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SMT-Mar2016

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14 SMT Magazine • March 2016 The way we measure and control this is by the time vs. temperature profile of the solder joints, PCB, and most importantly, the components. In other words, the oven needs to produce the correct profile. We can even think of the reflow oven as a profile-producing machine. A reflow oven has literally billions of alter- native recipes. Each different set of zone tem- peratures and conveyor speeds will produce a unique profile on a given assembly. It is impera- tive that the correct recipe is selected to estab- lish the acceptable profile for all the different PCB assemblies. Therein lies the challenge. It may take an oven 10, 20, 30 or even 40 minutes to stabilize on a new set of temperatures, espe- cially if the new recipe is cooler than the previ- ous. (It takes longer to cool down an oven than to heat it up.) The key to instant or fast oven changeover is intelligent selection of the oven recipe. Clearly, if two different PCB assemblies are very similar and have the same process win- dow, then it will be possible to select a single recipe that will produce an acceptable profile for both assemblies. An experienced process en- gineer can find such a recipe manually. But what if the PCB assemblies are quite dif- ferent in terms of size, thermal mass, and so on? This is a great challenge and it is not time-effi- cient to perform a trial-and-error experiment on billions of alternative oven setups to evaluate the resulting PCB profile for each one in order to find a common recipe. But such a challenge is perfect for data mining using computers. Pre- dictive software, even running on an old XP computer, is fast enough to do an exhaustive search on the recipes, and to select the optimal oven setup that will produce an acceptable pro- file for a wide range of different assemblies. This may seem complex, futuristic, and expensive, but the fact is that several competing technolo- gies have been able to do this for many years at low cost. The technology you choose, the ap- plications you have and the reflow ovens you use will affect your success. For example, if you enjoy a modern oven with 10+ zones and you have a range of PCB assemblies that do not in- clude the extreme low- as well as high-thermal mass, you may find a single recipe that can pro- duce an acceptable profile for all your assem- blies, hence the phrase in the title of this piece: InStant rEflow ovEn ChangEovEr In a world of Short ProduCtIon runS Figure 2: The reflow oven needs to solder the assembly within the tolerances/process window set by the relevant solder paste, components and substrate. And the way to measure and control this is by the time vs. temperature profile of the solder joints, PCB and, most importantly, the components.

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