The Circuit Designer's Companion - diagramas.diagram...

Printed circuits 47which they can work, and so that some uniformity of purchasing policy is possible. Ithas the advantage that the production and servicing departments are faced with areasonably consistent series of designs emanating from the design department, so thatinvestment in production equipment and training is efficiently used. At the same time,the rules should be reviewed regularly to make sure that they don’t unnecessarilyrestrict design freedom in the light of board production technology advances. Anycompany, for instance, whose design rules still specify minimum track widths of0.3mm is not going to achieve the best available packing densities. The design rulesshould not be enforced so rigorously that they actually prevent the optimum design ofa new product.BS6221 Part 3, “Guide for the design and use of printed wiring boards”, presents agood overview of recommended design practices in pc layout and can be used to formthe basis of in-house design rules.Factors in board design which should be considered in the design rules are• track width and spacing• hole and pad diameter• track routing• ground distribution• solder mask, component identification and surface finish• terminations and connections.Typical capabilities for PCB producers at the time of writing are shown in Table2.2. Having decided on a particular supplier, the first thing you should be clear about istheir capabilities in these key areas.Table 2.2 Typical best capabilities of PCB manufacturersBoard thickness range0.35–3.5mmMaximum number of layers 14–24Minimum track and gap width (1-oz copper)Minimum PTH hole diameterMaximum PTH hole aspect ratioRegistration: drill to padRegistration: layer to layer, incl. solder mask0.1mm possible, 0.15mm preferred0.2mm possible, 0.3mm preferred12:1 possible, 6:1 preferred0.03mm0.075mm2.2.1 Track width and spacingThe minimum width and spacing determine to a large extent the achievable packingdensity of a board layout. Minimum width for tracks that do not carry large currents isset by the controllability of the etching process and layer registration for multilayers,which may vary between board manufacturers and will also, at the margins, affect thecost of the board. You will need to check with your board supplier what minimum widthhe is capable of processing. Table 2.2 above suggests typical capabilities in this respectat the time of writing, for 1-oz thick copper; thicker copper will demand greater widthsbecause of undercutting in the etching. The narrowest widths may be acceptable inisolated instances, such as between IC pads, but are harder to maintain over longdistances. Figure 2.5 shows the trade-off in track width and spacing versus number oftracks that can be squeezed between IC pads.