PCB Stack-Up Principle

PCB stack-up has a great influence on the EMC (electromagnetic compatibility) performance of a product. A good stack can reduce radiation from the loops on the PCB and the cables attached to the board. On the other hand, a poor stack-up can increase the radiation from both of these mechanisms considerably.

To design multi-layer PCB circuit boards, designers need to confirm the circuit boards structure firstly according to the scale of circuit, the size of circuit boards, and the requirements of electromagnetic compatibility (EMC). That is to say, designers have to use 2, 4, 6, or more layers of circuit boards. If the design requires the high density ball grid array (BGA) devices, the minimal number of wiring layers must be considered. For years, people always think that the less PCB layers would spend the lower cost. However, many other factors affect PCB manufacturing costs. In recent years, the differences between costs of multi-layer boards have been reduced significantly. As soon as the number of layers been determined, the placement of the inner layer and different signals in these layers are decided --- this is the stack-up design of multi-layer PCB. Careful planning and rational stack-up designs beforehand will save a lot of efforts in the following wiring and future production.

After the number of circuit board layers determined, It needs to reasonably arrange the placement order of the circuit of each layer. In this part, there are two main factors to be considered. One is the distribution of special signal layers. The other is the distribution of power layer and ground layer.
The more layers circuit boards have, the more varieties arrangement on special signal layers, ground layers and power layers have. Thus, it is more difficult to choose the best combination method, but the general principles are as follows

(1) The signal layer should be next to an internal power layer (internal power/ground layer), shielded by the copper film of internal power layer.

(2) The internal power layer should be integrated with ground layer tightly. That means, the thickness of medium between internal power layer and ground layer should take the smaller value, improving the power supply capacitor between the internal power layer and ground layer, and increasing the resonant frequency. If the electric potential difference between internal power layer and ground layer is not important, a smaller insulation thickness can be used, like 5mil (0.127mm).

(3) To avoid the two signal layers directly adjacent. It is easy to introduce crosstalk between adjacent signal layers, leading to the fail of the circuit. To place a ground layer between two signal layers can avoid cross talk efficiently.

(4) Multiple grounded internal power layers can reduce the ground impedance effectively. For example, A signal layer and B signal layer use ground plane respectively can reduce common-mode interference effectively.

(5) The symmetry of layer structure.

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PCB Design Considerations

In recent years, electronic products have been around military equipment , computers, communications equipment , mobile storage equipment, mobile communications equipment, and other related fields , and in size, integration, function and other aspects of the increasingly high demand , and consumer electronics in several generations per year the pace of development update , as an important part of the printed circuit boards and electronic products are constantly improve and update .  

 China now is the world's printed circuit board (PCB) production superpower , PCB species from single-sided to double-sided, 4-28 layer to multilayer ; from the through -hole conduction technology to blind hole conduction technology , Then HDI (High Density Interconnection) high density interconnect technology and technology progress. System board means is endless , different processes are different system boards also have advantages and disadvantages depending on the application object. Currently , the production of printed circuit boards are mainly used in two aspects, One enterprise production , the second is the school teaching and research .

In PCB design should pay attention to :

1 . Making physical borders:  closed physical borders of a future component placement , alignment is a basic platform , and also the layout automatically plays the role of constraint , otherwise , coming from the schematic elements will be overwhelmed . Here we must note accurately , otherwise after installation problems will be big trouble . There is a corner where the best arc , on the one hand to avoid sharp corners scratched workers , but also can reduce stress. 

2 . The introduction of component and networks: the network element and the introduction of good painting border should be very simple , but there are often problems must be solved one by one carefully follow the prompts errors , or later have to spend more effort.

3 , The layout element: lifetime layout and trace elements of the product , stability, EMC has great influence , should pay special attention to the place.

4 . Placed the order : the first place and the structure of the fixed location of the components, such as electrical outlets , lights , switches, connectors category , the placement of these parts LOCK function after a good software lock , will not make it in the future mistaken move . Then place a special element on the line and large components , such as the heating element , transformers , IC and so on . Finally, place a small device .

5 .Pay attention to heat : component placement pay special attention to heat issues . For high-power circuit , the heating element should be those such as power transistors , transformers, etc. try to place aside dispersed layout , easy heat dissipation , not concentrated in one place , nor too high capacitance in order to avoid premature aging of the electrolyte .

6. The wiring principle : when two panel wiring , two wires should be perpendicular , oblique , or curved alignment, avoiding parallel to each other , in order to reduce the parasitic coupling ; as the input and output circuits printed wire should try to avoid rabbit adjacent parallel , in order to avoid feedback between these wires plus ground wire is best . Traces corner as greater than 90 degrees , 90 degrees or less to prevent the corner , but also minimize the use of 90 -degree corners.

7 .Adjustment and improvement: after the completion of wiring to do is to text, individual components , traces some adjustments and copper , in order to facilitate production, commissioning and maintenance .

8 , checked for network ; sometimes because of misuse or negligence painted board network of relationships with different schematics , checking is necessary. So after drawing the plate must not rush to the manufacturer .

9 . Using the simulation function : so you can find some of the problems in advance , greatly reducing the workload for subsequent commissioning

So for China printed circuit board production enterprises should use traditional chemical system board technology for teaching and research universities and research institutions can use mechanical system board technology.