PCB Tech - How to guarantee the quality of the four-layer PCB ?

The quality of the four-layer PCB circuit board is defined in IPC. The surface process is anti-oxidation. If the vacuum package is not opened, it will be used within half a year, and the vacuum package will be removed within 24 hours, and the temperature and humidity are controlled. In an environment where the board is not unpacked, it should be used within one year. After it is opened, it should be pasted within a week and hours. The temperature and humidity must also be controlled. The gold board is equivalent to the tin board, but the control process is stricter than that of the tin board.

Generally speaking, a four-layer circuit board can be divided into a top layer, a bottom layer and two middle layers. The top and bottom layers are routed with signal lines. The middle layer first uses the command DESIGN/LAYER STACK MANAGER to add INTERNAL PLANE1 and INTERNAL PLANE2 with ADD PLANE as the most used power layers such as VCC and ground layers such as GND (that is, connect the corresponding network labels. Note. Do not use ADD LAYER, this will increase MIDPLAYER, which is mainly used for multi-layer signal line placement), so PLNNE1 and PLANE2 are two layers of copper connecting the power supply VCC and the ground GND.

The four-layer board refers to the PCB Printed Circuit Board made of 4-layer glass fiber. Usually, SDRAM uses a 4-layer board. Although it will increase the cost of the PCB, it can avoid noise interference.

The general principles of multi-layer PCB circuit board layout and routing The general principles that PCB designers need to follow in the circuit board routing process are as follows:

(1) The principle of setting the spacing of printed traces of components. The spacing constraint between different networks is based on the principle of setting the spacing of the printed traces of the components by electrical insulation, manufacturing process, and components. Determined by factors such as size. For example, if the pin pitch of a chip component is 8mil, the [ClearanceConstraint] of the chip cannot be set to 10mil. PCB designers need to set a 6mil PCB design rule for the chip separately. At the same time, the spacing setting should also consider the production capacity of the manufacturer.

In addition, an important factor affecting components is electrical insulation. If the potential difference between two components or networks is large, electrical insulation needs to be considered. The gap safety voltage in a general environment is 200V/mm, which is 5.08V/mil. Therefore, when there are both high-voltage and low-voltage circuits on the same circuit board, it is necessary to pay special attention to sufficient safety clearance. When there are high-voltage circuits and low-voltage circuits, it is necessary to pay special attention to sufficient safety distance.

(2) The choice of the wiring form at the corner of the line. In order to make the circuit board easy to manufacture and beautiful, it is necessary to set the corner mode of the circuit and the selection of the wiring form of the corner of the circuit when designing the PCB. Can choose 45, 90 and arc. Generally, sharp corners are not used. It is best to use arc transition or 45 transition, and avoid 90 or more sharp corner transitions.

The connection between the wire and the pad should also be as smooth as possible to avoid small sharp feet, which can be solved by teardropping. When the center distance between the pads is less than the outer diameter D of a pad, the width of the wire can be the same as the diameter of the pad; if the center distance between the pads is greater than D, the width of the wire should not be greater than that of the pad diameter. When a wire passes between two pads without connecting with them, it should keep the largest and equal distance from them. Similarly, when a wire and a wire of a wire pass between two pads without connecting with them, it should be kept to the maximum And equal spacing, the spacing between them should also be uniform and equal and keep the maximum. The spacing between them should also be uniform and equal and kept to the maximum.

(3) How to determine the width of printed traces. The width of the trace is determined by factors such as the level of current flowing through the wire and anti-interference. The larger the overcurrent flowing through the current, the wider the trace should be. The power line should be wider than the signal line. In order to ensure the stability of the ground potential (the larger the change of the ground current, the wider the trace should be. Generally, the power line should be wider than the signal line, and the power line should have less impact than the signal line width), the ground line should also be longer. The wide ground wire should also be wider. Experiments have proved that when the copper film thickness of the printed wire is 0.05mm, the current carrying ground wire of the printed wire should also be wider and can be calculated according to 20A/mm2, that is, 0.05mm thick, 1mm wide wire can flow through 1A Current. Therefore, for the general, the general width can meet the requirements; for high voltage and high voltage, the width of 10-30 mils for signal lines can meet the requirements of high voltage and high current signal lines with a line width greater than or equal to 40 mils, line ~ The spacing between lines is greater than 30mil. In order to ensure the anti-stripping strength and working reliability of the wire, the widest possible wire should be used to reduce the line impedance and improve the anti-interference performance within the allowable range of the board area and density.

For the width of the power line and ground line, in order to ensure the stability of the waveform, if the wiring space of the circuit board allows, try to thicken it as much as possible. Generally, it needs at least 50mil.

(4) Anti-interference and electromagnetic shielding of printed wires. The interference on the wires mainly includes the interference introduced between the wires, the interference introduced by the power line) the anti-interference and electromagnetic shielding of the printed wires. The interference on the wires mainly includes the interference introduced between the wires, the crosstalk between the signal wires, and the crosstalk between the signal wires, etc. Reasonable arrangement and layout of the wiring and grounding methods can effectively reduce the interference source and make the PCB design The circuit board has better electromagnetic compatibility performance.

For high-frequency or other important signal lines, such as clock signal lines, on the one hand, the traces should be as wide as possible; for high-frequency or other important signal lines, such as clock signal lines, on the one hand, the traces should be as wide as possible. On the other hand, it can be adopted (that is, to wrap the signal line with a closed ground wire, and wrap" is equivalent to adding a package of ground to isolate it from the surrounding signal lines, which is to use a closed ground wire to separate the signal line. Wrap it up, ground the shield layer). Layer grounding shield layer).

The analog ground and digital ground must be wired separately and cannot be mixed. The analog ground and digital ground must be wired separately and cannot be mixed. If you need to finally unify the analog ground and digital ground into one potential, you should usually use one point grounding method, that is, only select one point to connect the analog ground and digital ground to prevent the formation of a ground loop and cause ground potential shift.

After wiring is completed, a large area of grounding copper film should be applied to the top and bottom layers where no wires are laid, which is also called copper, to effectively reduce the wiring. The large-area grounding copper film, also called copper, is used to effectively reduce the impedance of the ground wire, thereby weakening the high-frequency signal in the ground wire, and at the same time, the large-area grounding can inhibit electromagnetic interference. The low ground wire impedance can weaken the high-frequency signal in the ground wire, and the large-area grounding can inhibit electromagnetic interference. Large-area grounding can inhibit the parasitic capacitance of electromagnetic interference, which is especially harmful to high-speed circuits; at the same time, one via in a circuit board with too many vias will bring about 10pF of parasitic capacitance, which is very important for high-speed circuits. Said to be particularly harmful will also reduce the mechanical strength of the circuit board. Therefore, when wiring, the number of vias should be reduced as much as possible. In addition, when using penetrating vias for wiring, the number of vias should be reduced as much as possible. When wiring (through holes), pads are usually used instead. This is because when the circuit board is made, some penetrating vias (through holes) may not be penetrated due to processing, and the pads can definitely be penetrated during processing, which is equivalent to giving The production brings convenience.

The above is the general principle of PCB board layout and wiring, but in actual operation, the layout and wiring of components is still a very flexible job. The layout and wiring methods of components are not unique, and the result of layout and wiring is very To a large extent, it still depends on the experience and ideas of PCB designers. It can be said that there is no standard to judge the right and wrong of layout and wiring schemes, only the relative advantages and disadvantages can be compared. Therefore, the above layout and wiring principles are only used as a reference for PCB design, and practice is the only criterion for judging the pros and cons.