A METHOD FOR GRIDLESS ROUTING OF PRINTED CIRCUIT BOARDS. A. C. Finch, K. J. Mackenzie, G. J. Balsdon, G. Symonds

Transcription

1 A METHOD FOR GRIDLESS ROUTING OF PRINTED CIRCUIT BOARDS A C Finch K J Mackenzie G J Basdon G Symonds Raca-Redac Ltd Newtown Tewkesbury Gos Engand ABSTRACT The introduction of fine-ine technoogies to printed circuit board design and manufacture has stretched most previous automatic routing agorithms to the imit A different approach designed specificay to meet the present needs is detaied is not however reasonabe to reduce the grid size down to the order of 1 mi or ess to cope with current deveopments in the fied of printed circuit board manufacture Fine-ine routing requires a different data structure and a different agorithm suitabe for true gridess routing A data structure for gridess routing was suggested by U Lauther (RefB) and this is the basis of the data structure used by the router described here INTRODUCTION A program is described which has as its objectives: To produce an automatic printed circuit board router capabe of competey routing a range of different board types These shoud cover the spectrum from 50 mi gridded technoogy designs through to present day fine-ine technoogy from singe ayer to muti- ayer with pated through or buried vias In particuar the router shoud be abe to route effectivey on boards with surface mounted components The fina product shoud be compatibe with avaiabe CAD systems as we as future deveopments DISCUSSION Most commerciay avaiabe automatic routers are based on agorithms that rey heaviy on the concept of routing grids Their interna data structure or mode of the printed circuit board compements the methods used This is exempified by the ce structures used for the Lee or food type routers (Ref1) As eary CAD systems had gridded basic data structures this caused few probems Gridded modeing becomes difficut to impement successfuy as the routing becomes finer The data size increases by a square aw giving onger set-up and access times The seection of a suitabe grid itsef becomes more difficut as pad and pated through hoe sizes become arger compared to the track widths Some current impementations utiise specia purpose hardware to improve run times by providing a dedicated centra processor and penty of memory to overcome the data size probem (Ref2) It METHOD Before describing the method a definition of some of the terms used is given where these may not have their obvious meanings This is foowed by a brief description of the interna data structures an overa method and a detaied description of the method of routing a singe connection DEFINITIONS OF TERMS USED EDGE An item which can be expanded into an expansion area Edges incude source pads source track segments source vias free edges bocked edges or via site rectanges (figs -3) These items are defined by an encosing rectange a ayer a cost point the cumuative cost of reaching the cost point and a direction or directions in which they shoud be expanded DIRECTION The direction in which an edge may be expanded This may be any one of the directions N S E W NW NE SW SE or a four of the cardina directions FREE EDGE An edge of an expansion rectange forming part of its perimeter that does not abut onto any other obstace Free edges may continue around one corner FIXED EDGE An edge of an expansion rectange forming part of its perimeter that abuts onto a fixed obstace 22nd Design Automation Conference 073&100X/85/0509$ IEEE 509

2 BLOCKED EDGE An edge of an expansion rectange forming part of its perimeter that abuts onto a track segment or via of another net that is onto an unfixed obstace Bocked edges may be formed by merging adjacent free and bocked edges that are individuay too short to produce a ong enough edge to be usabe VIA SITE RECTANGLE An edge of an expansion rectange within or partiay via overapping it which coud hod a vaid FIXED OBSTACLES Obstaces to routing that must not be vioated as they wi not be atered on subsequent passes These incude the board outine or defined routing area pads and user defined fixed track segments and vias UNFIXED OBSTACLES Obstaces to routing that can be vioated if necessary as they may be atered subsequenty on the same or ater passes These are track segments and vias on other nets COST POINT A point associated with an edge used in the computation of the cost It is usuay the cosest point on the current edge to the cost point on the previous edge COST Consists of a basic cost and a notiona target cost The basic cost is a function of the distance traversed between the source point and the current cost poi:nt and additiona costs for using vias and bocked edges The notiona target cost is the owest cost from the current cost point to any target ignoring obstaces but incuding necessary vias NET Consists of component pads track segments and vias that are eectricay common ROUTED SUBNET Consists of a singe pad a singe fixed route segment or via; or two or more of these items competey connected by route segments and vias TARGET ITEM Any item in the net being routed that is not incuded in the current source subnet FIG 1: EXPANSION ABOUT A SOURCE PAD FIG 2:FREE EDGES AND BLOCKED EDGES FROM EXPANSION IN FIG 1 EXPANSION AREA BLOCKED EDGE e--+--d FREE EDGES FIG 3: EXPANSION FROM FREE EDGES IN FIG 2 CONNECTION An impied joining of two routed subnets The router does not have as input a defined frorn-to ist and routes may be found between any two items on routed subnets of the same net MIL One thousandth of an inch 510

