Insulating substrate: generally divided into organic substrate material and inorganic substrate material.
Organic substrate materials are made of reinforcing materials such as glass fibers, impregnated with resin binders, dried into blanks, and then covered with copper foils, made of high temperature and high pressure. Such substrates are called copper clad laminates ( CCL), abbreviated as CCL, is the main material for manufacturing PCB.
The inorganic substrate materials are mainly ceramic plates and enamel-coated steel substrates. The material of the ceramic substrate is 96% aluminum oxide. 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-covered steel substrate overcomes the ceramic substrate. The disadvantages of limited physical dimensions and high dielectric constants can be used as substrates for high-speed circuits in certain digital products.
In general, printed conductors are as wide as possible, which is conducive to withstand current and ease of manufacture. When determining the width of printed conductors, in addition to the need to consider 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 power line and grounding line have larger current carrying capacity. In general, the design principle of the line width is the signal line <power line <ground. The spacing of the printed conductors should be determined comprehensively based on factors such as the substrate material, working environment, and distributed capacitance. In general, the spacing of the conductors is equal to the width of the conductor. The direction of the printed conductors must be sleek and must not be at right angles or even acute angles. In general, the wiring of printed conductors should consider the signal line first, and then consider the power line and ground line. In order to reduce the parasitic coupling between the wires, the wiring should be arranged in the order in which the signals flow. The input and output ends of the circuit should be as far apart as possible, and the input and output ends should preferably be separated by ground.
When designing a printed wire and connecting it to an SMT pad, it is generally not allowed to proceed directly between the relative gaps of the two pads. It is recommended that the connection be made after both ends are led out; in order to prevent the integrated circuit from being deflected during reflow, integrated circuit (IC) bonding Wires connected from the disk are in principle drawn from either end of the pad, but the surface tension of the pad should not be excessively concentrated on one side. The solder tension on each side of the device should be balanced to ensure that the device does not occur on the pad. Deflection; When the width of the printed conductor is large and needs to be connected to the component pad, it is generally required to narrow the wide conductor to 0.25mm before connecting and not shorter than 0.65mm, and then connect with the pad. This avoids welding.
Printed circuit board quality inspection
Visual inspection refers to manual inspection of printed circuit board defects. The inspection includes the surface finish, whether the screen is clear, whether the pads are round and whether the solder void is in the center of the pad. The method is to use a negative made of a photographic substrate to cover the processed substrate. On the printed circuit board, it is determined whether the edge size of the printed circuit board, the width and the shape of the wire are within the required range.
2. Electrical performance inspection
The electrical performance test mainly includes the insulation and connectivity of the circuit board. The insulation test mainly measures the insulation resistance. The insulation resistance can be performed between the wires on the same layer or between different layers. Select two or more tightly spaced conductors. Measure the insulation resistance first, then humidify and heat for a certain period of time and measure again after returning to room temperature. Measurement of connectivity through a light panel tester is based on electrical schematics to see if the two points of the connection are connected.
3. Pad solderability test
Pad solderability is an important indicator of the printed circuit board. It mainly measures the wetting ability of the solder on the printed pad, which is divided into three indicators of 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 means that the solder first wets the surface of the pad, resulting in retraction of the solder due to poor wetting, resulting in a thin layer of solder on the base metal. Non-wetting means that although the solder is deposited on the pad, it does not form an adhesive connection with the pad.
4. Copper foil adhesion test
The adhesion of the copper foil refers to the adhesion of the printed wire and the pad on the substrate, the adhesion is small, and the printed wire and the pad are easily peeled from the substrate. Check the adhesion of the copper foil by using an adhesive tape, stick the transparent tape to the wire to be tested, remove the air bubbles, and quickly remove the tape from the printed circuit board in a direction of 90°. If the wire is intact, it indicates the printed circuit board. Copper foil adhesion is satisfactory.