The substrate properties that have an important effect on the printed circuit assembly are water absorption, thermal expansion coefficient, heat resistance, flexural strength, impact strength, tensile strength, shear strength and hardness.

All of these features affect both the function of printed circuit board structures and the productivity of printed circuit board structures.

For most applications, the dielectric substrate for printed circuit boards is one of several substrates:

1) Phenolic impregnated paper.
2) acrylic-polyester impregnated with irregularly arranged glass felt.
3) Epoxy impregnated paper.
4) Epoxy impregnated glass cloth.

Each substrate may be flame retardant or flammable. The above 1,2,3 can be punched. Metallic hole printed circuit board The most commonly used material is epoxy - glass cloth, its dimensional stability is suitable for high-density line use, and can make the metal hole in the case of cracks at least occur.

One drawback of epoxy-glass cloth laminates is that it is difficult to punch in the range of common thicknesses of printed circuit boards, and for this reason all the holes are usually drilled and imitated milling to form a print The shape of the circuit board.

PCB Electrical Considerations:

In the case of DC or low frequency AC, the most important electrical characteristics of insulating substrates are insulation resistance, electrical resistance and printed wire resistance and breakdown strength.

In the high frequency and microwave occasions are: dielectric constant, capacitance, dissipation factor.

In all applications, the current load capacity of printed wires is important.

Wire pattern:

PCB routing and positioning

Printed wire under the provisions of the provisions of the wiring rules, should take the shortest route between the components. Limit the coupling between parallel wires as much as possible. Good design requires the least number of layers to be routed, and the widest wire and maximum pad size are required at the required package density. Because rounded corners and smooth interior angles may avoid some electrical and mechanical problems that may arise, sharp corners and sharp corners should be avoided in the wires.

PCB width and thickness:

Rigid printed circuit board etched copper conductor current carrying capacity. For 1 ounces and 2 ounces of wire, take into account the etch method and the normal change in the thickness of the copper foil and the temperature difference, allowing to reduce the nominal value of 10% (to the load current meter); for the protective layer printed circuit board assembly (The thickness of the substrate is less than 0.032 inches and the thickness of the copper foil is more than 3 ounces) is reduced by 15% for components and 30% for submerged printed circuit boards.

PCB wire spacing:

The minimum spacing of the conductors must be determined to eliminate voltage breakdown or arcing between adjacent wires. The pitch is variable and it depends primarily on the following factors:

1) the peak voltage between adjacent wires.
2) Atmospheric pressure (maximum working height).
3) the coating used.
4) Capacitance coupling parameters.

Key impedance elements or high frequency components are generally placed very close to reduce the critical level delay. Transformers and inductance components should be isolated to prevent coupling; Inductive signal conductors should be arranged orthogonally; components that produce any electrical noise due to magnetic field motion should be isolated or rigid mounted to prevent excessive vibration.

PCB Wire Graphics Check:

1) Is the wire short and straight without sacrificing its function?

2) Is the wire width limit specified?

3) between the wires, between the wires and the mounting holes, between the wires and the pads ... the minimum wire spacing that must be guaranteed to stay out?

4) Is it possible to avoid the parallel arrangement of all wires (including component leads)?

5) Does the wire pattern avoid an acute angle (90 ° C or less)?

PCB Design Project Checklist:
1) check the rationality of the schematic and correctness;
2) check the correctness of the component package of the schematic;
3) the spacing between the strong and weak, the spacing of the isolation area;
4) schematic diagram and PCB map corresponding to check to prevent the loss of network table;
5) whether the components of the package and the physical match;
6) Is the placement of the component appropriate:
7) whether the components are easy to install and disassemble;
8) whether the temperature sensitive element is too close to the heating element;
9) can produce mutual inductance component distance and direction is appropriate;
10) Whether the placement between the connectors corresponds to smooth;
11) easy to plug;
12) input and output;
13) strong power weak;
14) whether the digital simulation is staggered;
15) arrangement of upper and lower air side components;
16) whether the directional component has undergone an incorrect rollover rather than rotation;
17) Is the mounting hole of the component pin suitable for easy insertion?
18) check the empty feet of each component is normal, whether the line;
19) check the same network table in the upper and lower wiring whether there is a hole, the pad connected through the hole to prevent disconnection, to ensure the integrity of the line;
20) check the upper and lower characters placed correctly and reasonable, do not put the elements to cover the characters, so that welding or maintenance personnel to operate;
21) very important connection between the upper and lower lines do not just use the straight element of the pad connection, it is best to use the hole connection;
22) socket power and signal line arrangements to ensure the integrity of the signal and anti-jamming;
23) Note that the ratio of pad and weld hole is appropriate;
24) the plug as far as possible on the edge of the PCB board and easy to operate;
25) to see whether the symbol is consistent with the components, the components placed as much as possible in the same direction and placed neatly;
26) Where the design rules are not violated, the power supply and ground should be as thick as possible;
27) Under normal circumstances, the upper horizontal line, the lower vertical line, and the chamfer is not less than 90 degrees;
28) PCB mounting hole size and distribution is appropriate, as much as possible to reduce the PCB bending stress;
29) Note the high and low distribution of components on the PCB and PCB shape and size, to ensure easy assembly.