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Understanding Reflow Soldering Oven and How it Works

A solder reflow oven is actually a useful piece of equipment for boards that comes with large quality of surface mount components or in making batches of the surface mount boards.

Reflow soldering in fact can help to assemble quickly high-density PCBs which could cause manual hand soldering challenges for even the experienced engineers and PCB technicians. To start of, a solder paste mixture that has a solder and flux will be applied with a stencil on the fabricated PCB. The components will then be placed in a position and will be held in place with the adhesive of the solder paste. The PCB then will be placed inside the reflow convection oven in order to start its heating process.

The solder reflow process is being based with convection heating, which is similar to home ovens. The temperature of hot air in the oven will follow a thermal profile which then will give the ideal heat and cooling rate on the given solder paste and components. The electronic manufacturers will provide the thermal profile recommendation which serves as an ideal starting point. However, sometimes the thermal profile needs to be modified to get the best possible soldering performance.

Smart or intelligent machines actually demonstrate capabilities in different areas. Reflow ovens are usually the ones that people have relevance the most on the principles.

Reflow ovens are usually thought as the black box because you can’t see it readily or be able to visualize what will happen to the PCB after this disappears inside the oven tunnel and blasting hot air to the boards for the next 8 minutes. The half step to making it transparent would be in making the machine transparent. In a reflow oven, it means a continuous machine that comes with monitoring capabilities in order to provide data for the chosen oven recipe and the actual conveyor speed and zone temperatures and also with the time stamped board and board out, board jam detection and others more. Though this kind of data is useful, it is however insufficient. Smart ovens would need process transparency.

The oven also comes with multiple zones that can be controlled individually for temperature. There are in fact several heating zones followed by cooling zones. The PCB will move through the oven through a conveyor belt, which then is subjected in a controlled time-temperature profile.

Heat sources are usually from ceramic infrared heaters that transfers the heat to the assemblies through radiator. Ovens that use fans to force the heated air to the assemblies are called as infrared convection ovens.

There are some ovens which are designed to reflow PCBs through an oxygen-free atmosphere. Nitrogen is the most common gas that’s used for such purpose. It will help to minimize the oxidation of the surface that will be soldered. The nitrogen reflow oven will take a few minutes in order to reduce the oxygen concentration towards acceptable levels in the chamber. This is why nitrogen ovens usually comes with nitrogen injection at all times that will help decrease the defect rates.

The primary purpose of a reflow oven would be in creating strong solder joints and at the same time adhering on the tolerances that were set by the relevant solder paste, substrate vendors and components. This is to make sure that the oven will produce every assembly in spec, where the PCB thermal profile is measured. For a process transparency, the profile as well as its relationship with the relevant process window should be measured continuously in real-time. In profiling an oven on a periodic basis, the process of the production is running blind and process transparency will be nonexistent.

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