Experience of the most popular printed circuit boa

2022-08-12
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Experience in PCB design

for electronic products, PCB design is a necessary design process from electrical schematic diagram to a specific product. The rationality of its design is closely related to product production and product quality. For many people who are just engaged in electronic design, they have less experience in this aspect, although they have learned PCB design software, However, the printed circuit boards designed often have such problems, and there are few articles in this regard in many electronic publications. The author has been engaged in the design of printed circuit boards for many years. Here, I will share some experience of printed circuit board design with you, hoping to play a role in throwing bricks and attracting jade. The author's PCB design software was Tango in early years, and now protel2.7 f or windows is used

board layout:

the usual sequence of placing components on the printed wiring board:

place components in fixed positions that closely match the structure, such as power sockets, indicator lights, switches, connectors, etc. after these components are placed, lock them with the lock function of the software, so that they will not be moved by mistake later; Place special components and large components on the line, such as heating elements, transformers, ICs, etc; Place small devices

distance between components and the edge of the board: if possible, all components and parts should be placed within 3mm from the edge of the board or at least greater than the thickness of the board. This is because in the mass production of assembly line plug-ins and wave soldering, they should be provided to the guide rail slot for use. At the same time, in order to prevent the edge part from being damaged due to shape processing, if there are too many components on the printed circuit board, they must be beyond 3mm, A 3mm auxiliary edge can be added to the edge of the plate, and a V-shaped groove can be opened on the auxiliary edge, which can be broken by hand during production. Isolation between high and low voltage: there are high-voltage circuits and low-voltage circuits on many printed circuit boards at the same time. The components and parts of the high-voltage circuit should be separated from the low-voltage part, and the isolation distance is related to the withstand voltage. Then turn the upper plate down to the adjusted spacing position within 1s. Generally, the distance on the plate should be 2mm at 2000kv, which should be increased in proportion. For example, if you want to withstand the withstand voltage test of 3000V, The distance between high and low voltage lines should be more than 3.5mm. In many cases, slots are also made between high and low voltage lines on the printed wiring board to avoid creepage

routing of printed wiring board: the layout of printed wiring should be as short as possible, especially in high-frequency circuits; The corners of printed wires should be rounded, and right angles or sharp corners will affect the electrical performance in the case of high-frequency circuits and high wiring density; When wiring two panels, the conductors on both sides should be perpendicular, oblique, or curved to avoid parallel to reduce parasitic coupling; The printed wires used as the input and output of the circuit should avoid adjacent parallel as far as possible to avoid feedback. It is best to add grounding wires between these wires. Width of printed wire: the width of the wire should meet the electrical performance requirements and be convenient for production. Its minimum value depends on the current it bears, but the minimum value should not be less than 0.2mm. In high-density and high-precision printed circuits, the width and spacing of the wire can generally be taken as 0.3mm; The wire width should also consider its temperature rise under the condition of high current. The single panel experiment shows that when the thickness of copper foil is 50 μ m. When the wire width is 1 ~ 1.5mm and the current is 2a, the temperature rise is very small. Therefore, generally, the wire with the width of 1 ~ 1.5mm may meet the design requirements without causing temperature rise; The common ground wire of the printed wire should be as thick as possible, and if possible, use lines greater than 2 ~ 3mm, which is particularly important in the circuit with microprocessor, because when the local line is too thin, due to the change of the current flowing, the change of the ground potential, the level of the microprocessor timing signal is unstable, which will degrade the noise tolerance; The principles of 10-10 and 12-12 can be applied to the wiring between the IC pins of dip package, that is, when two wires are passed between the two pins, the pad diameter can be set to 50mil, and the line width and line distance are both 10mil. When only one wire is passed between the two pins, the pad diameter can be set to 64mil, and the line width and line distance are both 12mil

spacing of printed wires: the spacing between adjacent wires must meet the electrical safety requirements, and in order to facilitate operation and production, the spacing should also be as wide as possible. The minimum spacing should be at least suitable for the withstand voltage. This voltage generally includes working voltage, additional fluctuating voltage and peak voltage caused by other reasons. If the relevant technical conditions allow a certain degree of metal debris between the conductors, the spacing will be reduced. Therefore, the designer should take this factor into account when considering voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased, and the signal lines with great difference between high and low levels should be as short as possible and the spacing should be increased

shielding and grounding of printed wiring: the common ground wire of printed wiring should be arranged at the edge of the printed wiring board as far as possible

