National Bureau of Standards. Absorbed moisture can also vaporize on heating, as during soldering, and cause cracking and delamination,  the same effect responsible for "popcorning" damage on wet packaging of electronic parts.
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Getting started is easy! Through-hole mounting may be used for some large components such as electrolytic capacitors and connectors. The pattern to be etched into each copper layer of a PCB is called the "artwork". The etching is usually done using photoresist which is coated onto the PCB, then exposed to light projected in the pattern of the artwork.
The resist material protects the copper from dissolution into the etching solution. The etched board is then cleaned.
A PCB design can be mass-reproduced in a way similar to the way photographs can be mass-duplicated from film negatives using a photographic printer. In multi-layer boards, the layers of material are laminated together in an alternating sandwich: Only the outer layers need be coated; the inner copper layers are protected by the adjacent substrate layers. FR-4 glass epoxy is the most common insulating substrate. Another substrate material is cotton paper impregnated with phenolic resin, often tan or brown.
When a PCB has no components installed, it is less ambiguously called a printed wiring board PWB or etched wiring board. However, the term "printed wiring board" has fallen into disuse. In informal usage, the term "printed circuit board" most commonly means "printed circuit assembly" with components. The IPC preferred term for assembled boards is circuit card assembly CCA ,  and for assembled backplanes it is backplane assemblies.
A PCB may be "silkscreen" printed with a legend identifying the components, test points, or identifying text. Originally, an actual silkscreen printing process was used for this purpose, but today other, finer quality printing methods are usually used instead. Normally the screen printing is not significant to the function of the PCBA.
A minimal PCB for a single component, used for prototyping, is called a breakout board. The purpose of a breakout board is to "break out" the leads of a component on separate terminals so that manual connections to them can be made easily. Breakout boards are especially used for surface-mount components or any components with fine lead pitch.
Advanced PCBs may contain components embedded in the substrate. The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or more compact double-sided boards, with components soldered on both sides.
Horizontal installation of through-hole parts with two axial leads such as resistors, capacitors, and diodes is done by bending the leads 90 degrees in the same direction, inserting the part in the board often bending leads located on the back of the board in opposite directions to improve the part's mechanical strength , soldering the leads, and trimming off the ends.
Leads may be soldered either manually or by a wave soldering machine. Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and it limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards, since the holes must pass through all layers to the opposite side.
Once surface-mounting came into use, small-sized SMD components were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB e.
Surface-mount technology emerged in the s, gained momentum in the early s and became widely used by the mids. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes.
Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities.
Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates.
Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper. However, prices of semiconductor surface mount devices SMDs are determined more by the chip itself than the package, with little price advantage over larger packages, and some wire-ended components, such as 1N small-signal switch diodes, are actually significantly cheaper than SMD equivalents.
Each trace consists of a flat, narrow part of the copper foil that remains after etching. Its resistance , determined by its width, thickness, and length, must be sufficiently low for the current the conductor will carry. Power and ground traces may need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane for shielding and power return. For microwave circuits, transmission lines can be laid out in a planar form such as stripline or microstrip with carefully controlled dimensions to assure a consistent impedance.
In radio-frequency and fast switching circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually undesired; conversely, they can be used as a deliberate part of the circuit design, as in Distributed element filters , obviating the need for additional discrete components.
The European Union bans the use of lead among other heavy metals in consumer items, a piece of legislature called the RoHS, for Restriction of Hazardous Substances, directive. PCBs to be sold in the EU must be RoHS-compliant, meaning that all manufacturing processes must not involve the use of lead, all solder used must be lead-free, and all components mounted on the board must be free of lead, mercury, cadmium, and other heavy metals.
Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form an integral final piece of uniform thickness.
The size can be up to 4 by 8 feet 1. Varying cloth weaves threads per inch or cm , cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate's type designation FR-4, CEM-1, G, etc. There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit.
Thermal expansion is an important consideration especially with ball grid array BGA and naked die technologies, and glass fiber offers the best dimensional stability. FR-4 is by far the most common material used today. The board stock with unetched copper on it is called "copper-clad laminate". With decreasing size of board features and increasing frequencies, small nonhomogeneities like uneven distribution of fiberglass or other filler, thickness variations, and bubbles in the resin matrix, and the associated local variations in the dielectric constant, are gaining importance.
The circuitboard substrates are usually dielectric composite materials. The composites contain a matrix usually an epoxy resin and a reinforcement usually a woven, sometimes nonwoven, glass fibers, sometimes even paper , and in some cases a filler is added to the resin e. The reinforcement type defines two major classes of materials: Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications.
The substrates are characterized by several key parameters, chiefly thermomechanical glass transition temperature , tensile strength , shear strength , thermal expansion , electrical dielectric constant , loss tangent , dielectric breakdown voltage , leakage current , tracking resistance At the glass transition temperature the resin in the composite softens and significantly increases thermal expansion; exceeding T g then exerts mechanical overload on the board components - e.
Below T g the thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require a matrix with a high T g.
The materials used determine the substrate's dielectric constant. This constant is also dependent on frequency, usually decreasing with frequency. As this constant determines the signal propagation speed , frequency dependence introduces phase distortion in wideband applications; as flat a dielectric constant vs frequency characteristics as is achievable is important here.
