Friday, March 18, 2011

【 Weak current College 】 with PROTEL DXP circuit board design with the General principles 】

Newbie guide--with PROTEL DXP circuit board design with the General principles General principles for the design of circuit boards including: the selection of the circuit board, circuit board size and component layout, wiring, pads, padding, cross-wiring, etc. Circuit board usually deposited copper laminates, lamellar selection from electrical performance, reliability and process requirements and economic indicators, and other considerations. Commonly deposited copper laminates are deposited copper paper-phenolic laminate, deposited copper epoxy paper laminates, deposited copper epoxy glass cloth laminate, deposited copper epoxy-phenolic glass cloth laminate, deposited copper PTFE glass cloth laminate and multilayer printed circuit board with epoxy glass cloth, etc. Different materials of the laminates have different characteristics. Epoxy resin and copper foil with excellent adhesion, copper foil adhesive strength and working temperature is higher, you can in the molten Tin 260 ℃ not bubble. Epoxy resin impregnated glass cloth laminate moisture less gas. UHF Board preferably deposited copper PTFE glass cloth laminate. The requirements for fire retardant to electronic devices, Circuit Board also needs to be flame-resistant, these boards are baptized into the flame retardant resin laminate. Board thickness should be based on the circuit board of the function, the weight of the loaded components, PCB socket specifications, circuit board size and withstand mechanical load, etc. Mainly should ensure sufficient rigidity and strength. Common board thickness 0.5mm, 1.0mm, 1.5mm, 2.0mm from cost, copper coating line length, robust capabilities, circuit board size as small as possible, but the Board size is too small, the cooling-performing and adjacent conductors easily cause interference. Circuit board production costs is and circuit board area related to the larger, higher cost in the area. In the design of the enclosure of the circuit board, circuit board size is also affected by the chassis cover size, be sure to determine the PCB size determines the size of the enclosure before, otherwise it is not possible to determine the size of the Board. Generally, prohibited wiring layer specifies the scope of the wiring is the size of the circuit board size. Circuit Board is in the best shape, aspect ratio 3: 2 or 4: 3, when the dimensions of a circuit board is greater than 200mm×150mm, consideration should be given to the Board of mechanical strength. In short, the pros and cons should be considered to determine the size of the Board. Though Protel DXP is able to auto-layout, but in fact almost all of the circuit board layout is performed manually. When you want to layout, follow the following rules: 1. a special component layout, the layout of special components based on these considerations: 1 high-frequency components: high frequency) component of the connection between the shorter the better, try to reduce the wiring of distribution parameters and mutual electromagnetic interference, susceptible to interference components can not be too close. Attached to the input and output components belong to the distances between some should be as large as possible. 2) have a high potential difference components: should increase with high potential difference components and the distance between the line to avoid accidental short-circuit damage components. In order to avoid the Creepage phenomena occur, 2000V potential difference between copper coating line distance should be greater than 2mm, for greater potential difference, distance should also be increased. With high-voltage devices, should try layout when debugging is difficult to reach them in hand. 3) weight too large components: these components should have a bracket, which is large and heavy, the calorific value of the component to be installed on the circuit board. 4) heating and thermal components: note the heating element should be kept away from the thermal components. 5) can be adjusted by the symbol: for potentiometer, adjustable inductance coils, variable capacitors, micro switch, adjustable components layout should consider the structure of the machine, if within the regulation, should be placed in the circuit board is easy to adjust, if it is outside the machine, its location to the chassis with the knob in position on the Panel. 6) circuit board mounting holes and bracket holes: should set aside a circuit board mounting holes and brackets for mounting holes, as these holes and holes nearby are not routed. 2. in accordance with the circuit function layout if there are no special requirements as much as possible in accordance with the principle diagram of components on the component layout, signals from left to right, from top of the output from the input, the output from the bottom. Follow the circuit processes, each functional circuit unit arrangements so that the flow more smoothly and signal remains consistent direction. To each function circuit as the core, built around the core circuit arrangements for layout, components should be uniform, neat, compact, principle is reduced and shorten the leader between the various components and connections. Digital circuit parts should be separated from layout of analog circuits. 