Printed circuit board

A printed circuit board ( PCB, printed circuit board or, in English printed circuit board, PCB) is a carrier for electronic components. It is used for mechanical attachment and electrical connection. Virtually every electronic device includes one or more circuit boards.

PCBs consist of electrically insulating material with them adhering, conductive connections (conductors ). As insulating material is fiber reinforced plastic usual. The interconnects are usually etched from a thin layer of copper. The components are soldered to solder pads (pads ) or in pads. So they are at these footprints held simultaneously mechanically and electrically connected. Larger components can also be secured with cable ties, adhesive or screw on the circuit board.

• 3.3.1 Production in the pool
• 3.3.2 Experimental boards
• 3.3.3 milling
• 3.3.4 toner transfer method
• 3.3.5 markers
• 3.3.6 Anreibesymbole
• 3.3.7 Oil Method

• 4.1 Manufacturing Technology
• 4.2 layout

• 5.1 SMD PCBs
• 5.2 Multi-layer boards
• 5.3 components on boards and in
• 5.4 microvia technology
• 5.5 Buried Via Technology
• 5.6 Plugged -via technology
• 5.7 thick copper
• 5.8 Thermal Management
• 5.9 Flexible PCBs
• 5:10 press-fit technology and other Lötalternativen

• 7.1 Continuity Test 7.1.1 operation

• 7.2.1 operation

PCB types

The PCB types range from one-sided printed circuit boards on multilayer bishin to special techniques.

• Standard PCB One-sided and two- sided PCB

• Rigid-Flex - The Flex-Rigid PCB provides particularly for systems that are subject to constant bending, a reliable and durable component.
• Flexlam
• High Current - In order to realize the transport of high currents and signal electronics on a circuit board
• Thick copper
• Dünnstleiterplatten
• Slip ring - A slip ring is used for the transmission and for tapping of power, signals and data in rotating systems. Application areas are for example in industrial robots and wind turbines. Requirement for the reliability and life of a slip ring surface is the correct application of the noble metal coating.
• HDI PCB
• IMS PCB
• PCBs on glass

Material

Simple printed circuit boards are made of an electrically insulating substrate (base material ), on which one or two layers of copper are applied. The layer thickness is typically 35 microns and for higher current applications between 70 microns and 140 microns. To enable thinner conductor elements, PCBs with only 18 microns copper are produced. In English-speaking countries sometimes takes the film thickness of the conductive layer, the mass per unit area in ounces per square foot ( oz / sq.ft ) specified. In this case corresponds to 1 oz / sq.ft approximately 35 microns thickness.

The base material was previously often Pertinax ( phenolic resin with paper fibers, so-called hard paper, material ID FR2). Today - except for cheap mass products - usually impregnated with epoxy resin glass fiber mats used ( material code FR4). This material has a better tracking resistance and better high- frequency characteristics and a lower water absorption than hard paper.

Material designations:

• FR1 = phenolic resin paper (cheap variety)
• FR2 = phenolic resin paper ( standard quality )
• FR3 = epoxy resin paper
• FR4 = epoxy resin fiberglass fabric
• FR5 = epoxy resin fiberglass cloth ( heat-resistant )

For special applications, other materials are used, such as Teflon, aluminum oxide or ceramic LTCC and HTCC in for Radio Frequency Technology and polyester film for flexible printed circuit boards. Manufacturer this special base materials are companies like Rogers Corporation and Arlon Materials for Electronics, which is also the colloquial name " Rogers " or " Arlon " derives in technical English for Teflon- based PCBs with application in high-frequency technology.

Recent developments also employ glass as a base material for printed circuit boards with high demands on the heat dissipation base materials are used with electrically isolated metal nuclei such as aluminum or copper, for example in the field of lighting technology with high-performance LEDs. These support materials are also referred to as a direct bonded copper or insulated metal substrate (IMS).

