Older single sided PCB might not prove a good foot-print for the designing of a multi-layer PCB. Anyone designing multilayer PCB needed to find-out any additional requirements necessary. Layers or attributes are required to add in foot-print of multi-layer PCB is completely dependent on CAD (Computer Aided Designing) system being used. Latest CAD systems with on-line library services having access to variety of source data is very much help full in designing of an accurate PCB in a shorter time period with least efforts. Layer stack-up planning is the major difference between the double-sided and multi-layer PCB.
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- Performance: Material used for the fabrication of multi-layer PCB my highly affect the parameters like impedance calculation. Speed of circuitry, how fast circuit will operate or the working environment where the completed board will operate also highly depend upon the material used for fabrication. There are more advanced materials than the FR-4 available for multi-layer fabrication but those materials can affect the other board material. Selection of material should be done after the discussion with the PCB fabricator, because selection of fabrication material highly depends on the needs of circuit board and the operating environment where the system will work after completion.
- Cost: Overall cost of PCB fabrication directly depends upon fabrication material used as well as number of layers and configuration. For consideration of all the options available again designer should contact to the PCB manufacturer to choose the most adequate options.
- Density: Determining the configuration of board layer stack up is highly dependent upon the routing density of the board. When a layout of the board is already been started it is very difficult and time consuming to add a new layer in layout, that not only needs to reconfigure the CAD libraries for that project but also need to make changes to layout as well. Starting with too many layers might leads to the extra expensive board fabrication than usual.
- Circuitry: Optimum layer configuration can only be done by completely understanding of the circuit needs. Additional ground planes might be required for the purpose of strip line layer configuration of some sensitive signals for performing at their best. Separate layer for analog and digital signal along their own individual ground planes and separate voltage supply layers for different voltage to the appropriate components requiring the corresponding voltage should considered while designing the multilayer PCB. All the circuitry consideration should be kept in mind and design a layer layout for the circuit before starting designing the PCB itself.
Placing and routing the board can be started after getting all the required knowledge, documentation and a basic sketch of layer layout.
A Different approach to Placing and Routing
Working with double sided PCB there are only two layers to play with are ‘TOP’ and ‘BOTTOM’ while working with multilayer PCB, there is whole new world of 3D. Playing safely in 3D world is necessary to expand a designer’s own thinking in 3D pattern where different things are happening in internal layers which may affect top or bottom of PCB. In some case designers doesn’t want to place a noisy component in a certain place because of sensitive routing in inner layer underneath.
Placing of components in multilayer PCB might seems to be same as in a double sided PCD design but the difference is the environment of landscape where the designer working despite of which CAD tool is being used. There is not much worrying about leaving of space for routing channels being used for the connectivity of components since much of them will be separated in inner layers. In some extent short and direct routing should be done at surface layers for the sensitive circuitry, but is much more space for routing is available in underneath layers. This wide space of working area provide a much more space to place more components in a smaller are of a circuit.
It’s a pleasure and excitement to work with internal place routing and power planes but some important considerations are really necessary to remember.
- Placing of more components over a multilayer board needs more routing as well. Specific routing widths and spaces or other requirements like impedance controlled traces or differential pairs depending on the technology being used for designing the PCB must be considered.
- Ground plane adjacent layer will be necessary for routing of strip line layer structure for some sensitive components. Cross perpendicular routing of sensitive routing lines must be done on the adjacent layers to reduce the noise like broadside coupling or crosstalk in circuit to minimal levels.
- Split planes should be placed in such a pattern so that the sensitive signal should not cross each other and destroy their own signal return path.
After completion of placement and routing checking done, remaining work will proceed like a double sided PCB and now the board is ready to go for fabrication.
Output Files, Documentation and finalizing the design
Finalizing the design out for manufacturing go same as for double sided PCD but only with some exceptions. At first instance more details must be needed on the manufacturing drawing. Stack up build details and explained extra notes on how the board will be built must be needed with fabrication drawing. Additional files for each layer of a multiple layer board must be needed while generating Garber file for building output files. Creating and managing manufacturing output files could be done very easily while using an advanced CAD tool.
Advanced CAD systems equipped with tools that accommodate cutting technologies are available for successful buildup of multilayer PCB in a shorter period of time. OrCAD PCB Designer is an advanced system having access to online CAD library services, board outline creation wizards and manufacturing and documentation generation utilities.
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