Detailed explanation of multilayer PCB laminate structure (2)

2.1 General principles of component layout


The general principles that designers need to follow during board layout are as follows.


(1) The best single-sided placement. If you need to place components on both sides, placing pin-type components in the collapse (bottom layer), it may cause the circuit board to be difficult to place, and it is not conducive to soldering. Similar to the commonly used computer PCBPCB component layout method. When placed on one side, it is only necessary to make a silk screen layer on one surface of the circuit board, thereby reducing costs.


(2) Reasonably arrange the position and direction of interface components. Generally speaking, as the connector components of the circuit board and external (power, signal line) connections, they are usually arranged on the edge of the circuit board, such as sockets and parallel ports. If it is placed in the center of the circuit board, it is obviously not conducive to wiring, and it may not be connected because of the obstruction of other components. In addition, pay attention to the direction of the interface when placing the interface, so that the connection line can be smoothly led away from the circuit board. After the interface is placed, use the character string (character string) of the interface component to clearly indicate the type of interface; for power-type interfaces, correctly indicate the voltage level to prevent the circuit board from being burned due to wiring errors.


(3) It is better to have a wide electrical isolation band between high-voltage components and low-voltage components. That way, do not put components with very different voltage levels together. This is not only conducive to electrical insulation, but also has great benefits for signal isolation and anti-interference.


(4) Components with close electrical connection are best placed together. This is the staggered layout idea.


(5) For components that are prone to noise, such as high-frequency devices such as clock generators and crystal oscillators, they should be placed near the clock input end of the CPU when placed. High-current circuits and switching circuits are also prone to noise. When laying out these components or modules, high-speed signal circuits such as logic control circuits and storage circuits should also be changed. If possible, try to use a control board combined with a power board and use an interface. To connect to improve the overall anti-interference ability and working reliability of the circuit board.


(6) Place series capacitors and filter capacitors around the power supply and the chip as much as possible. The arrangement of the de-series capacitor and the filter capacitor is an important measure to improve the power quality of the circuit board and the anti-interference ability. In practical applications, the routing, insertion of wires and wiring of printed circuit boards may cause parasitic inductance in series, resulting in high-frequency ripple and glitches in the power supply waveform and signal waveform, and placed between the power supply and ground For a 0.1F, if the circuit panel uses a chip capacitor, the chip capacitor should be connected close to the power supply of the component. For power conversion chips or power input terminals, it is best to arrange a 10F or higher capacitor to further improve the power quality.


(7) The number of components should be tightly arranged, uniform in size, neat in direction, not overlapping with components, vias, and alternately. The first direction of the component or connector; the positive and negative signs should be clearly marked on the PCB and allowed to be covered; power conversion components (such as DC / DC converters, linear conversion power supplies and switching power supplies) should be next to Sufficient heat dissipation space and installation space, and sufficient welding space left around.


2.2 General principles of component wiring


The general principles that designers need to follow during board layout are as follows.


(1) The principle of setting the pitch of printed wiring of components. The spacing constraints between different networks are determined by factors such as electrical insulation, manufacturing process and component size. For example, the pitch of a chip component is 8mil, the [Clear Limit] of the chip cannot be set to 10mil, and the designer needs to set a 6mil design rule for the chip separately. At the same time, the setting of the spacing must also take into account the production capacity of the manufacturer.


In addition, an important factor affecting the components is electrical insulation. If the potential difference between the two components or the network is on, the issue of electrical insulation needs to be considered. The gap safety voltage in the general environment is 200V / mm, which is 5.08V / mil. Therefore, when there are both high-voltage circuits and low-voltage circuits on the same circuit board, special attention needs to be paid to sufficient safety intervals.


(2) The choice of the corners of the line. In order to make the circuit board easier to manufacture and beautiful, you need to set the corner mode of the circuit during design. You can choose 45 °, 90 °, and arc. Generally, sharp corners are not used. It is best to use arc transitions or 45 ° transitions. Avoid 90 ° or sharper corner transitions.


The connection position between the lead and the insert should also be as smooth as possible to avoid small pointed feet, which can be solved by the method of trapping leakage. The width of the wire can be the same as the diameter of the coaxial line; if the center distance between the overlaps is greater than D, the width of the wire should not be larger than the diameter of the size.


When the wires pass between the two distances instead of communicating with each other, the maximum and alternating distance between them should be maintained, and the distance between the wires and the wires should also be evenly alternated and maintained at the maximum.


