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

Printed circuits 63soldering machinery which may require a clear area (sometimes called the“stacking edge”) on one or two edges• if the board is to be wave soldered, rows of adjacent pins are best orientedacross the direction of flow, parallel to the wave, to reduce the risk of solderbridges between pins or pads.2.3.3 Component identificationMost pcbs will have a legend, usually yellow or white, screen-printed on the componentside to describe the position of the components. This can be useful if assembly of thefinished unit relies on manual insertion, but its major purpose is for the test and servicedepartments to assist them in seeing how the board relates to its circuit diagram. Withlow-to-medium density boards you can normally find space beside each component toindicate its number, but as boards become more densely packed this becomesincreasingly difficult. Printing a component’s ID underneath it is of no use to theservice engineer, and if component placement is automatic it’s no use to the productiondepartment either. Particularly if the board consists almost entirely of small-outline orDIL ICs, these can be identified by a grid reference system with the grid co-ordinatesincluded on the component side track artwork. Therefore you should consider whetherthe extra expense of printing the legend onto a high-density board is justified.Assuming it is, there are a few points to bear in mind when creating the legendmaster. If possible, print onto flat surfaces, not over edges of tracks or pads; the unevensurface tends to blur the print quality. Never print ink over or near a hole (allowing fortolerances). Even if it’s not a solderable hole, the unprinted ink will build up on thescreen and after several passes will leave a blot, ruining the readability. If there are largenumbers of vias, these should be filled in or “tented” to prevent the holes remaining andrendering areas of the board useless for legend printing.Polarity indicationThere are several ways of indicating component polarity on the legend. Really the onlycriterion for these should be legibility with the component in place, so that they are asuseful to inspectors and test engineers as they are to the assembly operators. Once aparticular method has gained acceptance, it should be adhered to; using differentpolarity indicators on different boards (or even on the same board) is a sure way ofconfusing production staff and gaining faulty boards.2.4 Surface protectionThe insulation resistance between adjacent conductors on a bare pcb surface dependson the conductor configuration, the surface resistance of the base material, processingof the board and environmental conditions, particularly temperature, humidity andcontamination. For a new board with no surface contamination the expected insulationresistance between two parallel conductors can be derived fromR i = 160 · R m · (w/l)whereR m is the material’s surface resistance specification at a given temperature(see Table 2.1 on page 41)w is the track spacingl is the length of the parallel conductors