PCB Cabling Essentials and layout

I. Circuit board DESIGN steps In general, the basic process of designing a circuit board can be divided into three major steps.

(1). circuit schematic design: circuit schematic design is mainly PROTEL099 schematic design system (advanced Schematic) to draw a schematic diagram of the circuit. In this process, to make full use of the various schematic drawing tools provided by PROTEL99, various editing functions, to achieve our goal, that is, to get a correct, beautiful circuit schematic diagram.

(2). Generating a network table: The network table is a bridge between the circuit schematic design (SCH) and the printed circuit board design (PCB), which is the soul of the automatic circuit board. The network table can be obtained from the schematic diagram of the circuit, or it can be extracted from the printed circuit board.

(3). Printed Circuit board design: Printed circuit board design is mainly for the PROTEL99 of another important part of the PCB, in this process, we use PROTEL99 to provide a powerful function to achieve the layout of the circuit board, complete the work of high difficulty.

Two. Draw a simple circuit diagram

2.1 Schematic design process schematic design can be completed according to the following procedure.

　

(1) Design drawing size Protel 99/schematic, first of all to conceive a good part diagram, design a good drawing size. The size of the paper is based on the size and complexity of the circuit diagram, setting the appropriate paper size is the first step in designing a good schematic diagram.

(2) Set Protel 99/schematic design environment settings Protel 99/schematic design environment, including setting the size and type of the grid, cursor type and so on, most parameters can also use the system default values.

(3) Rotating parts users according to the needs of the circuit diagram, the parts from the part library to be taken out of the drawings, and the number of placed parts, parts package definition and set up and other work.

(4) schematic wiring using a variety of tools provided by Protel 99/schematic, the drawings on the components with electrical sense of the wire, symbols connected together to form a complete schematic.

(5) Adjustment line will be the preliminary drawing of a good circuit diagram for further adjustment and modification, making the schematic more beautiful.

(6) Report output through the various reporting tools provided by Protel 99/schematic to generate a variety of reports, the most important report is the network table, through the network table for the subsequent circuit board design preparation.

(7) file saving and printout The final step is to save the file and print the output.

The principle of microcontroller control Board design should follow the following principles:

　

(1) In the layout of components, should be related to each other components as close as possible, for example, clock generators, crystal, CPU clock input is easy to produce noise, when placed should be closer to them. For those prone to noise of the device, small current circuit, high-current circuit switching circuit, should try to keep it away from the microcontroller logic control circuit and storage circuit (ROM, RAM), if possible, these circuits can be made into a circuit board, which is conducive to anti-interference, improve the reliability of the circuit work.

(2) As far as possible in key components, such as ROM, RAM and other chips next to the installation of decoupling capacitor. In fact, printed circuit board traces, pin connections and wiring can contain a large inductance effect. A large inductor may cause severe switching noise spikes on the VCC line. The only way to prevent the switching noise spikes on the VCC line is to place a 0.1uF electronic decoupling capacitor between VCC and the power source. If a surface mount element is used on the circuit board, it can be fastened directly to the component with a chip capacitor on the VCC pin. It is best to use ceramic capacitors, because they have low electrostatic loss (ESL) and high-frequency impedance, and the dielectric stability of this capacitor temperature and time is good. Try not to use a tantalum capacitor because it has higher impedance at high frequencies. The following points need to be taken into consideration when placing the decoupling capacitor:

At the power input of the printed circuit board, the electrolytic capacitance around 100uF is bridged, and if the volume permits, the capacitance is better.

In principle, each IC chip next to the need to place a 0.01uF ceramic capacitor, if the gap is too small to place the circuit board, you can place a 1~10 tantalum capacitor around every 10 chips.

For components with a weak anti-jamming capability, large current changes during shutdown, and memory components such as RAM and ROM, the decoupling capacitor should be connected between the power line (VCC) and the ground.

The lead of the capacitor should not be too long, especially the high-frequency bypass capacitor cannot be leaded.

(3) in the single-chip microcomputer control system, there are many types of ground wire, such as systematically, shielded, logically, simulated ground, whether the ground is reasonable layout, will determine the anti-jamming ability of the circuit board. The following questions should be considered when designing ground and pick-up locations:

Logically and simulated ground to separate wiring, can not be combined, their respective ground wires are connected with the corresponding power ground. In the design, the analog ground should be as bold as possible, and as far as possible to increase the grounding area of the terminal. Generally speaking, the analog signal for the input and output, and the microcontroller circuit is best separated from the optocoupler.

When designing the printed circuit version of the logic circuit, the ground wire should form a closed loop and improve the anti-jamming ability of the circuit.

