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August 2015 • SMT Magazine 91 • Pros: Economical production system f or >100 boards/day; will typically accommodate board sizes up to 12"x12"; offers precise control of ramp tempera- tures to handle most common leaded and lead-free soldering profiles; can be reconfigured easily to run different board profiles, an important feature for contract manufacturers • Cons: More expensive than other methods Now back to the original question: Why is it always a good idea to buy as many zones as you can afford? In other words, when would you need more than three zones in an oven? The answer is a function of how much flex- ibility you need in your assembly operation, and that's driven by controllability, volume ef- ficiency, and cost. More zones, along with more length will provide greater flexibility to handle more complex profiles, greater board compo- nent densities, and higher speeds (number of boards processed). As a general rule of thumb, ramp temperatures must not exceed 2° to 4°C per second for proper profiling, so more zone flexibility will provide better control. Larger ovens can also be built with other features such as the ability to handle larger board geometries and special capabilities such as double-sided soldering. For the contract as- sembly operator who handles a wide variety of products, more zones provide the ability to cap- ture more business that can't be done by com- petitors with smaller ovens. Selecting a Reflow Oven Provider When evaluating proposals from reflow oven vendors, try to find references or reviews before you buy. You can often find them on In- ternet search, and while not altogether scientif- ic, you may get some clues regarding construc- tion quality or temperature stability that could influence your decision. Some offshore products may look attractive from a pricing perspective, but may not perform to your standards; plus, aftermarket support can be problematic if you need help. In my next column, we'll address specific heating technologies, conveyors, and software options. SMT figure 3: Discrete zone schematic in conveyorized oven. Robert voigt is vp of global sales at DDM novastar inc. To reach voigt, click here. smt QuICk tIPs SELECTING A REFLOW OvEN, PART 1 continues

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