Closed Loop Control of Atmospheres for Soldering

Well designed stable reflow ovens and wave machines can be operated at low rates of the inerting gas. One problem is that at the lowest flow rate settings changes in room draughts and in external venting conditions can affect the internal oxygen levels unless the flow rates are altered. The internal oxygen levels can drift or suddenly alter. This generally means that the ovens are operated at higher flows than necessary in order to ensure steady operations below some desired oxygen level without excessive operator intervention. A closed‐loop control system using the output of internal oxygen meters to vary the flow rates set by mass flow meters is one way to ensure steady operation. One such system — NITRAPILOT™ control system—has been developed by BOC Gases as a stand alone system that can be added to existing equipment. The results of small‐scale and production level testing on this system are described in the present paper. Reduced flow rates can be achieved for stable set point oxygen levels at critical locations. An equally significant benefit, particularly when used in conjunction with data logging, is that the system assists in statistical process control since it ensures, monitors and logs compliance with process recipes without operator intervention.