Reflow Soldering

Definition 

Reflow soldering permanently glues components that are first temporarily stuck to their pads on circuit boards through solder paste that will be melted through hot air or other thermal radiation conduction. Reflow soldering is implemented in reflow soldering machine which is called reflow soldering oven.

Soldering Process 

As its definition goes, electrical components are temporarily attached to contact pads prior to real soldering through solder paste. This process primarily contains two steps. First, solder paste is accurately placed on each pad through a solder paste stencil. Then, components are placed on pads by pick and place machine. Real reflow soldering won't start until those preparations have been fully made.

Step one: Pre-heating. It's not a tough job to understand the significance of pre-heating when comparing reflow solder oven with toaster or baking oven. To achieve well-baked bread, oven should be preheated in advance. Pre-heating features two purposes during reflow soldering. First, it allows boards to be assembled to consistently reach required temperature fully compatible with thermal profiling. Second, it is responsible for pushing volatile solvents contained in solder paste to expel. Otherwise, soldering quality will possibly be compromised.

Step two: Thermal Soak. Similar with wave soldering, reflow soldering also depends on flux that has been contained in solder paste. Accordingly, temperature has to climb to the extent that flux can be activated. Otherwise, flux fails to play its active role in soldering process.

Step three: Reflow Soldering. This phase witnesses the advent of peak temperature during the whole process. Peak temperature leads solder paste to be melted and reflowed. Temperature control plays a crucial role in reflow soldering process. Too low a temperature stops solder paste from sufficiently reflowing while too high a temperature may cause damage on SMT components or boards. For example, BGA (ball grid array) package contains lots of solder balls that will be melted during reflow soldering. If soldering temperature doesn't reach optimal level, those balls may be unevenly melted and BGA soldering may suffer from rework or crap.

Step four: Cooling. As a temperature curve is depicted, temperature will go down soon after top temperature is achieved. Cooling leads solder paste to solidify with parts permanently fixed on contact pads on boards.

Application Field 

Reflow soldering can be applied in both SMT and THT assembly but primarily used in the former. When it comes to application of reflow soldering on THT assembly, PIP (pin in paste) is usually relied on. First, solder paste is filled in holes on boards. Then, component pins are plugged into holes with some solder paste coming out at the other side of the board. Finally, reflow soldering is implemented to complete soldering.