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70 SMT Magazine • April 2016 So far in this series, we've covered stencil printers, pick and place machines, reflow ovens and multiple types of through-hole soldering. In this and the next couple of chapters, we'll discuss selective soldering machines. It's useful to start by understanding why and when selective soldering is used. There's a misconception that it is a newer, better way to go than wave soldering, and nothing could be further from reality. They are two entirely dif- ferent types of solder techniques for very differ- ent kinds of applications. Whereas wave soldering is used extensively for economical and secure soldering of boards with only through-hole components, selective is used on boards with a mix of surface mount and through-hole populations. Wave is still the fastest, most effective method for process- ing through-hole components because it comes in contact with the whole surface of the board at once; selective tends to be considerably slower because it's soldering individual leads or closely spaced components one joint—or set of joints—at a time. Hybrid boards exist for a number of reasons. In spite of the proliferation of surface mount components which are perfect for high density functions on smaller and smaller footprints, through-hole devices are still preferred for high power applications and those with connectors which require a very strong, stable joint. How Selective Soldering Works Selective soldering acts on the un- derside of the board without affecting anything on the top. A board is posi- tioned in a frame, and all subsequent operations occur automatically accord- ing to the process control system pro- grammed in advance for that board. For each solder point, the machine operator controls the flux, preheat dwell time, and solder position, reducing any side effect of other joints on the board, unlike a wave process, which swipes the entire board without discrimina- by robert voigt DDm NoVAstAr selecting a selective soldering system, Part 1 smT Quick-TiPs Figure 1: Selective soldering leads on a hybrid PCB both through hole and surface mount components.