Monday, 30 December 2013

Reflow Soldering Oven: Survival Of Assembly Plants

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.

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.

Cooling takes place in the last phase also known as the cooling zone from where the molten solder solidifies into the precise target points permanently fixing the electronic components on to the PCB. The temperature control is still significant as such problems like thermal shock or the excessive metallic formation must be avoided I order to obtain a mechanically sound devices attached with fine grained structured solder.

In the modern ovens with the most up to date technology, there is usually no need for solder to flow more than once as these advances techniques guarantees that the granules in the paste can surpass the reflow temperature of the solder used. The trick is therefore to select an oven that can perform optimally at all the phases resulting into the best possible PCB with attached components.

The business environment around us is rapidly changing particularly in regards to the customers, market, technology and competition hence the need to continuously review or operating methods to be sure they are optimal. The best reflow soldering oven guarantees maximum productivity and profitability for assembly firms and other businesses require soldering.




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