1. Thermal design of printed circuit board
Because the temperature resistance and thermal conductivity of printed circuit board substrate are relatively low, the peel strength of copper foil decreases with the increase of working temperature.
The operating temperature of printed circuit board shall not exceed 85 ℃. During the structural design of the main board, the heat dissipation mainly includes the following methods: evenly distributing the heat load, installing the radiator on the components, setting the strip heat conduction strip between the printed board and the components, and local or global forced air cooling.
2. Vibration damping design of printed circuit board
Printed circuit board is a kind of circuit component and device in electronic productsIt provides electrical connection between circuit elements and devicesconnect. In order to improve the vibration and impact resistance of the printed board, the load on the Board shall be reasonably distributed to avoid excessive stress. For large and heavy components (weight more than 15g or volume more than 27cm3), they shall be arranged close to the fixed end as far as possible, and their center of gravity shall be reduced or fixed with metal structural parts.
3. Anti electromagnetic interference design of printed circuit board
In order to minimize the mutual influence and interference of components on the printed board, the components of high-frequency circuit and low-frequency circuit, high-potential circuit and low-potential circuit shall not be too close. The input and output components shall be kept away from each other as far as possible, the wiring between high-frequency components shall be shortened as far as possible, and their distribution parameters and mutual electromagnetic interference shall be reduced.
With the development of high-density fine linewidth / spacing, the spacing between wires is becoming smaller and smaller, so that the coupling and interference between wires will bring stray signals or error signals, commonly known as crosstalk or noise. This coupling can be divided into capacitive coupling and inductive coupling. Stray signals caused by these coupling effects shall be reduced or eliminated by design or isolation methods:
(1) When the signal line and ground wire are staggered or the ground wire (layer) adopts double signal stripline, the adjacent two layers of signals
(2) Surround the signal line to achieve good isolation. Lines should not be arranged in parallel, but should be perpendicular and oblique to each other, so as to reduce the generation of distributed capacitance and prevent signal coupling. At the same time, it is not suitable to route at right angles or acute angles. It should use rounded arcs and oblique lines to minimize possible interference.
(3) Reduce the length of the signal line. At present, under the condition of maintaining high-density routing, the most effective way to shorten the signal transmission line is to adopt multilayer board structure.
(4) The highest frequency signal or the highest speed digital signal components shall be close to the input / output (I / O) at the connecting edge of the printed circuit board as far as possible to make their transmission line routing the shortest.
(5) For the pins of components of high-frequency signals and high-speed digital signals, BGA (ball grid array) type structure shall be adopted instead of dense QFP (square flat package) as far as possible.
(6) Adopt the latest CSP (bare chip packaging) technology.
4. PCB surface design
The components shall be arranged in a straight line according to the sequence of electrical schematic diagram,
Strive to be compact to shorten the length of printed wires and obtain uniform assembly density. On the premise of ensuring the electrical performance requirements, the components shall be parallel or perpendicular to the board surface and parallel or perpendicular to the main board edge. Evenly and orderly distributed on the board surface.
4.1.1 general principles of component wiring on printed circuit board
1. Power cord design
According to the current of the printed circuit board, thicken the width of the power line as much as possible, reduce the loop resistance, and make the direction of the power line and ground wire consistent with the direction of data transmission, which helps to enhance the anti noise ability.
2. Ground wire design
(1) The common ground wire shall be arranged at the most edge of the board to facilitate the installation of the printed board on the rack
(2) Digital ground and analog ground shall be separated as far as possible
(3) The ground wire of each level of circuit on the printed board shall generally form a closed loop, so as to ensure that the ground current of each level of circuit mainly flows in the ground circuit of this level and reduce the ground current coupling between stages.
3. Signal line design
(1) The low-frequency conductors are arranged close to the edge of the printed board, and the low-frequency and DC conductors such as power supply, filter and control are placed at the edge of the printed board. The high-frequency circuit is placed in the middle of the board surface, which can reduce the partial capacitance of the high-frequency conductor to the ground wire and the shell, and also facilitate the connection between the ground wire on the board and the frame.
(2) High potential conductors and low potential conductors shall be kept away from each other as far as possible. The best wiring is to minimize the potential difference between adjacent conductors.
(3) Avoid long-distance parallel wiring. The wiring on the printed circuit board shall be short and straight. Jumper wires can be used if necessary.
(4) Analog circuit and digital circuit shall be installed on the printed circuit board at the same time. At this time, the ground wire system of these two circuits shall be completely separated, and their power supply system shall also be completely separated.