The ground should be as coarse as possible. If the ground is very thin, then the ground resistance will be larger, resulting in the earth potential changes with the current changes, resulting in signal level instability, resulting in the circuit's anti-jamming ability to decline. In the case of space permitting, to ensure that the main ground wire width at least 2~3mm, the element pin on the grounding line should be around 1.5mm.

Be aware of the pick-up location. When the signal frequency of the circuit board is less than 1MHz, because the electromagnetic induction between the wiring and components is very small, and the circulation of the grounding circuit has a great influence on the interference, so we should use a little ground, so that it does not form a circuit. When the circuit board signal frequency above 10MHz, because the inductance effect of the wiring is obvious, the ground impedance becomes very large, at this time the circulation of the grounding circuit is no longer the main problem. Therefore, multi-point grounding should be used to minimize ground impedance.

Power line Arrangement In addition to the size of the current as far as possible to increase the thickness of the line width, in the wiring should also make the power line, the direction of the wire line and the line of data line alignment of the end of the wiring work, with the ground to the bottom of the circuit board without traces of the place covered, these methods are helpful to enhance the circuit

The width of the data line should be as wide as possible to reduce the impedance. The width of the data cable is at least 0.3mm (12mil), which is preferable if 0.46~0.5mm (18mil~20mil) is used.

As the circuit board of a hole will bring about 10pF capacitance effect, this for high-frequency circuit, will introduce too much interference, so in the wiring, should be as far as possible to reduce the number of holes. Again, too much vias can also cause the mechanical strength of the board to be reduced.

Additional:

One, the route:

Input and output lines should try to avoid parallel, so as not to cause interference. The two signal lines parallel traces should be added ground isolation, two adjacent layer wiring to try to perpendicular to each other, parallel prone to parasitic coupling. Power and ground should be divided in two layers perpendicular to each other.

Second, the line width:

Usual signal line width: 0.2~0.3mm, power and ground: 1.2~2.5mm

Power and ground should be widened as far as possible, three relations: ground power line Signal line

The digital circuit PCB can be a wide range of ground to do a circuit, that is, to form a network (analog circuit can not be used), with a large area of copper.

Iii. co-treatment of

If the analog and digital signals require a PCB to be co-located, then only the PCB and externally connected ports, analog and digital ground has a short contact. Note: Only a little bit short, the inside of the board is actually separate.

Layout of the board:

The usual order of component placement on a printed circuit board:

Placing components with a fixed position in close coordination with the structure, such as power sockets, indicators, switches, connectors, etc., these devices are placed and locked by the lock function of the software, so that they will not be mistakenly moved later;
Placing special components on the line and large components, such as heating elements, transformers, ICS, etc.;
Place the small device. The distance of the component from the edge of the board: if possible, all components are placed at the edge of the board 3mm or at least greater than the thickness of the plate, this is due to the high-volume production line plug-in and wave soldering, to provide to the guide groove use, but also to prevent the contour processing caused by the edge part of the defect, If there are too many components on the printed circuit board, it is necessary to exceed the 3mm range, the edge of the plate can be added with 3mm auxiliary edge, the auxiliary side Open V Groove, in the production of hand break can be.

Isolation between high and low voltage: In many printed circuit boards at the same time there are HV circuits and low-voltage circuit, high-voltage circuit part of the components and low-voltage parts to be separated from the place, the isolation distance and to withstand the pressure is related, usually in the 2000kV when the board to a distance of 2mm, above in proportion to increase, For example, to withstand the pressure test of 3000V, the distance between the high and low voltage lines should be above 3.5mm, in many cases to avoid creepage, but also on the printed circuit board between the low pressure groove.

Route of printed circuit board:
The layout of printed wiring should be as short as possible, in the high-frequency circuit should be the case, and the turn of the printed wire should be rounded, and the right angle or sharp angle in high-frequency circuit and wiring density of the case will affect the electrical performance; When the two panels are routed, the conductors on both sides should be perpendicular, oblique, or curved to avoid , as the circuit of the input and output of the printed wiring should try to avoid adjacent parallel, in order to avoid the return of the delegation, between these conductors preferably add ground wire.

Width of printed wire:
Wire width should be able to meet the electrical performance requirements and ease of production is appropriate, its minimum
Value to withstand the size of the current, but the minimum should not be less than 0.2mm, in high-density, high-precision printed circuit, wire width and spacing is generally preferable to 0.3mm; conductor width in the case of large currents also consider its temperature rise, single panel experiment shows that when the thickness of copper foil 50μm, wire width 1~1.5mm, With a current of 2A, the temperature rise is very small, so Generally choose the 1~1.5mm width of the wire may meet the design requirements without causing temperature rise; The common ground wire should be as coarse as possible, if possible, the use of lines larger than 2~3mm, which is particularly important in circuits with microprocessors, because the local line is too thin, due to the change of current flowing, ground potential change, microprocessor timing signal level Instability, will make noise tolerance deterioration; In the DIP package IC foot line, can apply 10-10 and 12-12 principle, that is, when the two feet through 2 lines, the pad diameter can be set to 50mil, line width and line distance are 10mil, when the two feet only through 1 lines, the pad diameter can be set to 64mil, Both the line width and the line distance are 12mil.

