Printed circuit board design experience

For electronic products, the printed circuit board design is a design process that must be passed from the electrical schematic to a specific product. The rationality of the design is closely related to the product's production and product quality, and many of them have just engaged in electronic design. For the personnel, there is little experience in this area. Although the printed circuit board design software has been learned, the printed circuit boards designed often have such problems, and many electronic publications have few articles in this regard. The author has worked on printed circuit board design for many years. Here he shares the experience of printed circuit board design with everyone. He hopes that he can play a role in attracting the attention of the public. The author's printed circuit board design software was TANGO for a few years and now uses PROTEL2.7F OR WINDOWS.
Board layout:
The general order of placement of components on a printed circuit board:

Place components in close proximity to the structure, such as power outlets, indicator lights, switches, connectors, etc. These devices are locked and locked with software's LOCK function so that they will not be misplaced later; Place special components and large components on the line, such as heating elements, transformers, ICs, etc.; Place small devices.

The distance from the edge of the component to the board: Wherever possible, all components are placed within 3mm from the edge of the board or at least larger than the board thickness. This is due to the fact that in the case of mass-produced assembly cards and wave soldering, rails must be provided The use of, but also to prevent the edge of the defect caused by the shape processing, if there are too many components on the printed circuit board, had to exceed 3mm range, you can add 3mm auxiliary edge in the edge of the board, auxiliary edge open V-shaped Grooves can be broken by hand during production. Isolation between high and low voltage: In many printed circuit boards, there are high-voltage circuits and low-voltage circuits at the same time. Components and low-voltage parts of the high-voltage circuit must be separated and placed. The isolation distance is related to the withstand voltage. Under normal circumstances, At 2000kV, the distance between the board is 2mm. Above this, the ratio must be increased. For example, to withstand a withstand voltage test of 3000V, the distance between high and low voltage lines should be 3.5mm or more. In many cases, the Creepage also slots between the high and low voltages on the printed circuit board.

Printed circuit board traces: Printed wire routing should be as short as possible, especially in high-frequency loops; printed conductor turns should be rounded, while right angles or sharp corners in high-frequency circuits and wiring density In the case of high, the electrical performance will be affected; when the two panels are routed, the conductors on both sides should be perpendicular to each other, skewed, or bent so as to avoid parallelism to reduce the parasitic coupling; as the input and output of the circuit is printed. The conductors should avoid parallelism as much as possible to avoid feedback. It is best to add a grounding wire between these conductors. Printed conductor width: The width of the conductor should meet the requirements of electrical performance and it is convenient for production. Its minimum value depends on the magnitude of the current to be accepted, but the minimum should not be less than 0.2mm. It can be printed at high density and high precision. Line, the width and spacing of the wire is generally desirable 0.3mm; wire width under high current conditions also need to consider the temperature rise, single-panel experiments show that when the thickness of the copper foil is 50μm, wire width 1 ~ 1.5mm, through the current 2A, The temperature rise is very small, so generally choose 1 ~ 1.5mm width of the wire may meet the design requirements without causing temperature rise; printed conductors of the public ground should be as thick as possible, if possible, use more than 2 ~ 3mm of the line, This is particularly important in circuits with microprocessors, because when the local line is too thin, due to changes in the current flowing, the ground potential changes, the level of the microprocessor timing signal instability, will make the noise margin deteriorated; In the DIP package IC between the feet, you can apply the principles of 10-10 and 12-12, that is, when the two feet through the two lines, the pad diameter can be set to 50mil, line width and line spacing are 10mil, when Only one root between the two feet For the line, the pad diameter can be set to 64 mils, and the line width and line spacing are both 12 mils.

Printed conductor spacing: Adjacent conductor spacing must meet electrical safety requirements, and spacing should be as wide as possible for ease of operation and production. The minimum distance must be suitable for the withstand voltage. This voltage generally includes the operating voltage, additional ripple voltage, and other causes of the peak voltage. If the technical conditions permit the existence of a certain degree of metal remnants between the wires, the spacing will be reduced. Therefore, designers should consider this factor when considering voltage. When the wiring density is low, the spacing of the signal lines can be appropriately increased, and the signal lines with high and low levels should be as short as possible and increased in spacing.

