With the advancement of electronic technology, the complexity and applicability of printed circuit boards (PCBs) have rapidly developed. Designers engaged in high-frequency PCBs must have corresponding basic theoretical knowledge and rich experience in the production of high-frequency PCBs. That is to say, whether it is the drawing of schematic diagrams or the design of PCBs, it is necessary to consider the high-frequency working environment in which they are located in order to design an ideal PCB.
This article mainly explains the design methods of PCB wiring, solder pads, and copper plating. Firstly, it introduces the design of PCB wiring from the direction and form of PCB wiring, as well as the requirements for power and ground wire wiring. Secondly, it introduces the design of PCB solder pads from the perspectives of solder pad and aperture, solder pad shape and size design standards in PCB design, PCB manufacturing process requirements for solder pads. Finally, it introduces PCB copper plating design from the perspective of PCB copper plating techniques and settings, Follow the editor for more details.
Design of PCB wiring
Cabling is the overall requirement for achieving high-frequency PCB design based on reasonable layout. Wiring includes two methods: automatic wiring and manual wiring. Usually, regardless of the number of key signal lines, manually route these signal lines first. After the routing is completed, carefully inspect the routing of these signal lines, fix them after passing the inspection, and then automatically route other wiring. The combination of manual and automatic wiring is used to complete PCB wiring.
Special attention should be paid to the following aspects during the wiring process of high-frequency PCBs
1. Route of wiring
It is best to use a full straight line for circuit wiring according to the signal flow direction. When turning is needed, a 45 ° broken line or circular arc curve can be used to reduce the external emission and mutual coupling of high-frequency signals. The wiring of high-frequency signal lines should be as short as possible. The length of signal wire routing should be reasonably selected based on the operating frequency of the circuit, which can reduce distribution parameters and signal loss. When making double-sided boards, it is best to route the wires perpendicular, oblique, or curved to each other on two adjacent layers. Avoiding parallelism can reduce mutual interference and parasitic coupling.
High frequency signal lines and low frequency signal lines should be separated as much as possible, and shielding measures should be taken when necessary to prevent mutual interference. For signal inputs with weak reception, they are susceptible to interference from external signals. Ground wires can be used as shielding to enclose them or to shield high-frequency connectors. On the same level, parallel wiring should be avoided, otherwise it will introduce distributed parameters and have an impact on the circuit. If unavoidable, a grounded copper foil can be introduced between two parallel lines to form an isolation line.
In digital circuits, for differential signal lines, they should be routed in pairs, as close and parallel as possible, with little difference in length.
2. The form of wiring
In the wiring process of a PCB, the minimum width of the wiring is determined by the adhesion strength between the wire and the insulation substrate, as well as the current intensity flowing through the wire. When the thickness of the copper foil is 0.05mm and the width is 1mm to 1.5mm, a 2A current can be passed. The temperature should not exceed 3 ℃. Except for some special wiring, the width of other wiring on the same level should be as consistent as possible. The spacing of wiring in high-frequency circuits will affect the size of distributed capacitors and inductors, thereby affecting signal loss, circuit stability, and causing signal interference. In high-speed switching circuits, the spacing of wires will affect the transmission time of signals and the quality of waveforms. Therefore, the minimum spacing of wiring should be greater than or equal to 0.5 mm, and as long as allowed, it is best to use relatively wide wires for PCB wiring.
A certain distance (not less than the board thickness) should be left between the edges of printed wires and PCBs, which not only facilitates installation and mechanical processing, but also improves insulation performance.
When encountering lines that can only be connected by winding large coils in wiring, flying wires should be used, that is, short wires should be directly used to reduce interference caused by long-distance wiring.
Circuits containing magnetic sensitive components are sensitive to the surrounding magnetic field, while high-frequency circuits are prone to radiating electromagnetic waves at the corners of the wiring. If magnetic sensitive components are placed in the PCB, it should be ensured that the wiring corners are at a certain distance from them.
