Generally, it is divided into organic substrate materials and inorganic substrate materials.

　　Organic substrate material refers to the use of reinforcing materials such as glass fiber, impregnated with resin adhesive, dried into a blank, and then covered with copper foil, which is made by high temperature and high pressure. This type of substrate is called a copper clad laminate ( CCL), referred to as copper clad laminate, is the main material for manufacturing PCB.

　　Inorganic substrate materials are mainly ceramic plates and enamel-clad steel substrates. The material of the ceramic substrate is 96% alumina. The ceramic substrate is mainly used in hybrid integrated circuits and multi-chip micro-assembly circuits. It has the characteristics of high temperature resistance, smooth surface and high chemical stability. The enamel-coated steel substrate overcomes the ceramic substrate. Due to the disadvantages of limited external dimensions and high dielectric constant, it can be used as a substrate for high-speed circuits and used in some digital products.

　　printed conductors

　　Usually, the printed wires are as wide as possible, which is conducive to carrying current and facilitating manufacturing. When determining the width of the printed wire, in addition to the current carrying capacity, attention should also be paid to the peel strength of the copper foil on the board. The width of printed conductors is recommended to be 0.5mm, 1.0mm, 1.5mm, and 2.0mm. Among them, the current carrying capacity of the power line and the ground line is relatively large. In general, the design principle of the conductor width is signal line < power line < ground line. The spacing of the printed wires should be comprehensively determined according to factors such as the substrate material, the working environment and the size of the distributed capacitance. In general, the pitch of the wires is equal to the width of the wires. The direction of the printed conductors must be smooth, and there should be no right-angled or even acute-angled direction. In general, the wiring of printed conductors should consider the signal line first, then the power line and the ground line. In order to reduce the parasitic coupling between the wires, the wiring should be arranged according to the order of signal flow, the input and output of the circuit should be as far away as possible, and the input and output should be separated by a ground wire.

　　When the design of the printed wire is connected to the SMT pad, it is generally not allowed to be directly connected between the relative gaps of the two pads. It is recommended to connect it after the two ends are drawn out. In principle, the wires connected to the pad are drawn from either end of the pad, but the surface tension of the pad should not be excessively concentrated on one side, and the solder tension on each side of the device should be balanced to ensure that the device will not occur relative to the pad. Deflection; when the width of the printed wire is large and it needs to be connected to the component pad, generally, the wide wire needs to be narrowed to 0.25mm before connection, and not shorter than 0.65mm in length, and then connected to the pad, This avoids false soldering.

　　Quality inspection of printed circuit boards

　　1. Visual inspection

　　Visual inspection refers to manual inspection of printed circuit board defects. The inspection contents include surface finish, whether the silk screen is clear, whether the pad is round and whether the solder void is in the center of the pad. The method is to cover the processed On the printed circuit board, determine whether the edge size of the printed circuit board, the width and shape of the wires are within the required range.

　　2. Electrical performance inspection

　　The electrical performance test mainly includes the insulation and continuity of the circuit board. Insulation test mainly measures insulation resistance, which can be performed between wires on the same layer or between different layers. Select two or more wires with close spacing, measure the insulation resistance first, then humidify and heat for a certain period of time and then measure again after returning to room temperature. Connectivity is measured by the optical board tester mainly to see whether the two points of the connection are connected according to the electrical schematic diagram.

　　3. Pad solderability inspection

　　The solderability of the pad is an important indicator of the printed circuit board. It mainly measures the wetting ability of the solder to the printed pad, which is divided into three indicators: wetting, semi-wetting and non-wetting. Wetting refers to the free flow and expansion of solder on the pad to form an adhesive connection. Semi-wetting is when the solder wets the surface of the pad first, and the solder shrinks due to poor wetting, leaving a thin layer of solder on the base metal. Non-wetting means that although the solder has accumulated on the pad, it has not formed an adhesive connection with the pad.

　　4. Copper foil adhesion test

　　Copper foil adhesion refers to the adhesion of printed wires and pads on the substrate. The adhesion is small, and the printed wires and pads are easily peeled off from the substrate. To check the adhesion of copper foil, tape can be used, stick transparent tape on the wire to be tested, remove air bubbles, and then quickly pull off the tape in the direction of 90° with the printed circuit board. If the wire is intact, it means that the printed circuit board has The adhesion of copper foil is qualified.

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