Spacing of printed conductors:
The distance between adjacent conductors must meet electrical safety requirements, and for ease of operation and production, the spacing should be as wide as possible. The minimum spacing must be at least suitable for the voltage to withstand. This voltage typically includes the operating voltage, additional fluctuating voltage, and other causes of peak voltage. If the technical conditions allow some degree of metal residue between the conductors, the spacing will be reduced. So the designer should take this factor into consideration when considering the voltage. When the wiring density is low, the spacing of the signal line can be enlarged suitably, and the signal line with high and low level should be as short as possible and increase the spacing.

Shielding and grounding of printed conductors:
The common ground wire of the printed wiring shall be arranged at the edge of the printed circuit board as far as possible. In the printed circuit board should be as much as possible to retain the copper foil to make ground, so that the shielding effect is better than a long wire, transmission line characteristics and shielding effect will be improved, in addition to reduce the role of distributed capacitance. The common ground wire of printed wiring preferably form a loop or mesh, this is because when there are many integrated circuits on the same board, especially the power consumption of many components, due to the limitations of the graphics generated ground potential difference, resulting in a reduction in noise tolerance, when the circuit, the earth potential decrease. In addition, the ground and the power of the graph as far as possible with the data flow direction parallel, this is the secret to suppress the noise capacity enhancement; Multilayer printed circuit boards can take several layers as shielding layer, the power layer, ground layer can be regarded as shielding layer, the general ground layer and the power layer is designed in multilayer printed circuit board inner layer, signal line design in

Solder Pads:
The diameter of the pad and the size of the hole: the inner hole size of the pad must be considered in terms of the component lead diameter and tolerance size, as well as the thickness of the tin-plated layer, the aperture tolerance, the thickness of the plating layer of the hole metallization, The inner hole of the pad is generally not less than 0.6mm, because less than 0.6mm of Kongkai punching is not easy to process, usually with the metal pin diameter and 0.2mm as the diameter of the pad hole, such as the resistance of the metal pin diameter of 0.5mm, the diameter of the solder pad is 0.7mm, the diameter of the pad depends on the diameter of the hole, as follows:
Hole Diameter: 0.4 0.5 0.6 0.8 1.0 1.2 1.6 2.0
Solder pad Diameter:1.5      1.5      2.0        2.5      3.0      3.5       4

1. When the diameter of the pad 1.5mm, in order to increase the strength of the pad peel, can be a length of not less than 1.5mm, width of 1.5mm and long round pad, this kind of pad in the IC pin pad is the most common.
2. For a pad diameter that exceeds the range of the above table, the following formula is available:
holes less than 0.4mm in diameter: d/d=0.5~3
holes with diameters greater than 2mm: D/d=1.5~2
in: (D-pad diameter, D-bore diameter)

Other points of note about pads:
The distance from the inner hole edge of the pad to the edge of the PCB is greater than 1mm, which can avoid the defect of the solder disc during processing.
The opening of the pad: Some devices are after the wave soldering after the solder, but because after the solder plate after wave soldering is sealed, so that the device can not be inserted, the solution is in the printed circuit board processing on the pad open a small mouth, so that the internal hole wave soldering will not be sealed, and will not affect the normal welding.
Pads to fill the tear drops: when the wire with the pad connection is thinner, the connection between the pads and the line is designed to be a teardrop-shaped, the advantage is that the pads are not easy to peel, but the line and the pads are not easy to break. Adjacent pads to avoid a sharp angle or large area of copper foil, into a sharp angle will cause the wave soldering difficulties, but also the danger of bridging, large area of copper foil due to heat dissipation will lead to difficult to weld.
Large area of copper: printed circuit board large area of copper is often used in two roles, one is heat dissipation, one for shielding to reduce interference, a beginner design printed circuit board often made a mistake is a large area of copper on the open window, and due to the PCB board and copper foil between the binder in the dip welding or long time heating , will produce volatile gas can not be ruled out, heat is not easy to distribute, resulting in copper foil expansion, shedding phenomenon. Therefore, when using large areas of copper, the open window should be designed to mesh.
Cross-wiring use: In the single-sided printed circuit board design, some lines can not be connected, often used to cross-wiring, in beginners, cross-wiring is often arbitrary, there is a short, this will bring inconvenience to the production. When placing the cross-wiring, the less the better, usually only set 6mm,8mm,10mm three, beyond this range will cause inconvenience to the production