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72 SMT Magazine • April 2016 tion. All process steps are independently done, one at a time, over the entire board population: 1. All selective areas are fluxed first 2. All areas are then preheated to activate the flux 3. Each joint is soldered one at a time according to the program In addition to a single point, selective can be designed to perform "dips" and "drags." These are closely spaced joints that can be handled by the same soldering operation either by covering multiple points in a single dip, or by swiping a row of joints in a long connector, for instance, by dragging the solder tip across them. pros and Cons of Selective Soldering While there is often no other practical way to handle boards with hybrid components ex- cept manual soldering, selective soldering has the usual set of advantages and disadvantages. Pros: Near-perfect joints, lower labor and rework costs than manual, reduced training and staffing costs for skilled hand labor, excellent joint consistency, high production efficiency Cons: Generally more expensive than wave, although their costs have been coming done re- cently as they've become more popular; speed is relatively slow compared with wave, but con- siderably faster than manual soldering in a pro- duction environment Options with Selective As with all other types of machine solder- ing, there are multiple types of fluxing, preheat- ing and soldering methods, which we'll dig into in later chapters. For now, here are some things to be aware of: Fluxing Technology Two types are available: spray and micro- drop. sElEcTinG a sElEcTivE soldErinG sYsTEm, ParT 1 Figure 2: Selective soldering leads on a through-hole circuit board.

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