Friday, April 8, 2011

【 Weak current College 】 achieve high-efficiency automatic routing of PCB design techniques and elements 】

Although now the EDA tool is very powerful, but with the PCB size requirements are smaller, higher density devices, PCB design of difficulty is not small. How to implement a PCB layout pass rate high as well as reduce design time? this article describes the PCB layout, the layout and wiring design techniques and elements. PCB design time now shorter and shorter, smaller and board space, higher density devices, extremely demanding layout rules and the large size of the components allows designers to work more difficult. In order to solve design problems, and accelerate product listing, now many manufacturers tend to use EDA tools for PCB design. But dedicated EDA tool does not produce the desired results, also not 100% pass rate of cloth, and messy, and often need to spend a lot of time to complete the remaining work.
Now available on the prevalence of EDA tools software a lot, but in addition to the use of terminology and location of the function keys are different things are similar, and how to use these tools to better achieve the PCB design? before beginning the wiring on design for serious analysis and tools software carefully set will make the design more in line with the requirements.   The following is the General design process and steps. 1. determine the PCB layer circuit board size and wiring layer needs to be determined at the design stage. If the design requires the use of high-density ball grid array (BGA) components, you must consider these device wiring required a minimum of wiring layers. The number of wiring and cascading (stack-up) have a direct impact on the printed line wiring and impedance.   Plate size helps to determine the cascade and printing line width to achieve the desired design effects. Over the years, people always think circuit layers less costs lower, but the impact of the circuit board manufacturing costs and many other factors. In recent years, multilayer PCB cost differences between has been greatly reduced. When you start to design the best use of circuit-layer and make deposited copper uniform distribution, to avoid when designing the end found that a small amount of signal does not conform to the defined rules and space requirements, thereby forced to add a new layer.   In the design before careful planning will reduce wiring in a lot of trouble. 2, design rules and limit automatic routing tool itself does not know what to do about nothing. For the complete wiring, wiring tool needs the right rules and restrictions. Different signal lines have different routing requirements, special requirements on all the signal lines to categorize different design category. Each signal class should have priority, the higher the priority, the more strict rules. Rule involves printing line width, the maximum number of holes, parallelism, signal line, the interplay between the layers, which rules on wiring tools has a great impact on performance.   Carefully consider the design requirements were successfully route important step. 3, component layout for optimized Assembly procedure, design for manufacturability (DFM) rules will have on the component layout. If the Assembly sector allows components to move, you can optimize the circuit due to automatic wiring easier.   The definition of rules and constraints affecting the layout design. When the layout regardless of the routing path (routing channel) and through-hole area.   The path and the area on the designer, is obvious, but the automatic routing tool once will only consider a signal by setting a wiring constraints and set to Chief signal line layer, you can make the wiring tools like designers foreseen complete wiring.   4, fan-out design at the design stage, fan-out to enable automatic routing tool can connect to the component pins, surface-mount devices of each pin should have at least one hole for more connections, circuit board to the inner connection, in-circuit test (ICT) and the circuit and then processed. In order to enable the automatic routing tools the most efficient, be sure to use the largest through-hole size and printing lines, the interval is set to 50mil more desirable. To use the routing path few largest through-hole type. For fan out design, taking into account the in-circuit test.   Test fixtures can be very expensive and is usually invested in the upcoming full production is ordered, if the time to consider adding nodes to achieve 100% Testability is too late. After careful consideration and projections, in-circuit test design can be carried out in the design of the early, late in the production process, according to the routing path and in-circuit test to determine the type of fan out of the hole, the power supply and grounding also affects the wiring and fan-out design.   To reduce the filter capacitor connector resulting gankang, through-hole should be as close to the surface-mount devices pin, if necessary, manual wiring, it could have on the original vision of wiring paths, or even may cause you to reconsider what the through-hole and must therefore be considered the hole and PIN inductance between resistance and set the priority of the hole specifications. 5, manual, wiring and key signal processing although this article primarily discusses automatic wiring problems, but manual wiring in present and future are printed circuit board design is an important process. Manual routing help autorouting tools complete wiring work.   Figure 2a and 2b illustrates, through the selection of the network (net) for manual routing and be fixed, can be formed automatically when routing path can be based. Regardless of the number of key signal, first of all on these signals are wiring, cabling, or manually combined with automatic routing tool. Key signal usually must through careful design of the circuit to achieve the desired performance. Wiring is complete, then from the project staff to check the signal wiring, this process much easier.  Check, if enacted, will these lines, and then start on the rest of the signal for autorouting. 6, autorouting on key signal wiring in some electrical wiring to control parameters, such as decreasing inductance and EMC, etc, for other signal wiring is similar. All EDA manufacturers will provide a way to control these parameters.   In understanding what the automatic routing tools, as well as the input parameter is an input parameter to the wiring, the automatic routing of quality, to a certain extent, can be guaranteed. Should adopt the General rules onSignal for autorouting. By setting restrictions and prohibition of wiring area to qualify the given signal layer and referred to a number of the hole, wiring tools can be thought of as an engineer design to automatic routing.   If the automatic routing tool used by the layer and the Chief of the hole number without restriction, automatic wiring will use to each layer, and will produce a lot of the hole. In the set of constraints and rules created by the application, the automatic routing will achieve the expected results of a similar, but of course you may also need to be some consolidation, but also the need to ensure that other signal and network cabling.   After the completion of the part design, fixed it, to protect against the effects of the process behind the wiring. Using the same steps for the remaining signal wiring. Routing number depends on the circuit of complexity and your definition of general rules. Every time you complete a signal, the rest of the network cabling constraints will be reduced. But followed many signal wiring requires manual intervention. Now automatic wiring tools are powerful and can usually be completed 100% of the wiring.   But when the automatic routing tool does not complete a full signal wiring is required on the remaining signal for manual routing. 7. automatic routing of design elements include: a slight change in settings, 7.1 try multiple path routing;
7.2 maintain basic rules, try different wiring layer, different printing line and interval width and different line widths and different types of through-hole such as blind and buried hole, observation of these factors on the results of the impact of the design;
7.3 let wiring tools on the default network are processed as required;
7.4 signal less important, automatic routing tools on its route, the greater the degree of freedom. 8, route finish if you are using EDA tools software is able to list the length of signal wiring, check these data, you may find some constraints are minimal length of signal wiring. This problem is easier to handle, by manually editing can shorten the length of signal wiring and reduce the number of the hole. In the finishing process, you need to determine which route is reasonable, what wiring unreasonable.   As with manual wiring design, automatic routing design can also be carried out during the check the collate and editing. 9, the appearance of the circuit board design often pay attention before the Visual effect of the circuit board, now is not the same. Automatic design of circuit boards than manual design of appearance, but in electronic characteristics to meet the requirements and design integrity cannot be guaranteed.

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