Reflow Soldering is the most widely used method of attaching surface mount components to Printed Circuit Boards (PCBs).
During the pick and place operation, components are placed on the board on the paste deposits. A reliable electrical and physical connection is then made during the reflow soldering process. The solder pastes melt during this process and cool down again to create a good solder joint. The aim of the process is to form solder joints between a PCB and the components on the top without causing damage by overheating.
Reflow soldering Cycle
The soldering cycle including melting and cooling is usually done in a Reflow Soldering Machine also call Reflow Oven where regulated temperatures at different stages also called Reflow Soldering Profile is set on the oven considering the type of solder paste, the number of layers in the PCB, the copper distribution on the board, the number and size of components, etc. Also, the design of the PCB assembly will influence the machine selections.
There are typically four stages for a Reflow Soldering Profile: Preheat, Thermal Soak, Reflow and Cooling.
Preheat: The temperature is raised in the oven, heating the boards and the components evenly. Here the temperature gradient is important because if temperature changes too quickly, it can cause damage to components.
Thermal soak: The flux within particles is activated to reduce oxidization of pads and leads of components and improve solder wetting.
Reflow: The solder pastes melt, and the process reaches its maximum temperature. And the component that has the lowest maximum temperature allowance will define the maximum allowable temperature.
Cooling: The solder alloys solidify to make the solder joints. If the profile is not well optimized, it can lead to poor or non-wetting of the solder joints, damaged components or cold soldering.
In Reflow Soldering, the quality of all items used during the process must be high because anything of a poor quality will lead to undesirable results. And in practice, setting the ideal Reflow Solder Profile for each assembly can be time consuming but is essential to ensure all components are fully soldered without being damaged, so it is worth the extra time and effort in order to result in a repeatable process which will consistently deliver the required results.
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Author: Guohua Wang Audited: By Berlyn Chen