3 OVERALL DESCRIPTION subsequent passes as opposed to simpy ignoring the faiures The router is a singe or muti-pass cost-driven mutiayer food and backtrack type router Its major difference to conventiona food type routers being DATA STRUCTURES the fact that it has no defined ce size Each ce or expansion area is as arge as possibe Movement Two main interna data structures are used by this from one area to another is across an edge forming router the obstace data structure being the mode part of the common perimeter or through a possibe of the PC board and the activity array or frontier via site Edges may aso be invaid or bocked edges ist aowing for invaid routes to be found where no vaid one is possibe This is utiised in the muti-pass mode The food part of the routing makes no detaied decisions about track paths merey defining possibe paths in terms of abutting or overapping expansion areas As the areas are aways rectanguar the router coud perhaps be described as a rectange-probe router SINGLE-PASS MODE A simpe fast method of testing the routabiity of a given pacement for a particuar printed circuit board design is necessary At present the ony rea test is to attempt to route it As the pace - test for routabiity process is necessariy iterative the test for routabiity itseif needs to be effective and fast Using a router to do this requires a fast router giving a reasonabe competion rate A simpe singe-pass of the connections through a router that has a mutipass mode shoud give answers that are representative of the fina routing Given a high competion rate the designer has the opportunity to compete the board manuay if he wishes to or pass it to the mutipass router to compete it automaticay Most currenty avaiabe routers eave him no aternative as they have ony a singe-pass mode MULTI-PASS MODE - LEAST INVALID ROUTING The router described here has a muti-pass mode of use This wii continue to attempt to route a board design unti 100% competion is attained then it wi tidy the routing produced reducing the tota ength of tracks number of via hoes and number of track segments and generay finishing with a truy manufacturabe board Obstace Data Structure -the obstace data structure used is a 4-dimensiona binary tree based on one proposed in Ref3 This was described as: 1 trees are defined by the foowing properties: The tree consists of nodes and eaves Each node has exacty two successors the eft son and the right son which may be nodes (subtrees) or eaves 0 A objects segments the router has to dea with (wirevias obstaces ) are stored as rectanges in the eaves 0 Each node contains the circumscribed rectange of the corresponding subtree Each node defines a partitioning of the corresponding subtree according to one of the four coordinate directions 0 For a rectanges R in the right subtree and for a rectanges L in the eft subtree the inequaities R [D] < Z L [D] >=Z hod where D is one of the four directions and Z is the partitioning vaue The coordinate Z is the same as the D component of the circumscribed rectange in the right son and the partitioning direction D is west for the root of the tree south for nodes on the first eve and so on (repeating the directions cycicay) There is a fundamenta difference between the two modes of use that shoud be expained in more detai here The singe-pass mode does not route connections for which it cannot find a vaid path This is common to most automatic routers The muti-pass router however aways finds a path for each connection on each pass; if it cannot find a vaid path it wi return an invaid path that is a path which impinges upon other nets For exampe the singepass router wi route 90% of the connections of a given board with 10% missing; the muti-pass router wi route 85% of the connections vaidv and the remaining 15% with minimum crossovers or spacing vioations Hence the singe-pass mode is simper than the first routing pass of the muti-pass mode and may even produce an apparenty higher competion rate The muti-pass mode uses the resut of the previous pass as the starting point for its next pass It is beieved that east invaid routing is an important key to improving the routing success on The structure used differs sighty from that proposed in that a singe tree is used to hod obstaces on a ayers this simpifies the searching for via sites on muti-ayer designs Additionay the connectivity information is hed in the tree by ringing a items on the same net and on separate rings a items on the same routed subnets This is done so that a the unfixed items on a singe net can easiy be identified for deetion or for marking as targets a non-source items This structure has the advantage that its size is dependent upon the number of obstaces it hods and not the size of individua obstaces or more particuary any defined grid size Activity array - this array is simiar in use to the frontier ist of a cassica food router It contains detais of each edge before and after expansion incuding costs and cost points back pointers and pointers to expansion areas in the obstace data structure 511

