between approximately 1947 and 1977.Removal of contaminated soil from Rogers Island was completed in December 1999.Reject capacitors were hauled and dumped in area salvage yards and landfills.Over 2 million pounds of PCBs were estimated to have been dumped in Monroe and Owen Counties, making it the biggest concentration of PCBs in the world.Although federal and state authorities have been working on the site remediations, many areas remain contaminated.Feasibility Study) and ROD (Record of Decision), although Westinghouse signed a US Department of Justice Consent Decree in 1985.Much of the Great Lakes area is still heavily polluted with PCBs, despite extensive remediation work.Locally caught fresh water fish and shellfish are contaminated with PCBs and their consumption is restricted.Typical urban atmospheric concentrations are in the picogram per cubic meter range.The sources of the PCBs were unknown though, because seabirds may die at sea and be washed ashore from a very wide area.The toxicity of PCBs varies considerably among congeners.PCBs because they are not substituted at the ring positions ortho to (next to) the other ring, (i.The most commonly observed health effects in people exposed to PCBs are skin conditions such as chloracne and rashes, but these were known to be symptoms of systemic poisoning dating back to the 1920s.Disease in over 14000 people.Other symptoms included fatigue, headache, cough, and unusual skin sores.Other studies suggest that the immune system was affected in children born to and nursed by mothers exposed to increased levels of PCBs.The most likely way infants will be exposed to PCBs is from breast milk.Transplacental transfers of PCBs were also reported.Biological magnification of PCBs has also led to polar bears and whales that have both male and female sex organs and males that cannot reproduce.This effect is also known as endocrine disruption.Rats that ate food containing high levels of PCBs for two years developed liver cancer.The Department of Health and Human Services (DHHS) has concluded that PCBs may reasonably be anticipated to be carcinogens.PCBs are also classified as probable human carcinogens by the National Cancer Institute, World Health Organization, and the Agency for Toxic Substances and Disease Registry.These effects may vary depending on the specific PCB.However, not all effects may be mediated by the AhR receptor.PCBs may disrupt thyroid hormone transport by binding to transthyretin.Large quantities of PCBs have been placed in landfill sites, mainly in the form of transformers and capacitors.No emissions above background are seen if the landfill is designed correctly.Methods of destruction
These can be separated into three distinct categories: physical, microbial, and chemical destruction.Although PCBs do not ignite themselves, they can be combusted under extreme and carefully controlled conditions.In a similar process to combustion, high power ultrasonic waves are applied to water, generating cavitation bubbles.These then implode or fragment, creating microregions of extreme pressures and temperatures where the PCBs are destroyed.If a deoxygenated mixture of PCBs in isopropanol or mineral oil is subject to irradiation with gamma rays then the PCBs will be dechlorinated to form inorganic chloride and biphenyl.The reaction works best in isopropanol if potassium hydroxide (caustic potash) is added.If oxygen, nitrous oxide, sulfur hexafluoride or nitrobenzene is present in the mixture then the reaction rate is reduced.Generally, these organisms work in one of two ways: either they use the PCB as a carbon source, or destruction takes place through reductive dechlorination, with the replacement of chlorine with hydrogen on the biphenyl skeleton.However, there are significant problems with this approach.Firstly, these microbes tend to be highly selective in their dechlorination, with lower chlorinated biphenyls being readily transformed, and with preference to dechlorination in the para and meta positions.Finally, while microbes work well in laboratory conditions, there is often a problem in transferring a successful laboratory strain to a natural system.This is because the microbes can access other sources of carbon, which they decompose in preference to PCBs.Further recent developments have focused on testing enzymes and vitamins extracted from microbes which show PCB activity.Especially promising seems to be the use of vitamin B12, in which a cobalt ion is in oxidation state (III) under normal redox conditions.Chemical
Many chemical methods are available to destroy or reduce the toxicity of PCBs.Substitution of chlorine by polyethylene glycols) occurs in under two hours under a blanket of nitrogen, to prevent oxidation of the oil, to produce aryl polyglycols, which are insoluble in the oil and precipitate out.This is analogous to the Wurtz reaction for coupling halogenoalkanes.The major pathway for atmospheric destruction of PCBs is via attack by OH radicals.Direct photolysis can occur in the upper atmosphere, but the ultraviolet wavelengths necessary to excite PCBs are shielded from the troposphere by the ozone layer.The Schwartz reaction is the subject of much study, and has significant benefits over other routes.See also
GE
Bay mud
Organochlorine compound
Polybrominated biphenyls
Neal Stephenson's novel Zodiac involves PCBs and their impact on the environment.Guidelines for the Identification of PCBs and Materials Containing PCBs.United Nations Environment Programme, p.Australian and New Zealand Environment and Conservation Council (ANZECC), pp.Breivik K, Sweetman A, Pacyna JM, Jones KC (2002).United States Environmental Protection Agency.Chapter 9: PCB Pollution in the Hudson."The Great Lakes cleanup effort".Contamination of rice bran oil with PCB used as the heating medium by leakage through penetration holes at the heating coil tube in deodorization chamber."Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs): biochemistry, toxicology, and mechanism of action".Simon T, Britt JK, James RC (2007)."Development of a neurotoxic equivalence scheme of relative potency for assessing the risk of PCB mixtures".Woods SL, Trobaugh DJ (1999).External links
ATSDR Toxicological Profile U.Note that biphenyl, while not technically a PCB congener due to its lack of chlorine substituents, is still typically included in the literature.All text is available under the terms of the GNU Free Documentation License.Read more information about the transfer.Japan for disposal in the United States.PCBs may be imported for disposal.The MCL has been set at 0.National Primary Drinking Water Regulations.What are the Health Effects?PCB materials, such as waste transformer fluid, to open areas.What happens to PCBs when they are released to the environment?The regulation for PCBs became effective in 1992.PCBs are present above some lowest detectable level.PCB, the system must continue to monitor this contaminant.If the levels of PCBs exceed the MCL, 0.Learn more about your drinking water!Environment is also a valuable source of information.PCB is an interactive printed circuit board editor for the X11
window system.PCBs) when they enter the environment?PCBs was stopped in the U.PCBs are taken up by small organisms and fish in water.High levels of toluene may affect your kidneys.The EPA has set a limit of 0.Our Free PCB software is a snap to learn and use.For the first time, designing circuit boards is simple for the beginner and efficient for the professional.Learning the ExpressPCB software is fast because of its familiar user interface.Most two layer boards are shipped the next business day!PCB is a printed circuit board, also known as a printed wiring board.It is used in electronics to build electronic devices.PCB serves two purposes in the construction of an electronic device; it is a place to mount the components and it provides the means of electrical connection between the components.The most common material used is a glass fiber epoxy laminate material.The next step is to "print" the connection diagram onto the PCB.The "printing" is usually done by photographically transferring the image to the PCB.Next, the PCB is put into an acid bath.The acid bath removes the copper from the board, excepting the areas protected by the resistant material.Finally, a protective coating is applied to the board to prevent corrosion of the copper traces.It is possible to make PCBs of almost any number of layers by repeating the process above and laminating the resulting PCBs into a single PCB.It is also more common now for parts to be soldered directly to the board, eliminating the need for holes into which leads (wires) from the components must be inserted.However, surface mounting also requires better quality control on the manufacture of the PCB.PCB is now used in almost every electronic product available.What is an OLED (Organic Light Emitting Diode)?Very good information about PCB, that helps people who are unknown about the wired circuit board.Post your questions or comments about this article!
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