If you are looking for professionally made surface mount PCBs, there is no doubt they are soldered in reflow oven.
The author of this project built a reflow oven controller that is easy to build, Android based reflow controller. By putting in a cheap bluetooth module on the controller, he was able to write an app which could control the oven using any bluetooth enabled Android phone or tablet. There is no big difference what type of oven you are going to use, the most important thing is that there were electric heating element and the controller. The single PCB is divided into the high voltage, mains powered section separated from the low power control electronics with cutout slots to take care of creepage issues.
The oven never has to apply 100% power except for a second or two during the ramp-up period. The rest of the time it just gently applies small nudges to the temperature.