In applications for low temperatures or high humidity and base materials with integrated heating elements can be used to prevent the lower cooling or condensation of the circuit. In addition, research is being conducted on alternative materials that are more environmentally friendly, currently there is still some problems with the moisture resistance.

Production

Design

The circuit board design (layout) is nowadays usually with a software, which also contains the schematic and bill of materials often, as well as data such as solder paste pattern or silkscreen next to the trace data. The circuit board design can be output from the PCB layout programs in a standard format. Most PCB manufacturers process the formats Gerber RS- 274X, Excellon or Sieb & Meyer. The project data of the circuit board are divided. The first part consists of Gerber data for the topography of the printed circuit boards. This, for example, of the interconnect performance and the localization of PADs, etc. are documented. The second part consists of the drilling data of Excellon or Sieb & Meyer- data

The PCB layout ( manually or with an autorouter ) is the main content of the design. There are also technological information such as copper thickness, PCB manufacturing technology and surface type. The end result is the transfer of data to the production.

Thence to the data, for example, a film for exposure, a printing, or a solder cream is prepared. The data can also be used to control a light scribe or a milling machine. The drilling data in the format of Excellon or Sieb & Meyer data are used to control an NC drill.

The production data are structured in separate according to functional levels:

• Pattern of one or more copper layers ( conductor lines and spaces)
• Drill holes ( location, depth and diameter)
• Contour and openings
• Assembly diagram above and below
• Solder mask top and bottom
• Silk screen up and down
• Adhesive dots and Lotpastenmuster for SMD components above and below
• Partial metallization (eg, gold plating for contact surfaces )

Series production

Photochemical processes

The majority of single-sided and double-sided through-plated PCB is produced photochemically.

The present sequence of processing steps is

Then, depending on demand post-processing steps.

Initially, the drilling and through-plating was performed after the etching of the printed circuit board. Since then, however, the photoresist has been replaced by a so-called dry film resist, a photosensitive film, the order of the production steps was varied. Advantage is that no longer a mask must be applied to the board prior to contact to prevent the growth of the copper to undesirable locations. At this time, the entire printed circuit board by copper is still covered, only the layer thickness of the copper foil increases. The metallized holes are terminated at both ends during the etching of the photoresist film.

The manufacture of the conductor tracks is effected photolithographically generally by a thin layer of photosensitive resist is applied to the surface of the still completely metallized disk. After exposure, the photoresist through a mask with the desired circuit board layout, depending on the photoresist used, either the exposed or the unexposed portions of the resist soluble in a suitable developer solution and removed. The circuit board so treated Bring you into a suitable etching solution (eg, dissolved in water, iron ( III ) chloride or sodium persulfate or hydrogen peroxide with hydrochloric acid ), only the exposed portion of the metallized surface is attacked; remain covered by the photoresist Shares, and because the paint is resistant to the etching solution.

Prototypes can also be structured by milling the copper layers ( " isolation milling " below image to Lötrasterplatinen ). Such boards are not made of conductive paths, but from surfaces which are separated from each other by milling tracks.

The copper layers may be reinforced by electroplating after etching.

The production of holes for receiving wired components and for vias requires because of the fiber component of the carrier material carbide tools. When holes are metallized on the inside walls, caused vias. The metallization of the holes ( insulating areas) requiring seeding, subsequent electroless deposition of a thin copper layer, and finally the electrolytic reinforcement.

In addition, electroplated on small areas or the entire copper surface metallic protective and contact layers of tin, nickel or gold are applied. Thin copper gilding require to back a diffusion barrier layer ( nickel barrier layer).

After that, a solder mask (green paint layer of the circuit board in the photo) is applied, covering the tracks and only the solder joints can be free. Solder defects thus have been avoided even when flow soldering saves tin and the conductors are protected against corrosion. The matching solder points (pads and lands ) may include a physical process (Hot Air Leveling ) with a tin layer and is additionally covered with a flux which allows better soldering.

