Today’s convection ovens are equipped with several key control elements - temperature, belt speed, and the atmosphere.  The more controls an oven can offer, the more ability the process engineer has to adjust for variations in his process, allowing for repeatability and increased reliability leading to improved yields.
To achieve the most effective and reproducible thermal transfer process, look to today’s forced convection ovens for applications such as flip chip, BGA and lead-free soldering.  A forced convection process to maximize thermal uniformity can be best accomplished by employing static pressure generation in what is known as closed loop convection.

Closed loop convection is defined as a system providing continuous feedback and measurement of convection rates under control with adjustments being made through the operation software.  Static pressure set points are input to the software and static pressure is continuously monitored via pressure transducers.  Pressure is continuously maintained by utilizing the oven controller and blower frequency controllers.  This provides precise process control to maintain consistent heating and cooling transfer rates.  It ensures repeatability and facilitates ease of recipe and process transfer, so profiles developed in one location can be transferred to a processing facility nearby, in another country or on another continent, and not be influenced by variation in temperature, voltage, or even altitude.

The closed loop convection control option found on the Pyramax 98 Reflow Oven from BTU, monitors convection rates and applies the signal to frequency controllers, which vary the blower speed of designated zones.  This type of additional control, which is applicable to both heating and cooling zones of the oven, is the most accurate method available for controlling the convection rate, and affords the widest range of control.  This is particularly true when it comes to lead free solders requiring higher thermal transfer rates and the processing of outsized boards providing little head room.

The key to closed loop convection systems is the constant sampling of static pressure generation.  Static pressure developed within the blower plenum produces true forced convection as gas is distributed from side to side within the oven’s process chamber.  Increased static pressure not only maximizes thermal transfer, it reduces zone-to-zone temperature set points.  This provides the means to control, zone by zone, the oven’s convection rates by increasing or decreasing the rate, as required.  Decreasing the static pressure and then holding it at that pressure is the key behind the control in closed loop convection.  This up and down control factor helps reduce ramp rates for both heating and cooling, eliminates component movement, and improves overall thermal uniformity. 

As parts move through the oven, the process might require a change in the rate of heat transfer or in the quantity of heat.  Sometimes, high- mass components are mixed with low-mass devices on a board.  With closed loop convection, the oven operator can apply more heat from the bottom of the oven than from the top by changing the set point.  A more precise heat transfer lessens chances for defects. 

Static pressure in the reflow is important because it directly relates to the amount of heat transfer an oven can deliver.  The greater the static pressure the greater the heat transfer to the product.  Static pressure with proper plenum design ensures uniform convection delivery from the plenum across the entire width of the product. The direct relationship between static pressure and uniform convection rate ensures repeatability in the reflow process.  Providing the ease of recipe transfer translates into time and cost savings because there is a reduced need to profile the oven.  With adjustments for temperature, altitude and voltage variations under constant control, there is no deviation in results.

Cooling is a very important part of the BTU reflow profile.  While still under reflow, tight uniformity is critical to achieving good results.  The ability to increase or decrease the cooling rate as the process dictates, is vital.  Controlled convection rates need to be maintained at temperatures below 217 C. 

Closed loop convection control provides constant and reproducible heating and cooling thermal transfer rates with the widest range of control.  It is possible to reduce the convection rate to inhibit component movement and to reduce temperature ramp rates. In lead-free applications, closed loop convection employs higher temperatures within a very controlled, tight process window without damaging board components.  It ensures repeatable heating and cooling convection rates while compensating for changes in temperature, voltage and altitude automatically to ensure process repeatability and easy recipe transfer. 

For more information related to this article, or to schedule a demonstration of the BTU Pyramax 98 located at the EMPF, contact Ken Friedman, 610-362-1200 x 279 or via email at  kfriedman@aciusa.org