3. a component from the PCB edge distance all components should be placed away from the plate edge mm, or at least the distance from the circuit board edge is equal to the thickness, this is due to mass production in the line of plug-in and soldering, to use provided to guide Groove, but also prevent the appearance of the edge of the damaged caused by circuit board processing, line break caused by copper lead scrap film. If the circuit board components too much, had to go beyond 3mm, you can add the circuit board edge 3mm aux auxiliary side, V-shaped grooves along the edge of the opening, in production with their hands when they broke up. 4. component placement order be placed and structural work closely with the fixed-position elements, such as the power outlet, led, switch and connect plug-ins, etc. Then place the special symbols, such as heating elements, power transformer, integrated circuits, etc. Last place small components such as resistors, capacitors, diodes, etc. Routing rules are as follows: 1) cord length: copper coating line should be as short as possible, in high-frequency circuit more so. Copper coating line should not bend to rounded or chamfered corners, square or sharp corners in the high-frequency circuits and wiring density of circumstances affect the electrical properties. When the dual panel wiring, both sides of the wire should be perpendicular or diagonal or bend alignment, avoid parallel to reduce parasitic capacitance. 2) line width: the width of the copper film should be able to meet the requirements of electrical properties and ease of production to its minimum value depends on the current flowing through it, but not less than 0.2mm in General. As long as the area is large enough, copper coating line width and spacing is best to choose 0.3mm. In General, 1 ~ 1.5mm line width, and allow current to flow through the 2A. For example the best ground and power cordSelect the line width greater than 1 mm. In integrated circuit block, pads go between two lines, the pad diameter 50mil, line width and line spacing are 10mil, when pads a line between the pad diameter 64mil, line width and line spacing are 12mil. Note metric and inch conversion between 100mil = 2.54mm. 3) line spacing: of adjacent copper film lines of space between the electrical safety requirements should be met, at the same time in order to facilitate production, spacing should be wider and better. The minimum spacing is at least able to afford the voltage peaks. In the wiring density low, spacing should be as big. 4) shielding and grounding: copper film lines of public ground should as far as possible on the circuit board edge. The Board should be retained as much as possible, so that the ground wire copper do allows masking capability enhancement. Another ground of shape best make loops or grid. Multi-layer circuit board as a result of the innermost do private power and ground layers, so that you can play better shielding effect. Pad welding plate dimensions pads of bore dimensions must be from the component lead diameter and tolerances of dimensions and the tin coating thickness, diameter tolerance, hole metal plating layer thickness, etc, normally to a metal pin diameter plus 0.2mm as pads of bore diameter. For example, the resistance of a metal pin diameter 0.5mm, pads hole diameter is 0.7mm, pad diameter should be added to the pad diameter 1.2mm, minimum aperture for pads with 1.0mm. When the pad diameter 1.5mm, in order to increase the pads of the anti-stripping strength, you can use a square pad. For the hole diameter is less than the pads, 0.4mm pad diameter/pads hole diameter = 0.5 ~ 3. For hole diameter greater than 2 mm pad, pad diameter/pads hole diameter = 1.5 ~ 2. The commonly used welding plate dimensions such as table 1-1 as shown in table 16-1 common welding plate dimensions
Pad diameter/mm 0.4 0.5 0.6 0.8 1.0 1.2 1.6 2.0
Pad diameter/mm 1.5 1.5 2.0 2.0 2.5 3.0 3.5 4
Design considerations for pads is as follows: 1) pads hole edge to circuit board edge distance to greater than 1 mm, this prevents machining defect: when the pads. 2) pad repair tears, when the connection with the pads of fine copper coating line, to pads and copper connections between membrane line designed to tears, so you can make the pad is not easy to be split, but copper film lines and lines that connect the pads are not easily broken. 