PCB Blog - General principles of PCB board design

Single-sided printed circuit boards are widely used in consumer electronic products, such as radios, tape recorders, televisions, and electronic game consoles. Its manufacturing cost is among all kinds of printed boards. Single-sided printed circuit boards may also be used in industrial electronic products. Mainly according to the complexity and density of the circuit and the assembly requirements of the whole machine, single-sided printed circuit boards should be used when all interconnections can be completed with one side. .

1. In the case where all the interconnection of the circuit cannot be completed with a single-sided printed circuit board, the design of a double-sided printed circuit board must be considered.
Most double-sided printed circuit boards use metallized holes to achieve through-connection of conductors on both sides. In a few cases, double-sided printed circuit boards with non-metallized holes can also be used. Its through-connection mainly depends on insertion holes. components lead to achieve.
Consider designing a multilayer printed circuit board when:
1) The full interconnection of the circuit cannot be completed with a double-sided printed circuit board, and more jumper wires need to be added.
2) Light weight and small size are required.
3) There are high-speed circuits. Due to the short interconnecting wires of the multi-layer printed circuit board, the attenuation of the high-speed pulse signal is reduced; the ground layer of the multi-layer printed circuit board can provide a good shielding effect on the high-speed signal layer; the connection between the ground layer and the power layer The high-frequency distributed capacitance between them has a good decoupling effect on the power supply.
4) High reliability is required. Several double-sided printed circuit board assemblies can be combined into one multilayer board assembly, resulting in improved reliability of the entire electronic product.
5) Simplified printed circuit board layout and photographic basemap design. Some double-sided printed circuit board layouts are very complex due to the complex interconnection. After using a multi-layer printed circuit board, the power supply or ground wire can be separated from the signal wire. The layer is laid out on the same layer as the ground plane.
In order to meet the needs of some special assembly of electronic equipment, reduce weight and improve assembly density, it is sometimes required to use flexible printed circuits or flexible-rigid printed circuits. Flexible printed circuits can also be used as connecting cables, saving time in soldering individual wires and eliminating assembly errors. For moving electronic components, flexible printed circuits can also be used. When the wires of the printed circuit board have to withstand a large current or the power consumption of the entire printed board is very large, so that it exceeds the allowable operating temperature, it is necessary to design a printed circuit board with a metal core or a surface heat sink. When using a printed circuit board to make a rotary switch or a contact encoder disc, the flush printed circuit should be designed.

2. Coordinate network system
When designing printed circuit boards, a co-ordinate network system is used that is commonly adhered to. The leads of the components are inserted into the assembly holes located at the grid intersections. According to the specified grid, computer-aided design (CAD) systems, automatic optical drawing machines, numerical control drilling devices, computer-aided testing (CAT) systems, and automatic component insertion devices can all easily compile or automatically generate control programs. For the manual layout of printed circuit board sketches and the production of photographic base maps, the use of a coordinate grid system is also convenient for calculation and operation. According to the national standard "Printed Circuit Grid System" GB1360-1998, the grid spacing of the basic grid is 2.5mm. If necessary, auxiliary grids can be set, and the grid spacing is 1, 2 (ie, 1.25mm) of the basic grid. ) or 1/4 (ie 0.625mm). For printed circuit boards for integrated circuits with a center-to-center distance of 2.54mm between adjacent leads, a 2.54mm basic grid can be used. The 2.54mm base grid system also has two secondary grids with 1.27mm and 0.635mm spacing.

3. Design enlargement ratio
According to the requirements of the dimensional accuracy of the printed circuit board, the photographic base map is made in the ratio of 1:1 or 2:1 or even 4:1. The layout sketch of the printed circuit board is also drawn to the same scale. This prevents redrawing the enlarged layout sketch when making the photographic basemap, and also makes it easier to check the photographic basemap. The commonly used magnification ratio for layout sketches or photographic basemaps is 2:1. According to this ratio, the wiring accuracy is relatively high and the operation is relatively convenient. The 4:1 magnification ratio is only used when the precision of the printed board is particularly high or when a digital instrument is used to input and compile a high-precision conductive pattern program. The 1:1 scale design layout sketch is only suitable for the power and ground planes of relatively simple double-sided printed circuit boards or multi-layer printed circuit boards.

4. Production conditions of printed circuits
Designing printed circuit boards should take into account and be familiar with production conditions. For example: the method of making a photographic plate (photographic reduction method, light drawing method, 1:1 mapping method, etc.), the size of the base image allowed by the photographic platesetter, the size and size of the printed circuit board processed by each equipment, the drilling hole of the drilling machine Accuracy, blanking requirements, fine wire pattern printing technology and etching accuracy, etc.

5. Standardization
The result of standardization is to simplify the printed circuit board production process, shorten the production cycle, reduce costs, and meet the quality control requirements of printed circuit boards. Therefore, designers must apply and strictly adhere to these standards. Applicable standards include National Standard (GB), International Electrotechnical Commission (IEC TC52), US Military Specification (MIL), British Standard (BS), Japanese Industrial Standard (JIS) and Japan Printed Circuit Association (JPCA) and other relevant standards .

6. Design documents
1) Circuit diagram (electrical schematic diagram)
In addition to using the common method to express the connection relationship of the circuit, the circuit diagram also needs to mark some special parts according to the design requirements of the printed board. For example: the relationship between the input, output and connectors of the printed board, the length of the key signal lines, the conductors protected by the ground wire, the conductors of special width, the devices that generate electromagnetic interference, the components that generate a lot of heat, and the thermal elements devices, etc.
2) Components table
The components list includes all resistors, capacitors, transistors, diodes, integrated circuits, transformers, inductors, heat sinks and metal parts in the circuit diagram. In the component table, the number corresponding to the circuit diagram (such as R1, R2, R3, ..., C1, C2, C3 ...), the specifications of the components, the matching requirements of the metal parts, the mechanical dimensions, etc. should be indicated . If necessary, the actual components of the components should be used for reference.
3) Component wiring table
This table is generally used in CAD automatic routing, and it only represents the connection relationship between various components.
4) Machining drawing
Machining drawings are important documents for machining printed circuit boards. The machining drawing includes the following parts:
a. The overall dimensions of the printed circuit board and their deviations, including the dimensions and deviations of the printed plug portion, the dimensions of the mechanical mounting holes, and their dimensions and deviations from the reference datum.
b. The name, symbol, thickness and thickness of copper foil of the copper clad laminate used. Sometimes more emphasis is placed on the thickness of the insulating substrate between the copper foils.
c. The technical requirements of the surface coating, for example, the thickness of the tin-lead coating, the ratio of tin-lead, the thickness of the nickel or gold coating, etc.
d. Surface coating requirements, eg, solderability coatings, solder mask coatings, etc.
e. The assembly drawing and the component table are both essential documents for the electrical assembly of the printed circuit boards.