You can easily solder a board with leaded components if you have a hand held soldering iron. Another method of soldering several leaded components in a short time is to pass the board through a wave soldering machine. However, component manufacturers are moving away from leaded components to making more of leadless components, and the soldering technology has had to follow through.
Soldering leadless components or surface mount components requires a different technique than solder wire and soldering iron. Usually, this needs a reflow oven. The process of soldering surface mount components involves applying solder paste to the pads on the board, carefully placing the components in their designated places on the solder paste, and passing the mounted boards through a reflow soldering machine.
Using surface mount technology has its own advantages over the use of through-hole components. Apart from the several electrical, mechanical, cost, and size benefits of using surface mount components, the use of a reflow machine for soldering the whole board within about three minutes is enough reason for switching over totally to surface mount technology.
Selecting a bench-top reflow soldering machine requires looking at different aspects. A practical wide drawer type design makes it easy to load and unload boards. A large window on the side should allow you to see inside the machine when soldering. Modern reflow machines usually feature digital controls along with a touch controlled display panel. Some even offer a USB port allowing you to connect to a computer for a more detailed control.
Reflow soldering machines are available in different sizes, the larger ones allowing a large enough surface to solder several boards at a time. However, larger surfaces are only useful if the machine can distribute the heat evenly over it, since you would like to have all the boards soldered properly, and not just the ones in the middle. This is true for large boards also, and an even heat distribution helps to solder all the parts in the same way.
For evenly distributing the heat, reflow soldering machines use full-width quartz infrared lamps, followed up with an air circulation system. They are set up in a special way to enable a minimum temperature difference over the soldering area. Most manufacturers of soldering machines include PCB holders with brackets that do not influence board heating.
An important factor influencing soldering of surface mount components is the thermal profile. As the board passes through different regions of the reflow machine, it must gradually heat up to the proper temperature to enable soldering, and subsequently, cool down at a defined rate. As the board remains within the machine for only a definite time, the temperature variation over time defines its thermal profile.
Most modern reflow soldering machines are computer controlled and allow creation of elaborate thermal profiles by adjusting the speed at which the board traverses the entire length of the machine and the temperatures of different zones during its journey. Users can define up to three preheating zones, a reflow peak, and a cooling down phase. You can also store a few profiles so the machine can be operated in a stand-alone manner.