Paste Islands for soldering SMD components are applied by means of a solder paste mask. It is made ​​of sheet metal, and includes holes at the positions where the solder paste should go. The masks are produced by laser fine cutting. Another possible step in the SMT assembly is the application of glue points, which ensures the fixing of the components when fitting ( pick and place ) by soldering.

Often circuit boards have a silkscreen produced by screen printing, which facilitates in conjunction with a circuit diagram assembly and service.

Die cutting and wire laying technique

Two other important production process for printed circuit boards are the stamping technology and wire-laying equipment.

In stamping technology printed circuit boards to very large quantities are produced. The technique is only suitable for single-sided printed circuit boards made ​​of Pertinax or unreinforced plastics. This base material is used without a copper layer, a copper foil with an adhesive layer is placed on the base material, and then punched out with a punch, the conductor track forms and simultaneously pressed onto the base material. In one operation, while the contour of the circuit board and the holes are punched, and punched the conductor pattern and bonded to the base material.

For small series and for special applications, which require a high current carrying capacity of the circuit board, the wire-laying technique is used. In this case, a machine insulated wires laid on the base material, which is connected by means of ultrasonic welding, both at the solder pads, as are also mounted on the surface of the base material.

By " benefit" is the grouping of several smaller layouts referred to in the manufacturing of printed circuit boards on a large board. The term comes from the printing technology. The entire processing chain as far as possible with this utility. The clever arrangement of different designs which are usually rectangular formats of the base material can be well exploited at different, for example, L -shaped geometries. For the subsequently required division of the board, the term depanelization is in use.

Screen printing

Instead of the photo- chemical process can be used to cover the conductive traces prior to the etching, the screen printing technique. This is particularly suitable for single-coated material and a low degree of difficulty of printed circuit boards.

Prototypes

Before starting a production, it is often advisable to test a circuit without compromising the high cost of preparing the photomasks.

There are the following options:

Production in the pool

Manufacturers offer the production of single pieces and small series in the "pool", ie several individual pieces than good (see above) to be drilled on a large plate, plated through, illuminated, etched and milled afterwards.

Experimental boards

Breadboards have holes or pads ( either unilaterally or through contact) in a grid on which is common for electronics components, ie 2.54 mm, which corresponds to 0.1 inches ( for the rarer metric components 2.5 mm) or half of it. Connections can be made by soldering jumper wire in Fädeltechnik or in winding technology. Often several eyes are already connected by conductor tracks (for example, for operating voltages ), or are longer or shorter conductive paths provided in order to come closer to practical requirements. Also completely provided with parallel tracks experimentation boards ( Lötstreifenplatine ) are common. These can be separated as needed by a tool, in that the conductor track is durchgeritzt. In addition, there are small experimental boards suitable for common SMD package shapes to adapt their connections to the grid.

Milling

When milling dividing lines between the conductor surfaces are made with a pin router. Everything stays are copper ( island method ). Account for the wet-chemical and photolithographic steps. With special CAD software, CNC programs can be generated, so that even prototypes are to be made quickly.

Toner transfer method

This layout is a mirror image printed on a laser printer onto suitable paper or a purpose made heat-resistant film ( catalog pages, etc.) and then " ironed " with iron or laminator on the board. The toner is thereby lower viscosity like in the printer's fuser and combines with the copper of the board. Then the paper is replaced with water, - the toner remains on the copper. This is followed by the etching process, the areas covered by the toner points stop. The toner can be removed with thinner then. In this process tolerances can occur due to the heating by the printer and paper transport by stretching and shrinking of the paper.

Markers

The conductive traces and pads can also be transferred directly with a permanent marker (so-called permanent marker ) to the base material. The paint protects the covered areas during the etching process. After etching, the ink is then removed with alcohol or acetone.

When this method is applied by analogy to a plastic film, this is how the resulting mask to use for the photo- positive process and thus enables mass production.

