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Achieve the design of the high efficiency automatic wiring of PCB
Date:2015-12-20 22:18:29   Source:   Click:
 Now the design time of PCB is shotrer and shorter,the circuit board space is more and more narrow,the device density is higher and higher,extremely harsh placement rule and large-sized assembly make that designer's work is more difficult.To solve the difficult in largedesign,accelerate the listing of product, now a lot of producers tend to adopt special-purpose eda tool to realize the design of PCB.But special-purpose eda tool can not produce ideal results, can not reach 100% completion rate, and in a mess, also need spend a lot of times to accomplish remaining work usually.Today, this paper introduces the design tactics and the main points of PCB planning, place and route.
  Now there are a lot of popular EDA tools in market, but all similar except the position of the term that uses and function key is different, how to use these tools to realize the desi-gn of PCB better?To analysis conscientiously the design and set the tool software conscientiously before wiring,which will make the design meet the requirements.The 
consciengeneral design process and step as below.
    1.Confirm the layer of PCB

 The circuit board size and the wiring number of plies need be confirmed in design initial  time. It must consider the needed minimized wiring number of plies of these device wires if  high density ball bar array (BGA) assembly is used in designing requirement.The quantity of wiring layer and range upon range of (stack-up) mode can directly have influence on the wiring and the impedance of track.The size of plate helps to confirm overlapped way and track width, realizes the design effect of expectation. 
  For many years, people always think the samller of circuit board number,the lower cost  but the manufacturing cost that influences circuit board also has many other factorses.In recent years, the cost difference between the multi-layer sheet reduced greatly.Preferably adopt more circuit layer and apply copper and evenly distribute when beginning to desin.To avoid closing on few signals does not meet rule and the space requirement that has defined  when finished, thereby compelledly add new layer in design.Conscientious planning will reduce a lot of trouble in the wiring before design.
    2.Design rule and restriction

 Auto-placement tool itself does not know what should be done.For accomplishing the wiring task, wiring tool need be worked under correct rule and restrictive condition.Different signal lines has different cabling requirements, classify to the holding wire of all special requirements, and different design categories is also different.Each class signal all should have priority, and the higher of priority,the more stricter of the rule.The Rule relates to influencing each other and the restriction of layer between the maximum quantity, the depth of parallelism, holding wire of track width, via hole,and these rules have very big influence to the performance of wiring tool.Thinking better of designing requirement is the essential step to connect up successfully.

    3.The layout of assembly

 To optimization assembling process,manufacture ability of design (DFM) rule can produce restriction to assembly layout.If assembling department allows assembly to move, can suitably optimize the self routing of being more convenient for the circuit.Defined regular constraints can influence layout designs.
  Need consider routing path (routing channel) and via area when layout.These paths and zones are obviously for the designers; But auto-placement tool once only considers a signal,can make wiring tool accomplished as the designer imagined through setting the layer of wiring constraints and the layer of holding wire.

    4.The fan-out design

 In the fan-out design phase,to make auto-placement connected the assembly pin,each pin of surface mount device should have a via hole at least, so that circuit board can carry  out internal layer connection, on-line testing (ICT) and circuit to be handled again when needs more connectors.

 In order to make auto-placement tool most effective, must use maximum via size and track as far as possible, it is comparatively desirable to be set at 50mil.Adopt the via hole type   that makes the routing path number maximum. It must consider the In-circuit Test problem when carrying out the fan-out design.Test fixture is may, be very expensive, and normally  just can order when being about to drop into overall production, if at this time consider to add node to realize that 100% test ability,just too late.
  Through consideration and prediction carefully, the design of In-circuit Test can be carried out in design initial, realizes in the production process later stage, confirms via hole fan-out type according to routing path and In-circuit Test, and power supply and ground connection also can have influence on wiring and fan-out design.For reducing the induction reactance that the filter condenser connecting line produces,Via hole is next to the pin of as closely as possible surface mount device,it can adopt hand wired in case of necessity.This may  influence to the routing path of original imagination, even may cause you to rethink  which kind of via hole to use, therefore must consider the relation between via hole and pin induction reactance and set the priority of the via hole specification.

    5.The processing of hand wired and key signal
  Although this paper mainly discusses the self routing problem,hand wired is a significant process of PCB design at the present and the future.Adopting hand wired helps auto-placement tool to accomplish installation work.Through the network of picking out (net) being  carried out hand wired and fixing, which can form self routing that path foundation.

 No matter how much the quantity of key signal, at first these signals should be connected up hand wired or combine auto-placement tool .Key signal usually must reach the performance of expectation only through meticulous circuit design.Wiring is come these signal routing should be checked by relevant engineering staff after accomplishing wiring,this process is relatively easier.These lines should be fixed and begun to carried out self routing all  the other signals After inspection is passed through.

    6.self routing
  It need consider to control some electrical parameters such as reducing distributed inductance and EMC etc.,when connecting up to the wiring of key signal, for other wiring lines are also similar..All EDA manufacturers can provide a kind of method to control these parameters.After understanding auto-placement tool which has input parameter and the influence of input parameter to wiring, the quality of self routing can be guaranteed to a certain extent.
  We should adopt general rule to carry out self routing for signal.Through restrictive condition being set and forbidding that wiring region limits employed layer of given signal and   used number of vias, wiring tool just can come self routing according to engineer's design philosophy.If the layer used to auto-placement tool do not add restriction with the quantity of institute's cloth via hole, will use each layer during self routing, and will produce a lot of via holes.
  After setting constraints and using the rule of being created, self routing will reach the result closely with expection, also need carry out some arrangement certainly, also need guarantee the space of other signal and network layout simultaneously.After part design is   accomplished,it should be fixed to prevent  receiving the influence of back wiring process.
  Adopting identical step that all the other signals are connected up.What of the complexity that the wiring number of times depends on circuit and your defined general rule.Each    finished one type of signal,the constraints of all the other network layouts will reduce.But a  lot of signal routing should be connected up with hands.At Present auto-placement tool function is very powerful,which can usually accomplish 100% wiring .But when auto-placement tool is not accomplished whole signal routing, the signal of remainder is carried out wired by hands.
    7.The key points in design of self routing comprises:
    1.Change the setting slightly multiple path wiring on probation;
    2.Keep primitive rule constantly, different wiring layers on probation, different tracks,interval width and different line width, different via hole such as blind hole, buried via hole   etc., these factors are observed in what influence to the design result.
    3.Let wiring tool deal with the acquiescence network according to the needs.
    4.The more inessential signal is,the bigger freely auto-placement tool to the degree of its wiring.

    8.The arrangement of wiring

  If you can list wiring lines length by employed EDA tool software, check these data, you may find that the length of some constraint signal routing seldom is very long.This problem ratio is easier to handle, can shorten signal routing length and reduce number of via hole  through the manual editing.In the arrangement process, you need judge rationally which wiring is reasonable , which wiring is unreasonable.The same with the hand wired design, the self routing design also can be put in order in checking process and edit.
    9.The outward appearance of circuit board
  The visual effect of circuit board is usually noted in design in the past, and now is different.Automatic designed circuit plate is attractive in appearance unlike manual designs, but can satisfy the requirements of regulation on characteristic electron, and the complete performance of design is guaranteed.


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