4 OVERALL METHOD The singe-pass mode is impemented as a singe pass of the router with east invaid routing disabed The foowing method describes the Imutipass router The overa program structure can be written as: WHILE the number of invaid routes is greater than zero: Order the nets FOR each net in turn: END END END Remove any unfixed track segments and vias from the net and the obstace array WHILE the net is not fuy routed: Seect a routed subnet and mark remaining subnets as targets Route a singe connection (see next section) Add the route found to the obstace data structure The router has two stages; mutipe routing passes and smoothing passes For a passes after the first the nets are ordered by decreasing tota net costs on the previous pass Routing passes terminate when a nets have been routed without conficts or after a user defined maximum number of passes If the routing was competed a user defined number of smoothing passes is done Each pass invoves a ripup and re-route of each net in turn whether it was routed vaidy or invaidy on the previous pass ROUTE A SINGLE CONNECTION 1 Set the current owest route cost to an impossiby high vaue Add each item of the source routed subnet to the activity array as edges to be expanded with zero costs and expansion directions set to a1 directions 2 Expand the next unexpanded item in the activity array with the owest cost Expansion consists of moving one side of the rectange defining the edge in the expansion direction unti an obstace is met If more than one direction is impied the order in which this expansion takes pace is dependent upon the ayer concerned and the size of the edge rectang1e The resut of an expansion is a free expansion rectange about or away from the defined edge This expansion rectange is added to the obstace array as an expansion area If the edge was a bbocked edge it is expanded by finding the free eements of its opposite side and aso the end edges where these are made wide enough if possibe for the current track width and associated spacings This aows a route to be found either crossing a previous route segment or foowing its path These new edges are added to the activity array with costs associated with routing with conficts 3 The expansion area represents an area within which route segments can be paced At this stage no decisions are made as to actua positions for route segments Provided that the area is arger than the current track width and associated spacing in both directions the definition of track segments can be deferred unti the backtrack phase The expansion rectange is then examined for escape edges that is edges from which it can be exited These are free edges target edges and bocked edges around its perimeter and target via sites and via sites within it If vias are aowed in the design the expansion area is examined for possibe via sites that coud be paced to reach target track segments and for possibe via sites to reach other ayers Via sites <are found as via site rectanges which are arge enough to contain ega vias pus their associated spacings Target via site rectanges are constrained such that a via paced anywhere within them wi contact the target track segment The method used is oosey based on a method for finding maximum empty rectanges in a given area given by Ref4 During smoothing passes or for singe-pass routing bocked edges are treated as fixed and hence no routing with conficts is aowed 4 Any target edges are considered for addition to the activity array If the ength of the edge is too short the adjacent areas are examined to determine if it is possibe to reach the edge egay The cost from the previous cost po:int is computed and the cumuative cost found If this cost is ess than or equa to the current owest route cost the item is added and the current owest route cost updated 5 Any free edges are next considered for addition to the activity array For addition the cumuative cost of reaching them pus the notiona cost of reaching the nearest target must be ess than or equa to the current owest route cost and the ength of the edge must be sufficient for the current track width and associated spacing 6 Bocked edges are treated in a simiar manner to free edges except that the computed cost incudes cost penaties for using them These penaties are increased if the bocked edge or part of a merged edge is on a net aready routed this pass rather than on a net remaining from the previous pass 7 Via site rectanges are added to the activity array as edges on a the ayers that they can reach if the cost of reaching them using them pus the notiona cost of reaching the nearest target is ess than or equa to the current owest route cost 8 If there are any edges in the activity array that have a cost ess than the current owest route cost that have not yet been expanded route searching continues from 2 above Otherwise if the current owest route cost is equa to its initia impossiby high vaue no route has been found This coud happen if it is impossibe to find a path because of intervening fixed obstaces For the norma case the current owest route cost is ower than its initia high vaue and a route has been found in terms of abutting or intersecting expansion rectanges and associated escape edges 512