Anreibesymbole

Some manufacturers distribute Anreibesymbole representing pads, conductor parts and electrical symbols. These are - similarly applied and rubbed as decals on the circuit board. Then the applied symbols protect the copper among them during the etching process. This method is also used in combination with a marker (eg pads with Anreibesymbolen, traces with felt-tip pen ). After etching, the icons are removed by acetone or by scraping. Solder resist, which is often applied in order to improve the flow properties of the lead, also solves the symbols.

Also this method can be applied to mass production of a film for the photo- positive process.

Oil Method

Especially for hobbyists the oil method for rapid implementation of projects with minimal effort is popular. In this case, the layout is printed with the highest optical density on plain paper, and then impregnated with oil, whereby the paper is largely transparent. The actual exposure can be performed with any source of UV light (sun, Solarium, ...).

History

Before the introduction of printed circuit boards electronic circuits were free wired, possibly with the additional use of Terminals. Mechanical bases were any components such as potentiometers, variable capacitors, switches with solder tags and their sockets of electron tubes. Depending on the manufacturer, efforts were made to clear rectangular arrangement of the components or always chose the direct, oblique connection. Since the components such as capacitors or resistors also were still very large and long, they could cover distances of a few centimeters.

Devices of this type were finished by hand and with regard to the wiring plan.

PCB precursor from the 1920s were punched circuit lines that were riveted on hard paper. Components (resistors, capacitors ) were worn without solder joint between the sheet metal springs. Paul Eisler, a Viennese electronics engineer left, in 1943 patented the principle of the printed circuit board, but the long time in addition to the regular hand wiring eked out a rather insignificant shadows. Only with the increasing miniaturization of electronics increased the importance of this technology.

In the early days around 1940 circuits have also been prepared by screen printing of conductive silver on the base plate. On ceramic substrates and printed branded conductors and resistors in contrast, under the term thick-film technology.

Manufacturing Technology

The use of PCBs began in the early 1950s by the Ruwel works founded by Fritz steel in Geldern am Niederrhein.

A still widely used technique - case of printed circuits, however, the leads of the components from above through holes are placed through the printed circuit board (English Through Hole Technology, THT ). On the underside ( soldering, wire or L side ) are the copper lines, where they are soldered. Which allows a simplified and automated production, at the same time, the error rate falls in production, since faulty wiring can be eliminated in order for the circuit on the circuit board.

More complex single-layer printed circuit boards require additional compounds that can not be produced in the layout. These are made by solder bridges by means of angled wire or zero ohm resistors. The latter can be better used in pick and place machines. Alternatively, use of these compounds of copper traces on both sides of the printed circuit board ( double-layered printed circuit board, DL ). Connections between the upper ( or pick-and- B- side) and lower side were produced by soldering pressed-in pins or rivets.

Only in the 1960s were these connections ( vias, DK, Eng. Vias ) generated chemically by the PCB through hole metallization of the walls of the holes.

Even single-layer printed circuit boards are made ​​for cost reasons even today, when the circuit is allowed. Compared with a double-sided, plated through printed circuit board, the cost of an equal -sided circuit board at 25-50 %.

A significant portion of the PCBs produced in the world is still populated by hand, although it has been around since about the mid-1970s insertion machines. However, modern printed circuit boards with a high packing density and surface- mount devices (SMD ) can not be loaded by hand. So-called " pick and place" machines take over the handling of the sometimes less than 1 mm ² large components. The THT components are increasingly assembled by hand and then selectively soldered after reflow soldering of SMD assembled on both sides.

Layout

In the 1960s, it drew the layout (conductors ) structure at a scale of 2:1 with ink or adhesive technique with symbols and layout, glue rollers ( Brady ) on grid sheets. Later they created to programming workplaces NC programs for controlling a light character device, which established the necessary for photolithography film. Thereafter, the computer used to generate the drawings the various layers of copper and pressure and the NC control program for the preparations of the holes.

