Selecting a Reflow Oven
There are many factors to consider when choosing a reflow oven. The type of production environment is an important factor, meaning the product mix and volume. Is it a very high volume production environment with very little change in board type or size? Or is it a very low volume production environment, with periods of low or no production, and products that vary greatly in size and weight. Are the boards particularly difficult thermally? Do they have very big components and smaller ones? How tight is the thermal process window (the difference between the minimum process temperature and the maximum allowed process temperature)? What is the required throughput of the line?
Reflow Oven Sizing
Reflow ovens are sized primarily based on the required throughput. The number of boards produced per minute in a conveyor based reflow oven is based on the belt speed, and then length of the board plus the between board spacing. The boards per minute can be doubled, tripled or even quadrupled for multi-lane systems (boards processed side by side). The reflow oven belt speed is limited by the process specification which dictates ramp rates, and time above liquidus values.
Typical solder reflow Profile Targets:
- Peak temperature = 240+/-5°C
- TAL/Time above liquidus (221) = 30 to 60 seconds
- Soak (activation) = 150 to 200°C for 0-60 seconds
- Ramps = 2.5°C/second max for heating, -1.5°C/second for cooling
Given the specification above the reflow process would require 104 seconds for ramp, 60 seconds to soak, 16 seconds to ramp to peak, and 60 seconds time above liquidus – this results in 240 seconds in the heated section of the reflow oven (4 minutes).
If the board size is 10 inches long, single lane, with 3 inch spacing and the factory requires 120 per hour (or 2 per minute) the required belt speed would be 26 inches per minute (IPM).
So with the required belt speed of 26 IPM and the process time of 4 minutes the required inches of heat would be 104”. With Reflow oven sizes of 75”, 100”, 125” or 150” heated length the best choice would be at 125” inch reflow oven. If a factory is tight on space you could consider a shorter reflow oven by processing your boards multiple lanes (dual lane or more) and slowing your belt speed, this would allow a shorter reflow oven to produce more boards.
How Many Zones are Needed?
The number of zones required is considerably less important than the overall oven heated length and the thermal control performance of the reflow oven. There is a balance that must be struck between complexity of operation of the oven and ability to fine tune. Reflow oven zone widths of 10 to 14 inches are standard in the industry, and are adequate for the vast majority of reflow process requirements. For lab or pilot production 6 zones are often enough. For production environments 8 zones or more should be considered.
Want to learn more? Contact BTU with any questions or to hear about the latest advancements in reflow ovens, furnaces or SMT technology.