Reflow Soldering Oven: The Techniques And Working

By Harriett Crosby

In the assembly industries, the process of attaching electronic components on to a printed circuit board (PCB) is usually done by soldering them onto the board. Reflow soldering oven is one of the most modern devices used for to achieve this attachment. The process starts with a sticky mixture of flux and powdered solder that helps to attach the components on to their correct positions on to the board. A process of controlled heating and cooling then follows to achieve permanent joints.

One or more ceramic infrared heaters can be used for heating the oven. The heat is then directed through the radiation process to the assembly compartments although infrared ovens use fans to direct heat to the assembly. The target is usually to expose PCB to the optimum heat conditions enough to melt the solder into the correct positions without damaging the PCB or devices on it.

In a conventional reflow soldering oven, there are four phases or zone with a unique thermal profile for each. It all starts in the preheat zone where the time/temperature relationship (ramp-rate) is determined. This is the rate at which the temperature changes on the PCB and it is important so that the PCB does not crack or components do not get destroyed. The solvent in the paste starts to evaporate at this phase.

The second phase is the thermal soak zone where the solder paste volatiles are removed. It also involves flux activation(removal of oxide from leads and pads) and the temperatures can range anywhere between 60 to 120 depending on the tolerance levels of mounting board and the components on it.

The third phase is the reflow zone where the maximum possible temperatures are reached. The objective is to reduce the surface tension of the flux at the points of metal juncture which leads to metallurgical bonding involving combination of all the available solder powder. The maximum possible operating temperature is set slightly below the maximum tolerable temperature of that component with the lowest operating temperatures. The oven should therefore be efficient in heat control and monitoring.

The last stage is the cooling zone where the circuit board and its component are cooled. This process too is done under efficient temperature control as sudden temperature changes may result to thermal shock. It is also important to avoid excessive metallic formation at this stage as the desired finished circuit board should have components attached with fined grained structured solder making it mechanically sound.

The modern high tech ovens allows for only one reflow at the third phase as the granules in the paste are made to surpass the temperatures at this phase. For optimality and lower consumption therefore, it is advisable to shop for an efficient oven that put in place the most recent technological advancement.

The changing customer needs, competition, market condition and the general technology all calls for adopting measures that optimizes operating efficiency in terms of yield and profitability. It is such measures that can ensure the survival of a firm into the future. For assembly firms in particular, an efficient and modern reflow soldering oven is more than necessary as it increases the production rate and minimizes on power consumption.

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