(3) The determination method of printed trace width. The trace width is determined by factors such as the current level and anti-interference flowing through the wire. The total current flowing, the wider the trace should be. Generally the power line should be wider than the signal line. In order to ensure the stability of the ground potential (less affected by the change of the ground current), the ground line should also be wide. The experiment proves: when the thickness of the copper film of the printed wire


When it is 0.05mm, the current carrying capacity of the printed wire can be calculated according to 20A / mm2, that is, 0.05mm thick and 1mm wide wire can flow 1A. Therefore, for general signal lines, a width of 10 to 30 mils can meet the requirements; high-voltage, high-current signal line widths are greater than or equal to 40 mils, and the spacing between lines is greater than 30 mils. In order to ensure the anti-interference strength and working reliability of the wire, within the range allowed by the board area and density, a wide wire should be used to reduce the line impedance and improve the anti-jamming performance.


For the width of the power and ground wires, in order to ensure the stability of the waveform, if the wiring space of the circuit board allows, make it as thick as possible. Generally, at least 50mil is required.


(4) Anti-interference and electromagnetic shielding of printed wires. The interference on the wires mainly includes the interference of appointments between wires, the interference of appointments of power lines and crosstalk between signal lines, etc. Reasonable arrangements and layout of routing and grounding methods can effectively reduce the source of interference and make the circuit board better EMC performance.


For high-frequency or other important signal lines, such as clock signal lines, the length of the traces should be as wide as possible, instead of using a ground-wrapped form to isolate the surrounding signal lines (that is, use a closed ground line to isolate the signal lines). "Wrapped" is equivalent to adding a layer of ground shielding).


If it is necessary to unify the analog ground and the digital ground to the same potential at the end, one-point grounding should be adopted, that is, picking up only one point to connect the analog ground and the digital ground to prevent the formation of a ground loop and the ground potential interference.


After the wiring is completed, a large area copper ground film, also called copper copper, should be laid on the top and where no wires are inserted. It can be effectively inserted into the ground wire to reduce high-frequency signals in the ground wire. The ground of the area can suppress the electromagnetic interference.


A via in the circuit board will bring about 10 pF of parasitic capacitance, which is especially harmful for high-speed circuits. At the same time, too many vias will also reduce the mechanical strength of the circuit board. Therefore, the number of vias should be minimized when wiring. In addition, when a through-hole (through-hole) is used, it is usually used instead. This is because during the production of the circuit board, some through-holes (through holes) may not be penetrated due to processing reasons, and they can certainly be penetrated alternately during processing, which is equivalent to producing Bring convenience.


The above are the general principles of PCB board layout and wiring. However, in actual operation, the layout and wiring of components is still a very flexible job. The layout and wiring of components are not unique. That said, there is no standard that can judge the right and wrong of the layout and wiring scheme, and can only compare the relative advantages and disadvantages. Therefore, the above layout and wiring principles are only used as a design reference, and practice is the only criterion for judging the pros and cons.


2.3 Special requirements for layout and wiring of multilayer PCB boards


Compared with simple single-layer boards and double-layer boards, the layout and wiring of multilayer PCB boards have their unique requirements.


For the layout of multilayer PCB boards, it is necessary to reasonably arrange the layout using different power and ground type components. One of the objectives is to bring convenience to the subsequent division of the internal electrical layer, and at the same time, it can effectively improve the anti-interference ability between components.


The so-called reasonable arrangement of components using different power and ground types is to put together the wizards that use the same power level and the same type of ground components. For example, when there are multiple voltage levels such as + 3.3V, + 5V, −5V, + 15V, −15V on the circuit schematic, the designer should concentrate the components using the same voltage level in a certain area on the circuit board . Of course, this layout principle is not the only standard for layout. At the same time, other layout principles (general principles of double-layer board layout) need to be taken into account. This requires designers to comprehensively consider various factors according to actual needs, and to meet the foundation of other layout principles As far as possible, put together components that use the same power level and the same type of ground. For the wiring of multi-layer PCB boards, it can be summed up in one point: the signal line first, and then the power line. This is because the power and ground of multilayer boards are usually achieved by connecting the internal electrical layers. It can simplify the routing of the signal layer, and effectively reduce the internal resistance of the ground lead and the power supply, and improve the anti-interference ability of the circuit through the large-area copper film connection method of the internal electrical layer; The maximum current is also increased.


In general, designers need to reasonably arrange the layout of components using different power and ground types, while taking into account other layout principles, and then route the components (only the signal lines) according to the method described in this section, and divide after completion Internal electrical layer, determine the network label of each part of the internal electrical layer, and finally connect via vias and replacements on the internal electrical layer and the signal layer. And when the vias and the vias pass through the internal electrical layer, the through-holes or vias with the same network label will be connected to the internal electrical layer through some copper films that overcome corrosion, and the adjacent surrounding copper films that do not belong to the network will be It is completely etched away, so that it is not conductive with the internal electrical layer.