Oxygen Doping or Closed Loop Controlled Nitrogen in Reflow Oven
An inert atmosphere opens the reflow process window. Reflow ovens that have a Nitrogen environment may have better yields because of the lack of oxidation during the process. The wetting properties are improved due to the absence of Oxygen. The drawback of a low Oxygen level is that there is a potential risk for tombstoning. Due to the surface tension this defect is more likely to happen in a Nitrogen oven than in air. To overcome this risk a controlled Oxygen level of 500 – 1000 PPM is preferred. It is not the objective of this paper to define the most efficient Oxygen level, but rather, the method to keep this level consistent during different process conditions.
Two concepts are used in the market to keep the Oxygen level in the reflow process constant. One method purges Oxygen into a Nitrogen environment. Depending on the Oxygen PPM level, more or less Oxygen will be doped into the system. The second concept controls the Nitrogen supply to maintain the Oxygen level required. If the Oxygen level goes up, more Nitrogen will be purged into the oven. When there are no boards entering the oven the Nitrogen supply can be reduced and still maintain the Oxygen level. Pro and cons of both concepts are compared.