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16 SMT Magazine • March 2016 "instant reflow oven changeover." The fastest oven changeover is no changeover. In practical terms, one method to achieve this is to attach both the largest and lowest thermal mass PCB to the profiler and to run this train simultaneously through the oven. The data mining prediction software will select the correct oven setup only seconds after a com- pleted profile. This is a simulated or predicted result so you are encouraged to verify the ac- curacy of the prediction by running a profile on the PCBs after the oven has stabilized on the new settings. Please note that this method also can be used when individual components on the same assembly have different process win- dows. The data mining software will search for an oven recipe that positions the profile inside every process window for every measured com- ponent on both PCBs. The probability is high that all the other PCBs that fall between the two extremes used for the recipe generation will have a profile in spec as well. For a less capable reflow oven or a wider range of either thermal mass assemblies or pro- cess windows, there likely will not be a single oven setup that can handle everything. In that case, follow the same method described above, but narrow the grouping of PCBs. Users may end up with two recipes (small vs. large PCBs) or three or four groupings. In that case, there will be oven changeover delays, but limited to the few recipes there now are. Even in this case, it may be possible to eliminate or reduce the changeover time. Prediction software or oven setup software allows con- trol over the variables used for searching on the ideal recipe. For example, search on new recipes while allowing conveyor speed but not zone tempera- tures variables to change. De- pending on the applications, it may be possible to get in spec on all or most assemblies sim- ply by changing the conveyor speed. While an oven may take a long time to stabilize on new temperatures, the change in conveyor speed is near instant. To avoid the risk of making the oven the bottle- neck in the throughput, limit the range of al- lowable conveyor speeds to a minimum speed that is faster than the rest of the line. If the temperature must be changed in the oven, production planning should consider that an oven heats up faster than it cools down. It may be possible to start production with the coolest recipe and progressively go hotter throughout the production day. conclusion It is not uncommon for high-mix electron- ics assembly factories to have more downtime due to changeover than actual production time. It is important to take a holistic view on the ef- fect of production changeover for each produc- tion line as well as the entire operation. When drilling down to the individual machines, you may find that the biggest drag on your produc- tivity is the reflow ovens. The use of more ef- fective production planning and scheduling as well as modern data mining software to identify one or a few oven recipes may provide dramat- ic improvements to line changeover time and, consequently, profits. SmT mB allen is a product manager at KIC Thermal. Figure 3: reflow oven setup. InStant rEflow ovEn ChangEovEr In a world of Short ProduCtIon runS