Printed circuit board structure design

1. Thermal design of printed circuit boards

Since the temperature resistance and thermal conductivity of the printed circuit board substrate are relatively low, the peeling strength of the copper foil decreases as the operating temperature increases.

The operating temperature of printed circuit boards generally cannot exceed 85 °C. When designing the main board structure, the heat dissipation mainly includes the following methods: uniformly distributing the heat load, disposing the heat sink for the components, and providing a strip-shaped heat conducting strip between the printed board and the component, and partially or globally forcibly cooling the air.

2. Damping buffer design for printed circuit boards

A printed circuit board is a support for circuit components and devices in an electronic product that provides an electrical connection between the circuit components and the device. In order to improve the anti-vibration and impact resistance of the printed board, the load on the board should be reasonably distributed to avoid excessive stress. For large and heavy components (weight more than 15g or volume over 27cm3), place them as close as possible to the fixed end and lower their center of gravity or metal structure.

3. Anti-electromagnetic interference design of printed circuit boards

In order to minimize the mutual influence and interference of the components on the printed board, the components of the high frequency circuit and the low frequency circuit, the high potential and the low potential circuit cannot be placed too close together. Input and output components should be kept as far as possible to minimize the wiring between high-frequency components, and try to reduce their distribution parameters and mutual electromagnetic interference.

With the development of high-density fine line width/pitch, the distance between the wire and the wire is getting smaller and smaller, so that the coupling and interference between the wire and the wire will bring spurious or error signals, commonly known as crosstalk or noise. This coupling can be divided into capacitive coupling and inductive coupling. The spurious signals caused by these couplings should be reduced or eliminated by design or isolation:

(1) Using signal lines and ground lines staggered or ground lines (layers) When two-signal strip lines are used, adjacent two-layer signals

(2) Surround the signal line to achieve good isolation. Lines should not be laid in parallel and should be perpendicular and oblique to each other to reduce distributed capacitance and prevent signal coupling. At the same time, it is not suitable for right angle or sharp angle. The arc and diagonal line should be rounded to minimize the possible interference.

(3) Reduce the length of the signal line. At present, the most effective way to shorten the signal transmission line while maintaining high-density wiring is to use a multi-layer board structure.

(4) The highest frequency signal or the highest speed digitized signal components should be placed as close as possible to the input/output (I/O) of the printed circuit board connection side, so that their transmission line traces are the shortest.

(5) For the components of high-frequency signals and high-speed digitized signals, the BGA (Ball Grid Array) type structure should be used instead of the dense QFP (square flat package).

(6) Adopt the latest CSP (Bare Chip Package) technology.

4. Printed circuit board design

Components should be arranged in a straight line in the order of electrical schematics.

Strive to be compact to shorten the length of the printed conductor and achieve a uniform assembly density. Under the premise of ensuring electrical performance requirements, the components shall be parallel or perpendicular to the board surface and parallel or perpendicular to the main board edge. It is evenly distributed on the board surface.

5. Printed board design steps and methods

(1) Material, thickness and board dimensions of the selected printed board The choice of printed board material must take into account the electrical and mechanical properties of the substrate, as well as price and cost. The rigid substrate may be selected from phenolic paper laminates, epoxy paper laminates, epoxy glass cloth laminates, and polytetrafluoroethylene glass cloth laminates. The first two sheets are suitable for use in generally less demanding electronic equipment; epoxy glass cloth laminates are suitable for electronic equipment with high operating temperatures and high operating frequencies.

The thickness of the printed board is determined by the size of the board and the weight of the components mounted. The plate thickness has been standardized and its size is 0.2 and 0. 5, 0.7, 0.8, 1.5, 1.6, 2.4, 3.2, 6.4 mm and so on. The rigid plate thickness is generally 1.5 mm. The large current plate is 2 to 3 mm thick. Small appliances are about 0.5mm thick. The best shape of the printed board is rectangular with an aspect ratio of 3:2 or 4:3. A small number of small printed boards (rectangular or profiled) are assembled into a large rectangle, which is to be assembled, welded and then cut along the process hole to reduce the production cost.

(2) Design the printed circuit board coordinate dimension drawing According to the electrical schematic diagram and considering the component external dimensions and layout and wiring requirements, draw the circuit from the input to the output sequence step by step, using the grid of the grid printed with 1mm or 2.5mm squares. Board coordinate size chart. First select the typical components as the basic unit of the layout. Typical components are geometrically representative components of all components to be mounted on the board, and then other component sizes are estimated to be equivalent to multiples of typical components.

(3) Drawing a layout diagram according to the electrical schematic diagram The layout diagram of the layout is a simple line indicating the direction of the printed conductor and the connection of the component devices. In the layout of the layout diagram, the intersection of the wires should be avoided as much as possible, but it can be crossed at the components. Because the component spans can be typeset through the printed circuit, the layout design can be drawn on the squared paper according to the size of the components. Sketch (generally 2:1 or 4:1).