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Hot air welding allows the welding of surface components which are very small or whose legs are inaccessible with a soldering iron.
Check out my Youtube video : Soudure à air chaud - Reflow SMD
The first element to take into account is the choice of the tool for heating the dough. It is possible to do this type of maneuver with a hot air gun or a reflow oven. The advantage with the hot air gun is that it reaches its temperatures quickly, but the disadvantage is that because it projects air, it is necessary to be careful that the components do not Take off the circuit. In the case of the reflow furnace, the advantage is that there is no air flow such as a hot air gun and that several components can be heated at the same time. On the other hand, it has the disadvantage of having a large space to heat so that it reaches its temperatures more slowly.
The second element to consider is the type of tin in paste. There are several kinds and depending on the choice of tin, the temperature profile will vary. In my case, I took tin paste Sn63Pb37. When there is lead in the composition of the alloy, the melting temperature is lower which makes the welding easier to perform. On the other hand, if you want to comply with the ROHS (Lead Free Alloy) standards, the welding will do well too, but it will take more heat to reach the melting point of lead-free tin.
The first step of the process is to investigate and analyze the temperature profile of the alloy used and the components used.
The second step is to extend the alloy on the pads of the printed circuit. There are several different techniques to perform this step. For example, some will glue the component with tape and then apply the alloy to the tabs of the components. For my part, I prefer to put the alloy directly on the pads of the printed circuit to then deposit my component on it. In both cases, care must be taken to align the component with the circuit.
The third step in the process is to preheat the printed circuit board and the components. This step will allow not to make cold junction. The printed circuit will be of the same temperature as the component and the melting of the alloy will be easier.
The fourth step is to raise the temperature of the reflow oven or the hot air gun to the maximum temperature that the component can withstand or at the melting temperature of the alloy (which comes first). This step will allow the melting of the alloy. On the other hand, it is not necessary to stay too long at this temperature since this can damage the component or even break it.
Finally, the last step is to cool the printed circuit. In fact, the circuit should not be cooled too quickly as this could damage the component. With the hot air rifle, simply do not send air directly to the circuit and allow it to cool to room temperature. For the reflow oven, simply turn the oven off and open the door to gradually lower the temperature.
It is always recommended to check the circuit contacts visually and with a multimeter in continuous mode. Make sure there are no tin bridges between two legs and that each tab is well welded.