Vapour Phase Reflow: In this method, the reflow chamber is filled with an inert hydrocarbon vapor. Using IR enhances heat flow as it also heats the ambient air inside the oven. Far-IR avoids the shadowing effect and is not sensitive towards color. Using IR for reflow allows greater control over the amount of penetrating energy exerted, which is very much required for an even temperature rise. The type of emitter used determines the wavelength of the IR used for reflow. 0.72 to 1.5 u is near-IR, 1.5 to 5.6 u is middle-IR and 5.6 to 1000 u is far-IR. Infrared is the electromagnetic radiation located between microwave and visible light. The heat from the emitted radiation is used to melt the solder paste and subsequently used to form solder joint post solidification. The distance between the lamp and the PCBA is 5cm/2in. Infrared Reflow: In this method, the PCBA is passed under an infrared lamp. There are many different types of reflow soldering. To avoid these problems the most common soaking time of 30 seconds to 2.5 minutes exist. As if the temperature change happens too quickly, the components on the PCB develop problems, such as, cold solder joints, charred boards, tombstoning and uneven wetting. It is important to ensure that the temperature gradient is not very steep. The PCB then enters the cooling phase which occurs at 4 degrees Celcius per second. During the reflow process the PCB is taken to a temperature approximately 5 degrees Celcius lower than the lowest temperature failure value of a component placed onto it, but 20-40 degrees Celcius higher than the temperature required to melt the solder and allow “ wetting”, as stipulated in IPC-7530, to take place. The PCB is now primed and the reflow process can begin. The PCBs then stay at the soak temperature for 60 to 120 seconds till all parts of the PCB are at the same temperature. This step is also important to allow any volatile liquid trapped in the solder or the PCB to evaporate. The PCBs undergo linear heating in the preheat phase where the goal is to get the PCBs to soak temperature without any cracking or deforming of the ICs, and the PCB. All four stages have a carefully controlled temperature gradient measured in Celcius per second which is stipulated in IPC-7530 to be around 3 Degrees Celcius/Second.
The reflow soldering process consists of four stages - preheat, soak, reflow and cooling. The PCB is then passed through a reflow oven which is maintained at a particular temperature that results in the formation of solder joints. The paste is sticky, so the components stick to the board. A mixture of solder paste and flux is deposited on the PCB where the component leads need to be connected to the copper traces. Reflow soldering is a soldering process that is used to solder components on a Printed Circuit Board.
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