Current layout programs for the so-called Electronic Design Automation ( EDA) allow the creation of a joint plan and the corresponding representation ( " rat's nest " ) of a circuit diagram and contain comprehensive component libraries, where for each component and the housing geometries, technical data and the location and size of the solder pads (english footprint for " footprint" ) are included. The Footprint describes the dimensions of the pads in the through hole technology (THT ) and the dimensions of the solder pads for Surface Mounted Devices ( SMD) for a particular component on the circuit board.

Automatic PCB layout based on a given circuit diagram and specification of design rules ( placement of the components (automatic placement ) and unbundling ( autorouting ) the electrical connections ) is now standard on simple circuit boards. At its limits, this process meets with complex circuit boards that require a lot of experience in unbundling (eg for mobile phones ). Also, an increase in computing power does not improve, since the input of the complex design specifications partially takes more time than manual unbundling.

The current carrying capacity (power density) of interconnects is an important design aspect. You may be substantially higher than that of solid wires lie, since the substrate is cooled by heat conduction. Layout software can take into account the current capacity.

The capacitive and inductive coupling of the traces, their susceptibility to external electromagnetic fields and radiation (emission ) is described under the general term electromagnetic compatibility (EMC). Modern software can now begin to take into account EMC aspects within the board.

Other aspects are:

• At high frequencies and steepnesses of the pulse wave impedance of the conductor tracks is of importance (see stripline ).
• For analog signals ( particularly audio applications with high dynamic range) have ground loops (including ground loops, ground loops ), should be avoided.
• At high voltages need for security between the interconnects certain minimum distances ( aura) are complied with.

Printed Circuit Board Technologies

A large part of the circuit boards in electronic equipment is still made ​​from single-lined material and with discrete components. With progressive miniaturization SMD components are used on the underside increasingly, while the through- hole components are loaded from the top. The SMD components may additionally be glued, so they do not fall off during soldering.

The more expensive through-hole boards, and even more expensive multi-layer boards are used in more complex (such as computers ), more reliable (eg industrial electronics) or miniaturized (eg mobile phones ) assemblies.

SMD PCBs

Mid-1980s they began to craft Leadless components that were soldered directly to the circuit traces. These surface- mounted components ( engl. Surface Mounted Devices, shortly SMD) made ​​it possible to increase the packing density and contributed to an enormous reduction of electronic devices at. It is also possible to place the SMD components on both sides of a printed circuit board to those on the bottom side ( secondary side) are initially bonded to be mounted components on the board and then is cured, the adhesive and the board upside down to fit the other side. The soldering process can be done either in the reflow process or in wave soldering, provided the attached on the bottom parts are suitable to run through the solder wave. If the Lötdepot melted to SMD components by means of a reflow oven, the cohesive enough that sticking of the SMD components on the secondary side is not necessary, which brings further cost reductions with it. (Note is simply that higher component weights are not suitable for placement on the secondary side of the LP. )

Another reason for the development of the SMD process were the ever-increasing frequency of electronic assemblies. By using SMD line lengths and the associated parasitic inductance and capacitance could be reduced.

A major advantage of SMD components is also the ease of use in automated assembly systems. For thru-hole components, it is always a significant problem, with all the connections to make the holes and keep the permissible bending radii of the connecting wires with a bending amount, which is why large leaded components are also still used today in otherwise automated manufacturing by hand.

Multi-layer boards

To the packing density in modern SMD components, especially in computers, to do justice, it is not sufficient if the conductor tracks on only one side of the PCB. After the double-sided printed circuit boards, which have a copper layer on both sides of the board, they began several thinner boards aufeinanderzukleben with prepregs. This so-called multi-layer multi-layer printed circuit boards can currently up to 48 layers, in some cases even longer have. Usual, for example, four to eight layers in machines and up to twelve layers in mobile phones. The interconnections between the layers are made with plated-through holes ( " VIA ").

In many cases, the use of multi -layer PCB is also needed at lower density, such as to ensure the low inductance power supply of all components.

Components on boards and in

Simple passive components can be integrated into the board. Inductors, Coils, small capacity, contacts or heatsink may be formed directly as a copper layer structure. Resistors can be printed by means of specific pastes to the surface or in the hidden layer. This saves on components and their assembly.