(5) Appropriate plug-in forms shall be adopted, such as connector, plug-in end and lead out.
4.1.2 dimensions and graphics of printed conductors
When the structural layout and wiring scheme of components are determined, the graphic of printed wires should be designed and drawn concretely.
1. Width of printed conductor
Copper clad laminate the thickness of copper foil is generally 0.02mm ~ 0.05mm. The minimum width of printed conductor depends on the current carrying capacity and allowable temperature rise of conductor. The working temperature of the printed board shall not exceed 85 ℃. After the conductor is heated for a long time, the copper foil will fall off due to poor bonding strength.
2. Spacing of printed wires
The minimum spacing of conductors is mainly determined by the insulation resistance and breakdown voltage between conductors under the worst conditions. Generally, the conductor spacing is equal to the conductor width, but not less than 1 mm. For micro devices, not less than 0.4 mm. The spacing of surface mount plates is 0.12 ~ 0.2mm, or even 0.08mm. The following three factors shall be considered in the specific design:
(1) The wire spacing of low-frequency and low-voltage circuits depends on the welding process. The time interval for automatic welding should be larger and smaller for manual operation.
(2) The wire spacing of high voltage circuit depends on the working voltage and the electrical strength of the substrate.
(3) The influence of distributed capacitance on signal is mainly considered in high frequency circuit.
3. Figure of printed conductor
There are two arrangement modes of components on the printed board: irregular arrangement and regular arrangement (as shown in Figure 4.7). Irregular arrangement is suitable for high-frequency circuits. It can reduce the length and distribution parameters of printed wires, but it is not conducive to automatic insertion. Regular (sitting grid) arrangement, neat arrangement, high automatic insertion efficiency, but the lead may be long. The width of the conductor on the same printed board should be the same, and the ground wire can be widened appropriately.
For large area copper foil, the central hole of the pad is slightly larger than the diameter of the device lead, but the pad is too large, which is easy to form false soldering. General pad outer diameter D (D + 1.3) mm, where D is the diameter of lead socket. For high-density digital circuits, the minimum diameter of pad can be Dmin = (D + 1.0) mm.
(1) Printed circuit board design steps and methods the circuit to be accommodated on the printed circuit board surface is known,
(2) The conditions for designing printed boards and the models, specifications and main dimensions of various components in the circuit.
(3) Specify the layout and wiring of components and wires, and determine the position and special requirements of the printed board in the whole machine (or extension). Its connection form.
5. Design steps and methods of printed board
(1) Select the material, thickness and surface size of the printed board. The selection of printed board materials must take into account the electrical and mechanical properties of the substrate, as well as the price and cost. The rigid base material can be phenolic paper laminate, epoxy paper laminate, epoxy glass cloth laminate and PTFE glass cloth laminate. The first two plates are suitable for electronic equipment with low general requirements; Epoxy glass cloth laminate is suitable for electronic equipment with high working temperature and high working frequency.
The thickness of printed board is determined by the size of board surface and the weight of installed components. The plate thickness has been standardized, and its dimensions include 0.2, 0.5, 0.7, 0.8, 1.5, 1.6, 2.4, 3.2, 6.4 mm, etc. The thickness of rigid plate is generally 1.5 mm. The thickness of high current plate is 2 ~ 3 mm. The plate thickness of small household appliances is about 0.5mm. The best shape of printed board is rectangle, and the aspect ratio is 3:2 or 4:3. Several small printed boards (rectangular or special-shaped) are assembled into a large rectangle, which is cut along the process hole after assembly and welding, which can reduce the production cost.
(2) Design the coordinate dimension drawing of printed circuit board. According to the electrical schematic diagram and considering the overall dimensions of components and layout and wiring requirements, draw the coordinate dimension drawing of printed circuit board from input to output level by level, and use the coordinate grid drawing printed with 1 mm or 2.5 mm square grid. Firstly, typical components are selected as the basic unit of the layout. Typical components are representative components in geometric dimensions among all components to be installed on the board (as shown in Figure 4.10), and then it is estimated that the dimensions of other components are equivalent to multiple of typical components.
(3) Draw the typesetting wiring diagram according to the electrical schematic diagram. The typesetting wiring diagram uses simple lines to represent the direction of printed wires and the connection of components and devices. In the layout wiring diagram, the crossing of wires shall be avoided as far as possible, but it can be crossed at the components, because the component span can be arranged through the printed line wiring diagram, and the layout design sketch shall be drawn on the square paper according to the size proportion of components (generally 2:1 or 4:1).