Benefits of closed loop temperature control in reflow soldering of circuit boards

A closed loop control in heating / oven applications refers to a process being monitored and feed back to a controller by way of a temperature sensor.  That temperature data allows the controller to adjust power at the heating source to execute a predetermined thermal profile.  In assembly of printed circuit boards (PCB), the soldering machine used the most is a reflow oven.   They are available in a variety of sizes and types, ranging from small benchtop units to large scale production ovens with a conveyor drive system spanning 20ft or more in length.  The most common and widely used solder reflow ovens for electronic assembly implement forced air or nitrogen convection as the heating source, and use a closed loop temperature control.

The traditional setup involves an operator selecting a reflow profile from the software screen. A profile is a set of temperature and time settings for the soldering machine.  It is derived based on specific attributes of the alloy used as well as the size and mass of the circuit boards being processed.  Once selected, the reflow oven must heat up to get internal temperature stabilized before use.  As it is heating up, a thermal sensor within the heat chamber or zones, monitors the air temperature and sends feedback to the controller.   Controller in turn makes adjustments to power output of the heat source to maintain set temperature values.  This is the standard closed loop control in most reflow ovens and is based on feedback of air temperature within the zones.  However, for limited production or prototype assembly run, there is a better and more efficient method of profile control.

A closed loop temperature control as measured directly on printed circuit board rather than the air around it is a more precise approach to soldering.  Rather than have an engineer spend time developing a functional reflow profile by tracking board temperatures and make adjustments to oven settings, they can simply input desired circuit board profile parameters and have the software control automatically adjust the heater to yield a soldering profile.   This type of closed loop control utilizes the circuit board as the measurement source and air to board temperature adjustments are automatically handled by the software on the back end.   Figure 1 is a software screenshot from PRO 1600 reflow oven soldering when configured for circuit board surface temperature control.  The blue line on the temperature (Celsius) Vs. Time (seconds) graph is the control of the profile based on the surface temperature of a circuit board.  The red line is internal air or nitrogen gas temperature that is self adjusted on the back end by oven’s control algorithm.


Reflow Oven Profile

               Figure 1: Closed loop reflow profile control based on circuit board surface



The benefits to this method of profile control include precise and direct feedback from the circuit board, no longer is there a need to compensate for thermal mass of the boards being processed, and saving a considerable amount of time by bypassing the traditional reflow oven profiling process.   As great as this method is for soldering, it must be noted that its is best used for high value prototype or low volume assembly applications.  It does require physically attaching a thermocouple to the surface of the PCB being processed or to at least one of the boards in the same batch.  Also, this method is ideal if limited quantities are processed in a batch reflow oven and cannot be applied to a conveyor type oven for large volume applications.  For production reflow soldering, the traditional profiling method with closed loop air temperature control is still preferred in both batch and inline reflow ovens.

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