There are boards, on or in which integrated circuits are placed directly (chip on board, chip in board). Often they are bonded directly to the board and only protected by a drop of resin (English Glob Top) ( example: quartz movements ).

Microvia technology

For multi- layer circuit boards, HDI PCB technology is now standard. This blind bores with 50 microns to 100 microns in diameter are introduced by means of laser or plasma in the outer layers and end on the copper of the next - or the next - Location. After cleaning the remaining resin Mikrobohrlöcher these in turn are copper plated and thus galvanically connected electrically.

There are several possibilities of the layer structure,

• Depending on a position symmetrical,
• A position unbalanced,
• Two layers symmetrically,
• Two layers unbalanced,
• Microvias over two layers (stacked via).

For PCBs, high density ( HDI PCB, High Density Interconnect) microvia technology is necessary because connected electrically due to lack of space and the small distance between the contacts, not all contacts eg ball grid array components (BGA ) could be. How to Tie the pads of the BGA to Microviabohrungen to that end in a different position, thus ensuring their unbundling.

Buried Via Technology

The vias ( vias ) connect here two or more copper layers, but are only to be made between inner layers and not accessible from the platinum surface. Buried Vias (German: buried vias ) are thus possible only in multilayer boards from four layers.

Plugged -via technology

Besides Buried and micro vias, there is also still the possibility of vias close ( " pluggen " ) permit. With this technique, vias can be placed directly into SMD pads, which greatly simplifies the intervals with small ball unbundling eg BGA packages. The technique is relatively expensive and is rarely used, since the surface must also be ground and polished to remove excess material. The different ways to close a via specified in the IPC 4761st

Thick copper

The use of copper thickness from 200 microns to 400 microns will be referred to as thick-copper. They allow higher current capacities and lateral heat transport. Due to the etching process can be realized only rough conductor structures.

Alternatively, a circuit board with low copper thickness photolithographically patterned and reinforced by electroplating with copper. Subsequent etching can then without resist expose the conductive traces, so that the entire thickness of copper, but only the thin base layer must be etched.

A further development of thick copper technology is the technology iceberg (English: iceberg technique). The still closed copper layers in film form are pre-structured by a photolithographic etching process: areas that do not require a thick copper are etched back here at 20 microns or 100 microns. The sheets are then pressed into the prepreg, and further processed by conventional methods. The remaining low elevation allows finer Struktierung and possibly more reliable coverage with solder resist.

Thermal Management

Thermal vias improve the heat transport perpendicular to the circuit board. The thermal conductivity of inexpensive base materials such as FR4 with 0.3 W / m · K is too low for heat dissipation of devices. Thermal vias are plated-through holes, whose primary mission is to improve the thermal conductivity; they use the high thermal conductivity (300 W / m · K) of copper, the material of the via. By a dense array, such as on a hexagonal grid of 0.5 mm and a diameter of the vias of 0.25 mm, can be inserted into the circuit board effectively up to 10% copper. This results in a thermal conductivity of 30 W / m · K results perpendicular to the circuit board.

Metal core (English metalcore ) and thick copper allow a higher lateral thermal conductivity. These are copper or aluminum sheets or incorporated up to 400 microns reinforced copper layers in the PCB.

In conjunction with a thermal paste printing can be achieved a reduction of heat and avoid in certain cases, the use of additional heat sinks; has a circuit board in Eurocard format by convection with a thermal resistance of 6 K / W and due to thermal radiation of about 5 K / W.

There are also water-cooled circuit board in which fine grooves are cut on the top and bottom of the inner layer prior to assembly of the individual layers. After the assembly, there remains a channel here, can be passed through the cooling water.

Recently, PCBs are also provided on the narrow sides with a thin layer of copper, which can serve to improve heat dissipation. They may also contribute to a reduced radiation of electromagnetic fields.

