Reflow Soldering Oven: Survival Of Assembly Plants

By Harriett Crosby

Assembling electronic components on to printed circuit board (PCB) traditionally depends on soldering them on to that motherboard. In a well functioning reflow soldering oven, the best possible results are usually achieved. The technique is one of the most modern the widely used for this kind of attachments although just like other forms of technologies, it is too undergoing revolution.

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.

A basic reflow oven has four stages through which the operation goes to be complete. The starting point is the preheat zone. This is where the temperature/time rate (ramp rate) is determined. It is the rate at which the temperature of the mounting board and the electrical components on it changes relative to time. This is significant as it helps determine the maximum temperatures possible that can be reached and for how long. The solvent in the mix also starts evaporating at this point.

Thermal soak zone with temperature ranging from 60 to 120 is the next stage for the circuit board. The purpose is the removal of solder paste volatiles and flux activation (oxide reduction from leads and pads). Temperature control at this phase is also very essential. Too high temperature leads to damage to the PCB and the components while too low temperature leads for failure of full oxidation of flux.

Maximum temperatures are reached at the reflow zone (the third stage). At this stage, the surface tension of the flux gets reduced at the metal juncture resulting to metallurgical bonding-all the solder powder combines. In considering the maximum tolerable temperature, the components with the lowest thermal damage is very significant as the maximum operating temperature is set slightly below this level. It is at this phase that full control of the temperatures and exposure time must be done for obvious reasons of avoiding device destruction.

The last phase for the PCB is the cooling zone where the board and its components cool and the soldier solidifies. The temperature control is also significant here to avoid thermal shock excess intermetallic formation. The primary goal here is to achieve a mechanically sound and fine grain structure.

With the modern sophisticated ovens, the most up to date technologies are employed to give the best possible yields. These are operated by modern setup processes that identify the best possible combination of zonal temperatures and conveyor speed. Among other things, the production time has been significantly minimized greatly increasing efficiency.

The high rate of the change in business environment reflected by changing consumer needs, shifting technology, market changes and increasing competition all calls for drastic measures such as investing in highly efficient and optimally performing devices such as a reflow soldering oven which should drive production levels and increase profitability for the survival of the business.

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