Crossing is not allowed for wiring on the same level. For lines that may intersect, the methods of “drilling” and “winding” can be used to solve the problem, that is, allowing a certain lead to “drill” through the gap under the pins of other resistors, capacitors, transistors, and other devices, or to “wind” through one end of a possible crossing lead. In special cases, if the circuit is complex, in order to simplify the design, it is also allowed to use wire bridging to solve the crossover problem.
When the high-frequency circuit operates at a high frequency, it is also necessary to consider the impedance matching and antenna effect issues of the wiring.
Due to the client changing the previous protocol and requiring the interface definition and placement according to their definition, the layout had to be changed to the diagram on the right. Actually, due to the fact that the entire PCB has an area of only 9cm x 6cm. It is difficult to change the overall layout of the board according to the customer’s requirements, so the core part of the board was not changed in the end. Only appropriate modifications were made to the external components, mainly completing the modification of the two connector positions and pin definitions.
But the new layout has clearly caused some trouble in the wiring. The originally smooth wiring has become somewhat messy, the length of the wiring has increased, and many through holes have to be used, making the wiring much more difficult.
From this example, it is evident that layout differences have an impact on PCB design.
3. Wiring requirements for power and ground wires
Try to increase the width of the power cord according to the different working currents. High frequency PCBs should use large area ground wires as much as possible and be arranged at the edges of the PCB to reduce interference from external signals on the circuit; At the same time, it can make the grounding wire of the PCB in good contact with the housing, making the grounding voltage of the PCB closer to the ground voltage. The grounding method should be selected according to the specific situation, which is different from low-frequency circuits. The grounding wire of high-frequency circuits should be grounded nearby or at multiple points. The grounding wire should be short and thick to minimize the grounding impedance, and its allowable current should reach a standard of 3 times the working current. The grounding wire of the speaker should be connected to the grounding point of the PCB amplifier output stage, and should not be arbitrarily grounded.
During the wiring process, some reasonable wiring should also be locked in a timely manner to avoid repeated wiring. By executing the EditselectNet command and selecting Locked in the pre wired attributes, it can be locked and no longer moved.
PCB pad design
1. Pad and aperture
Under the premise of ensuring that the minimum spacing of the wiring does not violate the designed electrical spacing, the design of the solder pad should be larger to ensure sufficient ring width. Generally, the inner hole of the solder pad is slightly larger than the diameter of the lead wire of the component, which is too large in design and can easily lead to false soldering during welding. The outer diameter D of the solder pad is generally not less than (d+1.2) mm, where d is the inner diameter of the solder pad. For some high-density PCBs, the minimum value of the solder pad can be taken as (d+1.0) mm. The shape of the solder pad is usually set to be circular, but for integrated circuits packaged with DIP, it is best to use a runway shaped solder pad, which can increase the area of the solder pad in limited space and facilitate the soldering of integrated circuits. The connection between wiring and pads should have a smooth transition, that is, when the width of the wiring entering the circular pad is smaller than the diameter of the circular pad, a teardrop filling design should be adopted.
It should be noted that the size of the aperture d inside the solder pad varies and should be considered based on the actual diameter of the component lead, such as component holes, installation holes, and slot holes. The hole spacing of solder pads should also be considered based on the actual installation method of components, such as resistors, diodes, tubular capacitors, etc. There are two installation methods: “vertical” and “horizontal”, and the hole spacing of these two methods is different. In addition, the design of pad hole spacing also needs to consider the minimum gap requirements between components, especially the gap between special components needs to be ensured by the hole spacing between pads.
In high-frequency PCBs, it is also necessary to minimize the number of vias as much as possible, which can reduce distributed capacitance and increase the mechanical strength of the PCB. In summary, in the design of high-frequency PCBs, the design of solder pads and their shapes, aperture, and pitch should not only consider their uniqueness, but also meet the requirements of production processes. Adopting standardized design can not only reduce product costs but also improve production efficiency while ensuring product quality.
2. Design standards for the shape and size of solder pads in PCB design
(1) The PCB standard packaging library should be called.
(2) The minimum diameter on one side of all pads shall not be less than 0.25mm, and the maximum diameter of the entire pad shall not be greater than 3 times the aperture of the component.
(3) Try to ensure that the distance between the edges of two welding pads is greater than 0.4mm.