Shielding and grounding of printed conductors: The common ground of printed conductors should be arranged as far as possible on the edge of the printed circuit board.

In the printed circuit board should be as much as possible to retain the copper foil as the ground, so the shielding effect is better than a long ground, the transmission line characteristics and shielding will be improved, in addition to play a role in reducing the distributed capacitance . The common ground of the printed conductors is preferably looped or meshed, because when there are many integrated circuits on the same board, especially when there are many power-consuming components, the ground potential difference is generated due to the restrictions on the patterns. This causes a reduction in the noise margin, and when made into a loop, the ground potential difference decreases. In addition, the grounding and power supply patterns should be parallel to the data flow direction as much as possible. This is the secret of enhanced noise immunity; several layers of the printed circuit board can be used as the shielding layer; the power layer and the ground layer can be visible. For the shielding layer, the general ground layer and the power layer are designed on the inner layer of the multilayer printed wiring board, and the signal lines are designed on the inner layer and the outer layer.

Pad: Pad diameter and inner hole size: The pad inner hole size must be considered from the component lead diameter and tolerance size and tin-silicon layer thickness, hole diameter tolerance, hole metallization layer thickness, etc., the inner hole of the pad Generally not less than 0.6mm, because the hole is less than 0.6mm hole punching is not easy to process, usually with the diameter of the metal pin plus 0.2mm as the diameter of the hole in the pad, such as the resistance of the metal pin diameter is 0.5mm, The diameter of the inner hole of the pad corresponds to 0.7mm, and the diameter of the pad 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 pad peel strength, a pad with a length of no less than 1.5mm, a width of 1.5mm, and an oblong pad may be used, and this kind of pad is the most common in IC pin pads. .

2. The pad diameter beyond the above table range can be selected using the following formula:

Holes less than 0.4mm in diameter: D/d = 0.5 to 3
Holes with a diameter greater than 2mm: D/d = 1.5 to 2
Where: (D-pad diameter, d-hole diameter)
Other notes about the pad:

The distance between the edge of the inner hole of the pad and the edge of the printed board should be greater than 1mm, so as to avoid the defect of the pad during processing.

The opening of the pad: Some devices are soldered after wave soldering, but due to the wave soldering, the inner hole of the pad is sealed by tin, so that the device cannot be inserted. The solution is to perform the soldering on the printed circuit board. A small opening is made so that the inner hole will not be sealed during wave soldering, and it will not affect the normal welding.

Pad fills the teardrop: When the trace connected to the pad is fine, the connection between the pad and the trace is designed to be drop-shaped. The advantage of this is that the pad is not easy to peel, but the trace is The pad is not easily disconnected.
Adjacent pads need to avoid sharp angles or large areas of copper foil. Sharp angles can cause wave soldering difficulties, and there is the risk of bridging. Large area copper foils can not be easily soldered due to excessive heat dissipation.

Large area of ​​copper: copper on a large area of ​​a printed circuit board is commonly used for two functions, one is heat dissipation, and the other is used for shielding to reduce interference. One mistake often made by a beginner when designing a printed circuit board is large There is no open window on the area of ​​copper, and since the adhesive between the substrate and the copper foil of the printed circuit board plate is dip-soldered or heated for a long time, volatile gas can not be eliminated and the heat is not easily dissipated, resulting in copper foil. Expansion, shedding phenomenon. Therefore, when using a large area of ​​copper, its opening window should be designed as a mesh.

The use of jumper: In the design of single-sided printed circuit board, when some lines can not be connected, jumper is often used. Among beginners, jumper is often random, there are length and length, which will give production bring inconvenience. When placing the jumper, the smaller the type, the better. Normally only 6mm, 8mm and 10mm are provided. Exceeding this range will bring inconvenience to the production.

The thickness of the printed circuit board should be determined according to the function of the printed board and the weight of the mounted components, the specification of the printed board socket, the external dimensions of the printed board, and the mechanical load that it bears. The total thickness of the multilayer printed board and the distribution of the thicknesses between the layers shall be selected according to the electrical and structural performance requirements and the standard specifications of the covered foil. Common printed circuit board thickness is 0.5mm, 1mm, 1.5mm, 2mm and so on. The above is the author's personal experience in designing printed circuit boards for many years, which inevitably will have some deficiencies. Welcome to peers to discuss or discuss together.