3) adjacent to the pads to avoid a sharp corner. Large area fill circuit board area fill purposes there are two, one is the heat, the other is a shielded to reduce interference, to avoid welding heat when the Board have no emissions of gases that copper film peeling, should be on large-area filled with window, which makes the fill for the grid. Use deposited copper can achieve the purpose of anti-jamming and deposited copper can automatically bypass the pads and connect the ground wire. Cross-wiring in the single-sided printed circuit board design, when some copper film cannot connect, the common practice is to use cross-wiring, cross-wiring length should select the following: 6mm, 8mm and 10mm. A total of grounding 1 ground impedance ground jamming circuit diagram on that circuit zero potential, and as a circuit other points of the common reference point, in the actual circuit as the ground wire (copper coating line) impedance exists, will bring total impedance interference, wiring, cannot have the ground symbol point casually connected together, this can cause unwanted coupling and the impact of circuit function. 2. how to connect the ground wire is usually an electronic system, the Earth is divided into the system, the enclosure (shield), digital (logically) and analog several connecting ground wire should note the following: 1) correctly select the single point of grounding and multi-point grounding. In the low-frequency circuits, signal frequency less than 1MHz, wiring and components between inductance can be ignored, and the ground circuit resistance of the pressure drop on a large influence on circuit, so it should be a single point of grounding. When the signal frequency greater than 10MHz, ground inductance of larger, so the use of a nearby ground of multi-point grounding method. When the signal frequencies between 1 ~ 10MHz, if single-point grounding, ground wire length should not exceed wavelength of 1/20, otherwise you should use more grounded. 2) digital and analog ones. The Board has both a digital circuit, analog circuit, you should make them separate from the ground as possible, but cannot be mixed, with the power of the ground wire-end connections (the best power-to-end respectively). To increase the area of linear circuit. General digital circuit of anti-interference ability, TTL circuits of noise 0.4 ~ 0.6V, CMOS Digital circuit noise-power voltage 0.3 ~ 0.45 times, and analog circuits part as long as the noise of the microvolt, is enough to make it work properly. So two circuits should separate the layout and wiring. 3) as far as possible, bold ground wire. If the ground is very thin, ground potential with current changes, causing the electronic system of signal interference, in particular the analog circuit, therefore the ground should be wide, generally greater than 3mm suitable. 4) will constitute a closed-loop Earth wire. When the Board only digital circuit, you should make the ground form loops, which can significantly improve the antijam capability, this is because when the Board have a lot of integrated circuit, if the ground is very thin, could lead to higher ground potential difference, and the circular ground can reduce resistance, thereby reducing ground potential difference. 5) received the same level circuit should be as close as possible to the place, and at the circuit of a power filter capacitor should take place at the next level. 6) ground connection. Total ground wire must be in strict accordance with hf, MF, Louisa was always low frequency order from weak to strong electrical connection. High frequency part of the large bracket is preferred, in order to ensure that the ground wire has a good shielding effect. Anti-jamming with microprocessor electronic systems, interference and electromagnetic compatibility is the design process must be considered, particularly for the clock frequency is high, the bus cycle fast systems;There is a high power, high current drive circuit system; weak analog signal and high-precision A/D converter circuit system. In order to increase system capacity of electromagnetic interference should consider the following measures: 1) selection of clock frequency lower microprocessor. As long as the controller performance to meet the requirements, the clock frequency, the lower the better, and lower clock can effectively reduce noise and improve system anti-interference ability. Since the square contains a variety of frequency components, its high-frequency components can easily become a source of noise, in General, the clock frequency 3 x high-frequency noise is the most dangerous. 2) reduce the distortion of the signal transmission. When the high-speed signal (signal frequency high = rising and falling edge of signal) in copper film line transport, due to the copper coating line inductance and capacitance effects, cause the signal distortion, and when the distortion is too large, it will make the system work is not reliable. General requirements, signal circuit board film on copper wires as short as possible, through-hole, the fewer the number the better. Typical values: length not exceeding 25cm, hole number not more than 2. 3) reduces signal interference between cross. When a signal cable with pulse signals, on the other has a high input impedance of weak signal interference, in which case needs to be isolated with weak signal line, the method is a ground contour would surround the weak signal, or increasing the line spacing, for interference between different levels can be used to increase the power and ground-level solutions. 