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

56 The Circuit Designer’s Companionmaterial applied to the board after all copper processing has been completed. Holes areleft in the resist where pads are to be soldered. It serves to prevent the risk of shortcircuits between tracks and pads during soldering and subsequently, and is alsosometimes used as an anti-corrosion coating and to provide a dark, uniform backgroundfor the component identification legend. It can be a screen printed and oven curedepoxy resin, a photographically exposed and developed dry film or a photo-cured liquidfilm.Screen printed resistsScreen printed epoxy resin is a well-established method and is inexpensive, butachievable accuracies are poor compared to modern etching accuracies. Theconsequence of this is that there has to be an allowance for mis-registration and resistbleeding of about 0.3–0.4mm between the edges of pads and the edge of the solderresist pattern. It is easy to generate the artwork for this – simply repeat the pad patternwith oversize pads, and generate a negative photographic image – but it cuts into thespacing between pads and if fine tracks are run between pads they may not becompletely covered by the resist. Figure 2.10 shows this effect. This nullifies thesupposed purpose of the resist, to prevent bridges between pads and adjacent tracks!Also, screen printed resists over large areas of copper that has been finished with tinleadplating may crack when the board is wave soldered, as the plating melts andreflows. This is unsightly but not normally dangerous, as long as you do not rely uponthe resist as the only corrosion barrier.track exposed by poor registrationFigure 2.10 Mis-registration of the solder resistPhoto-imaged filmPhoto film resists are capable of much higher registration accuracy and resolution(typically better than 0.1mm) and are therefore preferred for high-density boards. Theyhave their own problems, apart from expense, the main one being that dry films sufferlack of adhesion to poorly-prepared board surfaces; liquid films have become the usualmethod as a result.A solder resist should not always be regarded as essential (although for dense, wavesoldered surface mount boards, it is). It can be useful in reducing the risk of boardfailure through surface contamination or solder bridges, but is not infallible. There is adanger that it is specified without thought, or used as a crutch to overcome badsoldering practices. A well-designed board in a good production environment may beable to do without it.2.2.7 Terminations and connectionsAny pcb that is part of a system must have connections to it. In the simplest case this isa wire soldered to a pad. If the board is plated-through then this approach is acceptable,as the combined strength of solder in the plated hole plus the pad lands on both sideswill be enough to cope with any normal wire flexing. Wires should not be solderedstraight to non-PTH boards because the wire strains will be taken by the pad-to-board