copper foil should be reserved as much as possible on the printed circuit board as the ground wire, so that the shielding effect is better than a long ground wire, the transmission line characteristics and shielding effect will be improved, and the distributed capacitance will be reduced. The common ground wire of the printed wire is best made into the corresponding fixture according to the installation position of the real vehicle and arranged on the test bench to form a loop or shape. This is because when there are many integrated circuits on the same board, especially the components that consume much power, the ground potential difference is generated due to the graphic restrictions, which leads to the reduction of noise tolerance. When it is made into a loop, the ground potential difference is reduced. In addition, the graphics of grounding and power supply should be parallel to the flow direction of data as far as possible, which is the secret of enhancing the ability to suppress noise; Multilayer printed circuit board can adopt several layers as the shielding layer, and the power layer and ground layer can be regarded as the shielding layer. Generally, the ground layer and power layer are designed in the inner layer of multilayer printed circuit board, and the signal line is designed in the inner layer and outer layer

pad: pad diameter and inner hole size: the inner hole size of pad must be considered from the aspects of element lead diameter and tolerance size, tin lining thickness, hole diameter tolerance, hole metallized plating thickness, etc. the inner hole of pad is generally not less than 0.6mm, because the hole less than 0.6mm is not easy to process when punching. Generally, the metal pin diameter value plus 0.2mm is used as the inner hole diameter of pad, For example, when the metal pin diameter of the resistance is 0.5mm, the inner hole diameter of the pad corresponds to 0.7mm, and the pad diameter depends on the inner hole diameter, as shown in the following table:

hole diameter

0.4

0.5

0.6

0.8

1.0

1.2

1.6

2.0

pad diameter

1.5

1.5

2

2.5

3.0

3.5

4

1. When the pad diameter is 1.5mm, in order to increase the peel strength of the pad, Oblong pads with a length of not less than 1.5mm and a width of 1.5mm can be used, which are the most common in integrated circuit pin pads

2. For the pad diameter beyond the scope of the above table, the following formula can be used:

holes with a diameter less than 0.4mm: D/D = 0.5 ~ 3 some tension machine manufacturers are still using 1064 nm laser rangefinders

holes with a diameter greater than 2mm: D/D = 1.5 ~ 2

where: (D - pad diameter, D - inner hole diameter)

other notes about the pad:

the distance from the edge of the inner hole of the pad to the edge of the printed board should be greater than 1mm, This can avoid pad damage during processing

opening of the pad: some devices are repaired after wave soldering, but because the inner hole of the pad is sealed by tin after wave soldering, the device cannot be inserted. The solution is to make a small opening on the pad during PCB processing, so that the inner hole will not be sealed during wave soldering, and the normal welding will not be affected

pad repair teardrop: when the wiring connected with the pad is thin, the connection between the pad and the wiring should be designed into a water drop shape, which has the advantage that the pad is not easy to peel, but the wiring and the pad are not easy to disconnect

adjacent pads should avoid forming acute angles or large areas of copper foil, which will cause difficulty in wave soldering, and there is a risk of bridging. Large areas of copper foil will not be easy to weld due to rapid heat dissipation

large area copper coating: large area copper coating on printed circuit board is often used for two functions, one is heat dissipation, and the other is used for shielding to reduce interference. Beginners often produce an experimental force with high accuracy at the indenter when designing printed circuit board. A mistake made on the workpiece is that there is no window on the large area copper coating, and because the adhesive between the substrate of printed circuit board and copper foil is immersed in welding or heated for a long time, It will produce volatile gases that cannot be eliminated, and the heat is not easy to dissipate, resulting in copper foil expansion and falling off. Therefore, when using large-area copper coating, its open window should be designed into shape

use of jumper wires: in the design of single-sided printed circuit board, jumper wires are often used when some lines cannot be connected. Among beginners, jumper wires are often random, with long and short, which will bring inconvenience to production. When placing jumper wires, the less their types, the better. Usually, there are only three types: 6mm, 8mm and 10mm. Those beyond this range will bring inconvenience to production

the thickness of the printed circuit board shall be determined according to the function of the printed circuit board, the weight of the installed components, the specification of the socket of the printed circuit board, the overall dimension of the printed circuit board and the mechanical load it bears. The total thickness of multilayer printed boards and the distribution of thickness between layers shall be selected according to the needs of electrical and structural performance and the standard specifications of foil clad boards. Common PCB thicknesses include 0.5mm, 1 mm, 1.5mm, 2mm, etc. The above is the author's personal experience in designing printed circuit boards for many years. It is inevitable that there will be some deficiencies, which are welcome to be pointed out or discussed by peers

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