A secondary aspect of thermal management concerns the distribution of heat during soldering: EDA programs focus specifically called thermal pads, where the connection to copper- filled areas is deliberately weakened to keep the heat in the solder joint and not to distribute in the copper surface.

Flexible printed circuit boards

As an alternative to fixed circuit boards are also thin flexible printed circuit boards, for example based on polyimide films using. The constructed therewith Flex circuits are more expensive, but can be used to save space by folding in the tightest structures eg in cameras, video cameras, or smartphones.

Flexible connections for permanent strain, for example in ink jet printers are frequently also formed as a polyimide film printed circuit board.

However, only a non- permanently flexible portion in the circuit board is required to allow for example the installation in narrow space conditions, there is the approach of a plurality of prepregs (see below) built up stack of layers of a printed circuit board except for a few layers by milling or die-cut prepregs rejuvenate with recessed areas. The tapered portion is typically provided with a permanently flexible coating layer and can then turn a few times.

Press-fit and other Lötalternativen

As an alternative to soldering the component connections on a circuit board, there are press-fit technique. This elastic or rigid pins are pressed into close tolerances and metallized holes in the circuit board. Due to the plastic deformation of the metals involved to secure electrical connections arise even without soldering. As a main application, the injection of multi-pole connectors and threaded bolt has established. A further possibility is given by the use of adhesive. Here you choose between electrically non-conductive or conductive isotropic and anisotropic adhesives. Another technique is the bonding ( chip-on -board technology ). Thereby thinned (shallow etched or ground ) chips are bonded to the housing without the circuit board or soldered ( die-bonding ), and connected by means of thin wires to the respective contacts on the circuit board (see the wire bonding ). The bonded to circuit boards and chip bonding wires are protected by a light-absorbing resin.

Standards and regulations

There are a variety of rules and standards for the development and properties of printed circuit boards. In addition to DIN, IEC and standards of the Institute for Printed Circuits (IPC ) large companies have in some cases also own factory standards. In addition to these universal standards exist for rack systems standardized dimensions for printed circuit boards:

• Map of Europe (3 HE): 160 mm x 100 mm (DIN 41494 part 2), contacted on the narrow side
• Double Eurocard format (6U ): 233 mm × 160 mm, contacted on the wide side.

Test

PCBs are often subjected prior to delivery and assembly of a test. The visual control between the individual production steps (eg before applying another layer ) and at the end of the production is included in the printed circuit board manufacturers usually included in the price.

An electrical test at the end of the production is usually free of charge and requires the complete CAD data, and a test automaton, which contacts all signal paths and checks. When the automatic testing, a distinction between the in-circuit tester and the flying prober. The flying prober have several individual test finger, which abtesten the circuit boards. This technique has the great advantage that no adapters are required to contact and so even small series can be tested conveniently. A disadvantage includes the long test time for testing, and that this system usually not 100 - % strength test is carried out ( long time to test time ). In the in - circuit tester, the circuit boards with spring -tipped pen adapters or very fine so-called rigid needle adapters are tested. This technique has the advantage that all of the test points can be contacted at a time and so a very quick test with a 100 - % solution of test depth can be achieved. Today's MCA Micro Adapter (see rigid needle adapter) allow the Staggering contacting finest structures in microelectronics. The disadvantage here is the high cost adapter to mention, but which no longer worthy to be in greater numbers.

Ready assembled printed circuit boards can also be tested with an ICT test system, for which often additional contact pads layouted that are no longer needed in later use. To ensure that no such additional test points must be generated, a rigid needle adapter can also be used here, which allows the contact to the component ports, connectors, or even chips.

Often only a function of control at the end of the production is carried out, as the manufacturing technology of circuit boards is a matter of much more reliable than subsequent process steps.

Continuity test

Upon passing the test, the circuit board is tested for bad or missing links. These interruptions can be caused by mechanical damage or by film defects during exposure.