(4) In cases of dense wiring, it is recommended to use oval and oblong connecting discs. The diameter or minimum width of a single panel solder pad is 1.6mm; The weak current circuit solder pad of the double-sided board only needs a hole diameter of 0.5mm, and excessive solder pads can easily cause unnecessary bonding.
(5) Pads with an aperture exceeding 1.2mm or a pad diameter exceeding 3.0mm should be designed as diamond or plum shaped pads
(6) For plug-in components, in order to avoid copper foil breakage during welding, and the single-sided connecting plate should be completely covered with copper foil; The minimum requirement for double panels is to fill in tears; As shown in the figure:
(7) All machine plug-in parts need to be designed as drip solder pads along the bending direction to ensure full solder joints at the bending position.
(8) The solder pads on a large area of copper sheet should use chrysanthemum shaped solder pads to avoid false soldering. If there is a large area of ground wire and power line area on the PCB (the area exceeds 500 square millimetre), a window should be opened locally or the grid should be designed to fill (FILL). As shown in the figure:
3. PCB manufacturing process requirements for solder pads
(1) If there are no plug-in components connected at both ends of the SMD component, a test point should be added, with a diameter equal to or greater than 1.8mm, for the convenience of online testing by the tester.
(2) If the IC pin pad with dense foot spacing is not connected to the hand plug-in pad, a test pad needs to be added. If it is a SMD IC, the test point cannot be placed inside the SMD IC screen. The diameter of the test point is equal to or greater than 1.8mm for the convenience of online testing with the tester.
(3) If the spacing between pads is less than 0.4mm, white oil must be applied to reduce continuous welding during wave peaks.
(4) The two ends and ends of the SMD components should be designed with lead wires. The width of the lead wire is recommended to be 0.5mm, and the length is generally 2 or 3mm.
(5) If there are hand soldered components on a single panel, the tin groove should be opened in the opposite direction to the direction of passing the tin, and the width depends on the size of the hole from 0.3MM to 1.0MM; (50-70% of the aperture) is shown in the following figure:
(6) The spacing and size of conductive rubber buttons should match the actual size of the conductive rubber buttons. The PCB board connected to this should be designed as a gold finger and the corresponding gold plating thickness should be specified.
(7) The size and spacing of the solder pad should be the same as the size of the SMD component (1:1).
(8) For solder joints with a distance of less than 0.4mm between pads (with more than 4 pads) on the same straight line, on the basis of adding white oil, if the long side of the component is as parallel as possible to the direction of the wave peak, an empty pad should be added at the end of the pad or the end of the pad should be enlarged to reduce continuous soldering.
PCB pad design
1. Pad and aperture
Under the premise of ensuring that the minimum spacing of the wiring does not violate the designed electrical spacing, the design of the solder pad should be larger to ensure sufficient ring width. Generally, the inner hole of the solder pad is slightly larger than the diameter of the lead wire of the component, which is too large in design and can easily lead to false soldering during welding. The outer diameter D of the solder pad is generally not less than (d+1.2) mm, where d is the inner diameter of the solder pad. For some high-density PCBs, the minimum value of the solder pad can be taken as (d+1.0) mm. The shape of the solder pad is usually set to be circular, but for integrated circuits packaged with DIP, it is best to use a runway shaped solder pad, which can increase the area of the solder pad in limited space and facilitate the soldering of integrated circuits. The connection between wiring and pads should have a smooth transition, that is, when the width of the wiring entering the circular pad is smaller than the diameter of the circular pad, a teardrop filling design should be adopted.
It should be noted that the size of the aperture d inside the solder pad varies and should be considered based on the actual diameter of the component lead, such as component holes, installation holes, and slot holes. The hole spacing of solder pads should also be considered based on the actual installation method of components, such as resistors, diodes, tubular capacitors, etc. There are two installation methods: “vertical” and “horizontal”, and the hole spacing of these two methods is different. In addition, the design of pad hole spacing also needs to consider the minimum gap requirements between components, especially the gap between special components needs to be ensured by the hole spacing between pads.