5 9 A backtrack routine is then used to define the actua path from the owest cost target reached through the expansion areas and edges During the routing passes this backtrack process contour foows the perimeters of the expansion areas where possibe in order to maintain the argest free spaces for future route paths The contour foowing keeps the track segments and vias formed one space away from the perimeter of the expansion rectanges During smoothing passes the backtracking is atered to attempt to continue in one direction for as ong as possibe to reduce the number of segments and to aim for the centre of the expansion areas to even out the spacing between tracks The cost parameters are aso adjusted to reduce the number of vias COST FUNCTIONS As the router is cost-driven the definition of and the baance between the various costs used affects the routing success very consideraby The costs currenty used by the router at any given point are a compex function of: Length in the preferred direction for each ayer from the source to the current point Length in the non-preferred direction for each ayer from the source to the current point Via usage Conficts with other nets not yet routed this pass Confict with other nets aheady aid down this pass Notiona cost of reaching the nearest target Competion rate on the previous pass Pass number Some of the above items are reativey straightforward inear functions others are non-inear graphica functions GRIDDING In practice there are severa reasons for constraining the route segments and vias produced to be on grids NC equipment eg photopotters and driing machines used to produce the printed circuit board may be ess accurate than the printed circuit design coordinate resoution Typicay these have accuracies in the order of 01 to 10 mi Manua designers prefer a coarser grid for manipuating route segments on the graphics screen A vaue of 5 mi is considered reasonabe To be compatibe with earier products that have a coarse coordinate resoution typicay 25 mi 4 With muti-ayer board designs in particuar a via grid is often a manufacturing requirement To satisfy these requirements both a route segment grid and a via grid have been incorporated into the router These are user-definabe so that for earier products a coarse grid can be used for new products a much finer grid can be chosen to meet the needs of the photopotter ony for exampe Using a coarse grid has the effect of increasing the run times and/or reducing the competion rates because it is imposing restrictions upon an essentiay free system and does not have the norma advantage of reducing the data structure size requirement IMPLEMENTATION A prototype progam was impemented on a VAX 1 /780 computer running under UNIX and written in C This version was used for eary deveopment of the method and to do initia evauation of cost functions Subsequent deveopment of the production version is argey independent of the hardware and is currenty avaiabe on VAX and APOLLO computers RESULTS Figure 4 shows the routing of a sma conventiona 2- ayer digita board which aows two signa tracks between DIL pads The main characteristics are: Board area 1725 square inches 285 connections 96 nets 20 mi grid Component density 047 square inches/ equivaent 14 pin DIL Running on a VAX 11/750 the singe-pass mode routed 968% of the connections in 13 minutes which compared we with our own VAX-based routers For the muti-pass mode the resuts were: % Vaid No of No of Time Pass Routing Vias Segments Length (Mins) Smoothing Figure 5 shows part of a arger design with mainy surface-mounted components routed on a 125 mi grid to 100% competion CONCLUSIONS The data structure deveoped for this router is amenabe for other purposes than the routing 513

6 method outined above It has been used for other routers in particuar a pattern router for memory area routing Without much ateration it is aso very suitabe for dimensiona checking of a competed ayout The cost functions for the current router contain many variabes and further deveopment of them is expected to pay dividends in competion rates and run times Some specia purpose hardware is under consideration to speed up the operation of the router This invoves the use of parae processing of the obstace data structure and computation of target costs REFERENCES 1 Lee C An Agorithm for Path Connections and its Appications IRE Transactions on Eectronic Computers September 1961 pp Damm E and Gethoffer H Hardware Support For Automatic Routing Proceedings of the 19th Design Automation Conference 1962 pp Lauther U A Data Structure For Gridess Routing Proceedings of the 17th Design Automation Conference 1980 pp Jayakumar V A Data Structure For Interactive Pacement of Rectanguar Objects Proceedings of the 17th Design Automation Conference 1980 pp October 1984 ** re p-:** $ 10 b! II qi 0 T--J I I i 1 T 7 t a I r7 a* 11 - e e? * a - J FIG 4 514

7 FIG 5 I *