Analysis of the PCB design routability and design efficiency techniques

  Introduction: PCB layout design, a complete set of methods for the improvement of cloth pass rate, in this, we provide all the PCB design routability as well as design efficient and effective techniques, customers can not only save the project development cycle, but also can the maximum guarantee of the quality of the finished design.
1 to determine the number of PCB layers
   Board size and wiring layers are determined early in the design. If the design requires the use of high-density ball grid array (BGA) components must be considered the minimum wiring layers in the device wiring. The number of wiring layers and cascading manner (stack-up) will directly affect the wiring and impedance of the trace. The size of the board to help determine the cascading manner and printed line width, to achieve the desired design effect.
    Over the years, people always think the circuit board layers, the less the lower the cost, but the impact on the circuit board manufacturing costs, there are many other factors. In recent years, the cost difference between the multilayer has been greatly reduced. More uniform distribution of the circuit layer and the copper began to design the best to avoid until the end of a small amount of signal does not meet the defined rules and space requirements in the design approaches, which was forced to add a new layer. Careful planning before the design will reduce the wiring in a lot of trouble.
2, the design rules and restrictions
    Automatic routing tool itself does not know what to do Shiyao. To complete the routing task, cabling tools need to work under the correct rules and restrictions. Different signal lines have different wiring requirements, to classify all the special requirements of signal lines, different designs are not the same classification. Each signal class should have priority, the higher the priority, the more stringent rules. Rules involve the printing of line width, the maximum number of vias, the parallelism between the signal line layer limit, these rules have a significant impact on the performance of the routing tools. Seriously consider the design requirements is an important step in successful wiring.
3 the layout of the components
   In order to optimize the assembly process, the component layout of the design for manufacturability (DFM) rules will limit. Assembly department allows components to move, the appropriate optimization of the circuit, more user-friendly automatic routing. Defined rules and constraints will affect the layout.
   To consider the layout of the wiring path routing channel and through-hole area. These paths and regional designers is obvious, but the auto-routing tool once only consider a signal by setting the wiring constraints, and set the layer of cloth signal line wiring tool as envisaged by the designer as to complete the wiring.
4, the fan-out design
   Design phase of the fan-out, to make automatic routing tools, component pins connect each pin surface mount devices have at least one through-hole, so that more connections are required, the circuit board to carry out within layer connected to the online test (ICT) and the circuit re-treatment.
   To the most efficient automatic routing tools, be sure to use the largest possible hole size and trace interval setting is ideal for 50mil. Routing path to the largest number of through-hole type. Fan-out design, taking into account the circuit the online test. The test fixture may be very expensive, and usually is about to put into full production will be ordered, this time to consider adding a node to achieve 100% testability is too late.
    After careful consideration and prediction of online testing of the circuit design can be carried out early in the design and realization late in the production process, according to the wiring path and line circuit test to determine Kong fans will also affect the type, power and ground to the wiring and fan-out design . Order to reduce the inductive reactance of the filter capacitor, cable, the hole should be as close to the pin surface mount devices can be used manually when necessary cabling, wiring path impact of which may have originally envisaged, and may even cause you to re- consider the use of what kind of hole, and therefore must consider the relationship between the hole and pin inductance and set the priority of the hole specifications.
5, manual routing, and the key signal processing
    Although this article discusses the auto-wiring problem, but an important process of manually wiring in the present and future of the printed circuit board design. Manual wiring help of automatic routing tools to complete the cabling work. Regardless of the number of key signal, the first of these signal routing, manual routing or in combination with automatic routing tools can. The key signal is usually through careful circuit design to achieve the desired performance. Routing is complete, and then by the relevant engineering staff of these signal wiring check, the process is much easier. Check through these lines, fixed, and then the rest of the signal for automatic routing.
6, automatic routing
    The key signal wiring need to consider the control wiring with some electrical parameters, such as reducing the distributed inductance and EMC also similar for the other signal wiring. All the EDA vendors will provide a way to control these parameters. Which input parameters and input parameters of the cabling for automatic routing tools, automatic routing quality can be guaranteed to a certain extent.
    General rules should be used for automatic routing of the signal. By setting restrictions and the prohibition of the routing area to qualify a given signal used by the layer and used the number of vias, routing tool will be able to cabling in accordance with engineers design ideas. Automatic routing tools, layers and the number of cloth over the hole without restrictions, automatic routing will be used to each layer, and will generate a lot of vias.
   Set of constraints and rules created, automatic routing will reach and expected similar results, of course, may also need to do some finishing work, but also need to ensure that the other signal and network cabling space. After the completion of part of the design will be fixed in order to prevent the influence behind the wiring process.
    Using the same procedure on the remaining signal wiring. Wiring frequency depends on the number of circuit complexity and the general rules you define. After each completion of a class of signals, the rest of the network cabling constraints will be reduced. Followed by a lot of signal routing requires manual intervention. Automatic routing tool is very powerful, and can usually be completed 100% of the wiring. When automatic routing tools are not complete all signal wiring on the need for manually wiring the remaining signal.
7, the design features automatic routing automatic routing design features include:
  7.1 slightly change the setting, try multiple path routing;
  7.2 The basic rules to keep the same, try out different wiring layer different printed line and the interval width, and different line widths, different types of vias, such as blind holes, buried vias, etc., to observe the impact of these factors on the design results;
  7.3 and route tools to deal with the default network as needed;
  7.4 signal is less important, the greater the freedom of automatic routing tool route.
8, wiring finishing
   If you are using EDA tools listed in the wiring length of the signal, check the data, you may find that a long length of the small signal wiring of the number of constraints. This problem is rather easy to handle, can be shortened by manually editing the signal wiring length and reduce the number of vias. In the finishing process, you need to determine which wiring is reasonable, what wiring unreasonable. Manual layout design, the automatic layout design can organize and edit in the inspection process.
9 Summary
In this paper we provide the PCB design routability and design efficient and effective techniques to help the followers of the PCB layout.