It is easy to think that the most important part of the reflow curve is the maximum temperature of 255°C (480°F). It is difficult to build an oven that is capable of reaching those temperatures in such a short duration. However, another important number is the rate-of-rise of temperature at the higher temperatures. At those temperatures it becomes increasingly difficult to achieve the required rate-of-rise without running the heating elements at full blast. You run the risk of damaging sensitive components if the reflow takes too long, and you can create hot spots (damaged components) and cold spots (no solder reflow) if the elements glow red hot.
There are a lot of other problems that can occur if the reflow profile isn't followed. Components can be damaged from thermal shock or cracking, solder paste can splatter which can lead to voiding in array type packages, and the solder paste could oxidate or the flux not fully activate. If the maximum allowable temperature of IC's is exceeded, it can damage the internal dies of components and foster intermetallic growth leading to shortened MTBF (mean time before failure) numbers. The time-above-liquidus and cooling rate also affect the quality of the resulting solder joint.
These ovens have infrared elements located directly above the boards. There are no heating elements below the boards.
You can build an oven with a single ON/OFF relay controlling power to the oven, but there are drawbacks:
Here all the heat comes from the bottom, which is great for keeping heat away from sensitive components. Unfortunately:
The Controleo3 controller and the oven built in the build guide should be considered top-of-the-line. A lot of engineering and experience has gone into making the Controleo3 reflow oven, and there are currently more than 1500 customers world-wide using either ControLeo, ControLeo2 and the latest Controleo3.
If you are only doing small boards and using leaded solder paste you can get away with a less-capable controller and poorly insulated oven. But how much is your time worth? What is the value of the components you plan to use? What is the turn-around time to order new boards or components when they get heat-damaged? Do yourself a favor and build yourself a great reflow oven. Do it once, and do it right!