4) to reduce noise from power supply. Power to the system to provide energy, but also its noise added to the power supply system, system reset, interrupt and other control signal the most susceptible to interference from ambient noise, therefore, be appropriate to increase the capacitance to filter out the noise from the power supply. 5) takes note of the circuit boards and components of high frequency characteristics. In the high-frequency, circuit board copper film line, pads, hole, resistance, capacitance, plug in distributions of inductance and capacitance is not ignored. As a result of these distribution effect of inductance and capacitance, when the length of copper coating line for signal or noise wavelength of 1/20, the antenna effect, on the internal, external electromagnetic interference emission of electromagnetic waves. In General, through-hole and pads will produce the capacitor, a 0.6pF integrated circuit package produces 2 ~ 6pF capacitor, a circuit board connector generates 520mH inductance, and has a DIP-24 socket 18nH inductance, capacitance and inductance on low clock frequency circuit does not have any impact for high clock frequency circuit must give attention to. 6) component layout to reasonable partition. Components in the placement of the Board should give full consideration to the electromagnetic interference issues. Principle is one of the various components of copper film between the lines to short, in the layout, the analog circuit, digital circuit and produce large noise circuit (relay, high current switch, etc.) reasonable separate, making them among the smallest of signal coupling. 7) handle the ground wire. In accordance with the preceding point grounding or more grounded approach to ground potential. Will analog, digital, high power devices with separate connections, and then aggregated to the power of the grounding. Circuit board other than the leader you want to use shielded cables, high-frequency and digital signal, shielded cable ends are ground, low-frequency analog signals with shielding line, single and grounded. On noise and disturbance sensitive circuits or a high-frequency noise particularly serious circuit should use the metal shielding enclosures shield. 8) decoupling capacitor. Decoupling capacitor to ceramic capacitor or multilayer ceramic capacitor of high frequency characteristics. Design of circuit boards, each integrated circuits between the power and ground to a decoupling capacitor. Decoupling capacitor serves two purposes: first, the integrated circuit capacitor energy storage to provide and absorb the IC's door and closing moments of the charge and discharge of electrical energy, on the other hand, bypass the devices produced by high-frequency noise. Digital circuit in typical decoupling capacitor as than 0 μ F, this capacitor inductance of 5nH, you can have the following noise 10MHz good decoupling effect. In General, select 0.01 ~ than 0 μ F capacitor can be. General requirements no 10 tablets of IC an additional 10 μ F capacitor of charging and discharging. In addition, power-, circuit boards, such as location of four corners should be a crossover 10 ~ 100 μ F capacitor. High frequency and high frequency circuit board wiring in order to make the design more reasonable, anti-interference performance better, PCB design should be based on these considerations: 1) reasonable choice of number of layers. Use the inner plane as a middle power and ground layers, you can play the role of the shield, effectively reducing parasitic inductance, reduced signal cable length, lower signal cross-interference between, typically, 4-layer Board than the two-tier Board of low noise 20dB. 2) alignment. Alignment must be in accordance with 45 ° angle corner, this will reduce the high frequency signal emission and coupling between each other. Length 3) alignment. Alignment length as short as possible, two lines parallel distance as short as possible. The number 4) hole. Through-hole, the fewer the number the better. 5) direction of interlayer wiring. The direction of interlayer wiring should be taken vertically, is the top level is horizontal, the bottom for a vertical orientation, which reduces signal interference between. 6) deposited copper. Additional grounding of deposited copper reduces signal interference between. 7) package. On important signal cable for package deal, you can significantly improve the signal of a antijamming ability, of course you can also make the source of interference with the package so that it does not interfere with other signals. 8) signal cable. Signal alignment cannot loop, you need to follow the Daisy way wiring. 9) decoupling capacitor. In the IC power end crossover decoupling capacitor. 10) HF choke. Digital, analog, connection to public ground received high frequency choke devices, usually the center hole to wear a high-frequency ferrite beads of traverse.

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