Function when the design of PCB (printed circuit board), a basic problem to be considered is the need to achieve the required number of circuit wiring layer, ground and power planes, and the printed circuit board wiring layer, ground and power determining the number of layers and the planar circuit function, signal integrity, EMI, EMC, manufacturing cost requirements. For many conflicting requirements of most designs, the performance requirements of the PCB, the target cost, the complexity of the manufacturing techniques and systems other factors, the PCB stack design is usually determined after considering a compromise of various factors. High-speed digital circuits and RF circuits usually multilayer design.
Listed below are the eight principles of design should pay attention to the stack
1. stratified
In a multilayer PCB typically includes a signal layer (S), the power (P) and a ground plane (GND) plane. Power and ground planes are generally planar undivided entity, the current signal traces are adjacent will provide a good low resistance current return path. Most of these signal layer positioned between the reference power or ground plane layer, constituting the symmetric or asymmetric stripline stripline. Top and bottom layers of the multilayer PCB for placing components and a small amount of generally alignment, these requirements can not be too long signal traces to reduce direct radiation traces generated.
2. The power supply reference plane determination unit (power plane)
Decoupling capacitors is an important measure to solve the power integrity. Decoupling capacitors can be placed in the top and bottom layers of the PCB. Decoupling capacitor traces, pads, vias and seriously affect the effect of the decoupling capacitors, which requires the designer must consider the decoupling capacitor traces should be as short and wide, connected to the via hole conductors should be as short as possible. For example, in a high-speed digital circuits, decoupling capacitors may be placed on the top of the PCB, the second layer 2 to the high-speed digital circuit (processor) as a power supply layer, the second layer 3 as a signal layer, the first layer 4 high-speed digital circuit board is arranged.
Further, to ensure as far as possible by the same high-speed digital signal device driven by the power traces in the same plane as a reference layer, and this layer is a power layer of the power supply high-speed digital devices.
3. Determine the multiple power reference plane
Multi-power reference plane is divided into several different entities voltage region. If the power supply layer is close to the multi-level signal, then the signal current on the signal layer in the vicinity thereof will encounter undesirable return path that returns the gap occurs on the path. For high-speed digital signal, such an unreasonable return path design may lead to serious problems, it is required to be away from the high-speed digital signal wiring multiple power reference plane.
4. determining a plurality of ground reference plane (ground plane)
A plurality of ground-reference plane (ground layer) provides a good low impedance current return path, it is possible to reduce the common mode EMl. Ground and power planes should be tightly coupled, the signal layer may also be adjacent to the reference plane and tightly coupled. Reducing the dielectric thickness between the layers can achieve this purpose.
The combination of rational design wirings
Two layers of a signal path across which is called a "routing combination." The best combination of the wiring design is to avoid a return current from flowing to the reference plane the reference plane to another, but the stream to another point (surface) of a reference plane from a point (plane). In order to complete the complicated wiring between the conversion layer traces it is inevitable. When converting between signal layers, to ensure that the return current can smoothly flow from one reference to another reference plane plane. In one design, the adjacent layers as a combination of the wiring is reasonable. If a signal path spans multiple layers, as a wiring which is usually not a combination of rational design, as a path through layers for purposes of the return current is not smooth. While ground bounce can be reduced by placing in the vicinity of the via decoupling capacitors or decreasing the dielectric thickness between the reference plane and the like, but also a non-good design.
6. Set direction wirings
On the same signal level, should ensure that most of the wiring direction is the same, the wiring should also be orthogonal to the direction of adjacent signal layers. For example, a signal wiring layer, the direction may be set to "Y-axis" direction, and the direction of the other wiring layer adjacent to the signal "X-axis" direction.
7. Using even layer structure
It is found from the PCB stackup design, classic design almost all laminate layers is an even number, not the odd layers, this phenomenon is caused by various factors, as shown below.
Can be understood from the manufacturing process of the printed circuit board, all of the conductive layers on the circuit board on the core layer, the core material is typically a double-sided cover plate, when the full use of the core layer, the conductive layer of the printed circuit board it is an even number.
Even layer printed circuit board having a cost advantage. Since the layer of dielectric and copper less, so the cost of odd-layer printed circuit board material of the printed circuit board is slightly lower than the cost of an even layer. But because the odd-layer printed wiring board laminated core need to increase the adhesion of non-standard process on the basis of the core structure of the process, so processing costs caused by the odd-layer printed circuit board is significantly higher than the even-layer printed circuit board. Compared with ordinary core structure, the core layer added to the outer copper clad structures would result in reduced efficiency of production, extend the production cycle. In the lamination bonding before, outside of the core layer requires an additional process, which increases the risk of scratches and the outer layer is etched wrong. Increase of the outer layer processing will be greatly improved manufacturing costs.
When the printed circuit board after the bonding process of the multilayer circuit, which upon cooling inner and outer layers, different lamination tension causes the circuit board to bend printed on different levels. And with the increase of the thickness of the circuit board, the risk of having two different structures of the composite printed circuit board is bent greater. Odd-layer circuit board is easily bent, the circuit board can be avoided even layers of the circuit board is bent.
At design time, if the occurrence of odd-layer laminate, the following methods can be employed to increase the number of layers.
If the power supply layer of the printed circuit board design is an even number and an odd number of signal layers, it can be adopted a method of increasing the signal layer. Increased signal level does not lead to increased costs, but the processing time can be shortened, improving the quality of the printed circuit board.
If the power supply layer of the printed circuit board design is odd and an even signal layer, power supply layer can be used to increase this method. And in the case of another simple method without changing other settings plus a ground layer laminated on the intermediate, i.e., odd-layer printed circuit board to press the wiring, and then copying a ground layer in the middle.
In microwave circuits, and mixed media (media have different dielectric constants) circuit, a blank signal may be added in the laminated layer close to the center of the printed circuit board, so that the imbalance can be minimized laminate.
8. cost considerations
In manufacturing costs, in the case of a PCB having the same area, the cost of the multilayer circuit board and certainly higher than single layer circuit boards, and more layers, the higher the cost. However, when considering the circuit function and achieve miniaturization of the circuit board, to ensure signal integrity factor, EMl, EMC performance indicators and the like, should be used a multilayer circuit board. Comprehensive evaluation, the cost difference with the multilayer circuit board both single-layer circuit board and not much higher than expected.
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