In high-frequency PCBs, it is also necessary to minimize the number of vias as much as possible, which can reduce distributed capacitance and increase the mechanical strength of the PCB. In summary, in the design of high-frequency PCBs, the design of solder pads and their shapes, aperture, and pitch should not only consider their uniqueness, but also meet the requirements of production processes. Adopting standardized design can not only reduce product costs but also improve production efficiency while ensuring product quality.
2. Design standards for the shape and size of solder pads in PCB design
(1) The PCB standard packaging library should be called.
(2) The minimum diameter on one side of all pads shall not be less than 0.25mm, and the maximum diameter of the entire pad shall not be greater than 3 times the aperture of the component.
(3) Try to ensure that the distance between the edges of two welding pads is greater than 0.4mm.
(4) In cases of dense wiring, it is recommended to use oval and oblong connecting discs. The diameter or minimum width of a single panel solder pad is 1.6mm; The weak current circuit solder pad of the double-sided board only needs a hole diameter of 0.5mm, and excessive solder pads can easily cause unnecessary bonding.
(5) Pads with an aperture exceeding 1.2mm or a pad diameter exceeding 3.0mm should be designed as diamond or plum shaped pads
(6) For plug-in components, in order to avoid copper foil breakage during welding, and the single-sided connecting plate should be completely covered with copper foil; The minimum requirement for double panels is to fill in tears; As shown in the figure:
(7) All machine plug-in parts need to be designed as drip solder pads along the bending direction to ensure full solder joints at the bending position.
(8) The solder pads on a large area of copper sheet should use chrysanthemum shaped solder pads to avoid false soldering. If there is a large area of ground wire and power line area on the PCB (the area exceeds 500 square millimetre), a window should be opened locally or the grid should be designed to fill (FILL). As shown in the figure:
3. PCB manufacturing process requirements for solder pads
(1) If there are no plug-in components connected at both ends of the SMD component, a test point should be added, with a diameter equal to or greater than 1.8mm, for the convenience of online testing by the tester.
(2) If the IC pin pad with dense foot spacing is not connected to the hand plug-in pad, a test pad needs to be added. If it is a SMD IC, the test point cannot be placed inside the SMD IC screen. The diameter of the test point is equal to or greater than 1.8mm for the convenience of online testing with the tester.
(3) If the spacing between pads is less than 0.4mm, white oil must be applied to reduce continuous welding during wave peaks.
(4) The two ends and ends of the SMD components should be designed with lead wires. The width of the lead wire is recommended to be 0.5mm, and the length is generally 2 or 3mm.
(5) If there are hand soldered components on a single panel, the tin groove should be opened in the opposite direction to the direction of passing the tin, and the width depends on the size of the hole from 0.3MM to 1.0MM; (50-70% of the aperture) is shown in the following figure:
(6) The spacing and size of conductive rubber buttons should match the actual size of the conductive rubber buttons. The PCB board connected to this should be designed as a gold finger and the corresponding gold plating thickness should be specified.
(7) The size and spacing of the solder pad should be the same as the size of the SMD component (1:1).
(8) For solder joints with a distance of less than 0.4mm between pads (with more than 4 pads) on the same straight line, on the basis of adding white oil, if the long side of the component is as parallel as possible to the direction of the wave peak, an empty pad should be added at the end of the pad or the end of the pad should be enlarged to reduce continuous soldering.
Summarize
In Rule Clearance, create a new Rule Clearance1 (with a customizable name), then select ADVANCED (Query) in the Where First Object matches option box, click QueryBuilder, and the Building Query from Board dialog box will appear. In the first row of this dialog box, select Show All Levels (default) from the drop-down menu, and then select Object Kinds from the drop-down menu under ConditionType/Operator, Then select Ploy from the dropdown menu under ConditionValue on the right, so that IsPolygon will be displayed in QueryPreview on the right. Click OK to save and exit. The next step is not yet complete. In the FullQuery display box, change IsPolygon to InPolygon (bugs in DXP must be changed like this, it seems that there is no need to change in the 2004 version). The final step is now, Now you can modify the spacing you need in Constraints (based on your plate making process level). This only affects the spacing of copper laying and does not affect the spacing of wiring in each layer.
