With the rapid development of high-speed theory, PCB routing can no longer be regarded as a simple interconnection carrier, but to analyze the influence of various distribution parameters from the transmission line theory. A distributed parameter circuit is a circuit in which the parameter distribution of circuit components must be considered. The distribution of parameters means that the potentials and currents of two adjacent points in the circuit at the same instant are different. This shows that the voltage and current in a distributed parameter circuit are not only a function of time, but also a function of space coordinates. At the same time, the complexity and density of PCBs are also increasing from copper-paved through-hole design to micro-hole design and to multi-level buried blind hole design, and now there are buried resistance, buried capacitance, buried device design, etc. While high density brings great difficulties to PCB wiring, it also requires PCB design engineers to have a more deeper understanding of the PCB production process and its process parameters.
Wiring methods are divided into automatic wiring and manual wiring. At present, automatic wiring cannot meet the high standards of hardware engineers in many aspects, so it is generally realized by manual wiring.
DFM requirements in wiring
1、 hole
Mechanical drilling is generally recommended to be above 8mil, and the limit is 6mil. Try to ensure that the ratio of thickness to diameter is generally 10:1 because the higher the ratio of thickness to diameter, the more difficult it is to process. The width of the hole ring of the device is at least 8 mils on one side, and the width of the via ring is at least 4 mils on one side. The processing manufacturer will automatically optimize during the cam processing, and the solder mask window is 50um on one side.
The pitch of vias in the same network can be 6mil and the pitch of via holes in different networks is 275um, and the pitch of via holes in different network devices is 425um.
The production is that the drill bit is generally 150um larger than the original hole, the drill bit is increased by 0.05mm, and the larger drill bit will be increased by 0.1mm. Then through holeization, electroplating meets the final product hole diameter requirements.
The distance between the non-metallized drilling and the board edge is 150um, which will not break holes, and the normal frame tolerance. The metalized drill hole is at least 10mil to the edge of the board.
2. ETCH
0.5oz copper thickness, and the smallest line width can be 3mil, and the smallest spacing is 2mil.
1oz copper thickness minimum line width is 3.5mil and minimum spacing is 4mil
2OZ copper thickness minimum line width is 4miland minimum spacing is 5.5mil.
The internal electrical layer should avoid copper at least 20mil.
For small discrete devices, the wiring on both sides should be symmetrical.
When the SMT pad pins need to be connected, they should be connected from outside of the solder leg, and it is not allowed to connect inside the solder leg.
For SMT pads, it is necessary to connect the pads when laying copper on a large area.
ETCH lines are evenly distributed to prevent warping after processing.
Electrical characteristics requirements in wiring
1、Impedance control and impedance continuity
Avoid sharp and right-angle routing.
Use as few vias as possible for key signal wiring.
High-speed signal lines should consider circular arc wiring appropriately
2、 crosstalk or EMC and other interference control requirements
High-speed signals and low-speed signals should be wired in layers and partitions
Digital signal and analog signal No. 1 layered zoned wiring
Digital signal and analog signal No. 1 layered and zoned wiring
Sensitive signal and interference signal layered and zoned wiring
The clock signal must go to the inner layer first
Do not lay copper wires under the projection area of inductive components such as power inductors and transformers. (Because there will be parasitic capacitance between the coils, which will generate parallel resonance with its inductance, As a consequence, there will be SRF and SRF is related to EPC, So the smaller the EPC, the wider the inductive frequency range can be ensured. And the SRF needs to be at least DC -Ten times the switching frequency of the DC Converter. For example, if the switching frequency is 1.2MHz, the SRF needs to be at least 12MHZ. Therefore, when Layout, the bottom of the power inductor should be hollowed out, and there should be no metal to avoid generating extra EPC, which will reduce the inductive frequency range)
The key signal should be placed on the optimal layer, with the ground as the reference plane
Critical signals are considered to be processed in packets.
Any signal, including the return path of the signal, must avoid forming a loop, which is one of the important principles of EMC design.
3w principle of high-speed wiring
Topological structure and timing requirements
Satisfying the timing requirements is the key to the normal and stable operation of the system. The delay control reaction to the PCB design is the equal length control of the wiring and the equal length has even become a term that the wiring engineers talk about you. Timing design is also a very complex system requirement and PCB design engineers must not only know about the isometric length, but also truly understand the timing requirements behind the isometric.
Wiring requirements for power supply and power signal
The power inlet circuit should be protected after the filtering principle
The pins of the chip and its filter capacitor should be as short and thick as possible, and the energy storage capacitor should be punched more to reduce the installation inductance caused by wiring
Considering the safety requirements, the power supply network needs to be kept away when the voltage difference is large, and the pins and vias of the high-voltage network plug-in need to be hollowed out.
Heat dissipation considerations in wiring
There is another important trend in electronic design, that is voltage drop and power consumption increase. As an important part of board thermal design, PCB wiring has become more important. It is necessary to use related electrical and thermal simulation tools to assist in thermal design.
Strictly calculate the wiring channels to meet the current-carrying requirements.
Pay attention to the current-carrying capacity of the vias, and plan the number and location of the vias reasonably.
The vacant position under the chip with large heat generation can be covered with copper on a large area, and ground holes can be added to enhance heat dissipation.
In the projection area of high-power devices with large heat generation, do not run high-speed lines and sensitive signal lines on all layers.
If high current power supply ’s wiring path is long to compensate, it is necessary to strengthen its wiring path to reduce heat loss.
Heat-generating devices with heat-dissipating pads have been added, and vias have been added to the heat-dissipating pads to enhance heat dissipation.
Wiring summary
PCB wiring is a systematic project and design engineers need to have comprehensive knowledge of multiple disciplines, as well as strong analytical and processing capabilities, and a good balance needs to be achieved in all aspects of integration.
PCB design is not a myth and also not a black box,and there is no one-size-fits-all method.Behind all reasonable specification requirements ,there can be found the real theoretical manufacturing process. We should think more, ask more, learn more in our normal work, and this is the road to becoming a master.
