Guidelines for optimum Oven Selection

Author: Stephen Dow

Revision 1, September, 1998

1. Determine the "Baseline" Heating Interval

Using solder paste data sheets, establish the minimum processing interval measured from the beginning of the heating curve to the point where peak temperature is achieved. Typical heating intervals fall between 2.8 and 3.5 minutes. If you are just getting into SMT reflow processing, an interval of "no more than" 3 minutes is recommended. Component heating/cooling requirements should also be considered at this point. Avoid using processing intervals that are longer than 3.5 minutes. They can be misleading.

2. Determine the "Corrected" Conveyor Speed for Various Oven Models

Measure the actual distance from the beginning of the first heating module (i.e., zone) to the end of the last. Divide this by the solder paste processing interval time in minutes and you will get the correct conveyor speed in inches/minute.

Utilization of the correct conveyor speed for all testing is a critical parameter since heat chamber lengths vary tremendously from vendor-to-vendor. Determining a "corrected" value for these differences ensures that you will equally assess heat transfer efficiency, zone profiling capabilities and true production throughput capabilities.

3. Match Customer’s Loading Interval to the Correct Oven Model

Using a "standard" 1 inch/2.54 cm load-spacing between PCB’s and the "corrected" conveyor speed from steps #1 and #2, determine which machine model is equal to, or above the actual, board-to-board loading (production) requirements of your process. For example, if a 1-inch spacing cannot be maintained, then the machine is too small for the job. Also, if excessive spacing exists then you may wish to evaluate a smaller model or, enjoy the "excess capacity" you will be purchasing.

4. Compare Actual Heating Efficiency of the Selected Oven Models

Based on the initial screening process described in steps 1-3, your final (and most important) evaluation is to compare actual heating uniformity and repeatability of your finalists. This should be done with, and without load. Also, if a nitrogen will be used then each machines’ thermal capabilities should be studied in both air and nitrogen modes. There should be no difference in performance. Finally, evaluate oxygen level stability under load as well as actual nitrogen consumption.

The optimum oven is the one that possesses the greatest overall performance efficiency in the smallest total length.

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