Operation

Upon passing the test, all belonging to a network points are tested against each other. Case of point connection can not be checked. Dirt on the contact points, the measurements may show a high-impedance result. Possible contaminants include dust, Fräsrückstände or oxidation on the bonding pad. Through a re-contacting ( retest ) these phantom errors ( errors that do not exist) are often excluded.

The measurement results are product specific classified at two-pole measurement, eg as follows:

• Measurement < 10 Ω → Good connection
• Measurement> 10 Ω → High-impedance connection
• Measurement> 2 milliohms → interruption

For measurements of compounds or below 10 Ω resistors often a four-wire measurement must be used, this does not distort the cable and contact resistance, the measurement result.

Short-circuit test

A short circuit is a connection between two points, which may not exist according to the circuit. Shorts are compounds that are caused for example by Zinnfäden, bad etching or mechanical damage of the insulating layer between the layers.

Operation

For each network, a test point is set as the primary test point. After the insulation is measured between all networks. If a board has 3 networks, Netz1 against Line2, Netz1 against Netz3 and Line2 is measured against Netz3. Are other networks available, the number of measurements behave according to:

• 2 = 1 measurement networks
• 3 networks = 3 measurements
• 4 nets = 6 measurements
• 5 networks = 10 measurements
• 6 nets = 15 measurements
• Networks N = N * (N -1) / 2 measurements

If an interruption detected when passing the test, there is another primary point is set and another sub- network generated ( net 3a). Thus, the circuit board can be 100% tested for shorts.

The measurement results are product-specific interpreted as follows:

• Measurement> 2 milliohms → No short circuit
• Measurement < 2 milliohms → High impedance short circuit
• Measurement < 100 Ω → short circuit

X-ray test

Especially for multi-layer boards and x-rays are used to perform a visual inspection can, for example, the fit of the different layers.

Load of interconnect structures with large currents

Frequently, particularly in thick-film hybrid technology, there is a need to bare boards with a larger test on constrictions to test bad vias etc.. Such imperfections are then destroyed and can be recognized as an interruption. A non-destructive means of testing of printed circuit boards and in operation is the thermography.

Circuit board base material

Important characteristics:

• Chemical Properties
• Electrical properties
• Mechanical properties
• Thermal Properties

Base material producing a circuit board

In the first impregnating the base resin, solvents, hardeners, accelerators are mixed. The other substances may be added, such as color pigments, flameproofing agents and flexibilizers. The carrier materials ( eg paper, glass fabric, aramid fabric ) are delivered in rolls, so that the process can be carried out continuously. After the carrier is pulled through the bath, over guide rollers ( impregnation ), the material is dried in the oven. In this case, not only the solvent but also the resin is achieved by the supply of heat to evaporate an intermediate stage - that the resin is not yet fully cured, upon renewed supply of heat, it is first again, and then tacky only hardens. This semi-finished resin and carrier is called prepreg. It is used for the preparation of printed circuit boards by the layers are pressed under the influence of heat. For multi -layer circuit boards more layers base material and copper are successively pressed and etched.

Compounds

Mechanical connections

In the assembly of printed circuit boards in a housing, a distance must be ensured between the metal mounting base and possibly the board. On the one hand, so that no short circuits occur, on the other, so that the uneven bottom of the board with the many solder points and partially protruding wire ends does not lie directly, which would lead to mechanical stresses. Is used, inter alia, long screws with spacers and nuts or plastic elements that are snapped into holes in the board and on the other side in the housing. Sometimes also described in the following electrical connection takes over with the mechanical part.

Electrical connections

If the circuit board is a plug-in card that sits on another board, it mostly uses direct connectors and female connectors.

Other multi-pin cable connections are realized via conductive rubber or power strips and headers, the contacts can be arranged in one or more rows. When it comes only a few poles and slats or small socket or coupling parts are placed on solder pins.

In special environments such as within a mechanical connector film cameras are selected that represent convenient flexible printed circuit boards, possibly with direct connectors at one or both ends, or alternatively, direct soldering.