The impedance of transmission lines decreases with frequency, therefore faster edges of signals reflect more than slower ones. Dielectric breakdown voltage determines the maximum voltage gradient the material can be subjected to before suffering a breakdown conduction, or arcing, through the dielectric.
Tracking resistance determines how the material resists high voltage electrical discharges creeping over the board surface. Loss tangent determines how much of the electromagnetic energy from the signals in the conductors is absorbed in the board material.
This factor is important for high frequencies. Low-loss materials are more expensive. Choosing unnecessarily low-loss material is a common engineering error in high-frequency digital design; it increases the cost of the boards without a corresponding benefit.
Signal degradation by loss tangent and dielectric constant can be easily assessed by an eye pattern. Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water also may be soaked by capillary forces through voids in the materials and along the reinforcement.
Epoxies of the FR-4 materials aren't too susceptible, with absorption of only 0. Teflon has very low absorption of 0. Polyimides and cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuit board materials.
Absorbed moisture can also vaporize on heating, as during soldering, and cause cracking and delamination,  the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required to dry them prior to soldering. Copper thickness of PCBs can be specified directly or as the weight of copper per area in ounce per square foot which is easier to measure. One ounce per square foot is 1. Heavy copper is a layer exceeding three ounces of copper per ft 2 , or approximately 0.
Heavy copper layers are used for high current or to help dissipate heat. Flexible substrates typically have thinner metalization. Safety Standard UL covers component safety requirements for printed wiring 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.
Initially PCBs were designed manually by creating a photomask on a clear mylar sheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout.
The finished photomask was photolithographically reproduced onto a photoresist coating on the blank copper-clad boards. Modern PCBs are designed with dedicated layout software, generally in the following steps: Manufacturing starts from the fabrication data generated by computer aided design , and component information. CAM performs the following functions:.
Several small printed circuit boards can be grouped together for processing as a panel. A panel consisting of a design duplicated n -times is also called an n -panel, whereas a multi-panel combines several different design onto a single panel. The outer tooling strip often includes tooling holes , a set of panel fiducials , a test coupon , and may include hatched copper pour or similar patterns for even copper distribution over the whole panel in order to avoid bending.
The assemblers often mount components on panels rather than single PCBs because this is efficient. The panel is eventually broken into individual PCBs along perforations or grooves in the panel. Laser depaneling reduces stress on the fragile circuits, improving the yield of defect-free units. The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper. Alternatively, a conductive ink can be ink-jetted on a blank non-conductive board.
This technique is also used in the manufacture of hybrid circuits. Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. In additive methods the pattern is electroplated onto a bare substrate using a complex process.
The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged exposed to light through a mask and then developed which removes the unexposed film.
The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished. Semi-additive is the most common process: 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 bare original copper laminate from the board, isolating the individual traces.
Some single-sided boards which have plated-through holes are made in this way. General Electric made consumer radio sets in the late s using additive boards. The semi- additive process is commonly used for multi-layer boards as it facilitates the plating -through of the holes to produce conductive vias in the circuit board.
Chemical etching is usually done with ammonium persulfate or ferric chloride. The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such as ferric chloride. Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate.
In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching.
In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates. As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as grams of copper per litre of solution.
In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content. The etchant removes copper on all surfaces exposed by the resist. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short-circuits between adjacent traces when closely spaced.
Overhang can be removed by wire-brushing the board after etching. Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil.
It is then drilled, plated, and etched again to get traces on top and bottom layers. The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected.
The automatic optical inspection system compares an image of the board with the digital image generated from the original design data. Holes through a PCB are typically drilled with drill bits made of solid coated tungsten carbide. Coated tungsten carbide is used because board materials are abrasive. High-speed-steel bits would dull quickly, tearing the copper and ruining the board.
Drilling is done by computer-controlled drilling machines, using a drill file or Excellon file that describes the location and size of each drilled hole. Holes may be made conductive, by electroplating or inserting hollow metal eyelets, to connect board layers. Some conductive holes are intended for the insertion of through-hole-component leads. Others used to connect board layers, are called vias. 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 laser drilled —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 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. The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes. These holes electrically connect the conducting layers of the PCB. For multi-layer boards, those with three layers or more, drilling typically produces a smear of the high temperature decomposition products of 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. The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma.
The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper. Proper plating or surface finish selection can be critical to process yield, the amount of rework, field failure rate, and reliability. PCBs are plated with solder, tin, or gold over nickel and a resist for etching away the unneeded underlying copper. Matte solder is usually fused to provide a better bonding surface for bare copper.
Treatments, such as benzimidazolethiol , prevent surface oxidation of bare copper. The places to which components will be mounted are typically plated, because untreated bare copper oxidizes quickly, and therefore is not readily solderable.
Traditionally, any exposed copper was coated with solder by hot air solder levelling HASL. The HASL finish prevents oxidation from the underlying copper, thereby guaranteeing a solderable surface. 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.
One of these lead-free compounds is SNCL, made up of It is important to use solder compatible with both the PCB and the parts used. An example is ball grid array BGA using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.
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