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90 SMT Magazine • August 2015 Conduction With a hot plate (a direct contact device), it is nearly impossible to achieve three discrete stages with any degree of accuracy, so this tech- nique is recommended only for the hobbyist who makes a couple boards a day and isn't par- ticular about the thermal profile. A dual-stage hot plate can do a little better, because with two digital temperature controls, the user can pre- set temperatures and manually move the board from one plate to another for specific times to accomplish each stage. However, no hot plate is really a technically profes - sional method. • Pros: Price • Cons: Low volume; no profiling ability, thus no reasonable quality control Convection Single zone batch ov- ens can improve quality and speed by virtue of a built-in multi-stage tem- perature controller. This system is appropriate for the serious hobbyist, bud- get-conscious customer, or test lab. It works by pre- setting the temperatures for each stage, placing the board in the oven, closing the lid, and removing it when complete. Adding a single or multi-device shuttle helps to stage one board for loading as soon as the prior board is done and eliminating the need to open the lid. Obviously, this technique is limited by speed but also by size, since batch ovens typi- cally handle boards only up to 8" x 8". • Pros: Inexpensive; fair controllability for lead and lead-free profiles; small foot- print (tabletop); low power consumption • Cons: Max board size 8"x8"; limited profiling capability due to single heat zone construction; low volume Multi-zone Ovens Conveyorized multi-zone ovens, starting with three zones to achieve each stage of pre- heat, soak and reflow, are the best solution for any type of production volume. They provide complete control of each stage for temperature and timing. Moving via a constant speed con- veyor, each board spends just the right amount of time in each stage based on the zone length (or alternately, the number of zones), to achieve the perfect ramp temperature rate for the mate- rial profile, board density and geometry. figure 1: Dual-stage hot plate. figure 2: Multi-zone oven with convection heating. smt QuICk tIPs SELECTING A REFLOW OvEN, PART 1 continues