Reflow oven
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A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards.
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[edit] Types of Reflow Ovens
[edit] Infrared and Convection Ovens
The oven contains multiple zones, which can be individually controlled for temperature. Generally there are several heating zones followed by one or more cooling zones. The printed circuit board moves through the oven on a conveyor belt, and is therefore subjected to a controlled time-temperature profile.
The heat source is normally from ceramic infrared heaters, which transfers the heat to the assemblies by means of radiation. Ovens which also use fans to force heated air towards the assemblies (which are usually used in combination with ceramic infra-red heaters) are called Infrared Convection Ovens.
Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. Nitrogen (N2) is a common gas used for this purpose. This minimizes oxidation of the surfaces to be soldered.
[edit] Vapour Phase Oven
The heating of the PCBs is sourced by thermal energy emitted by the phase change of a heat transfer liquid condensing on the PCBs. The liquid used is chosen with a desired boiling point in mind to suit the solder alloy to be reflowed.
Some advantages of vapour phase soldering are:
- High energy efficiency due to the high heat transfer coefficient of vapour phase media
- Soldering is oxygen-free. There is no need for any protective gas (e.g. Nitrogen)
- No overheating of assemblies. The maximum temperature assemblies can reach is limited by the boiling point of the medium.
This is also known as condensation soldering.
[edit] Thermal profiling
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In the electronics manufacturing industry, a statistical measure, known as the Process Window Index (PWI) is used to quantify the robustness of a thermal process. PWI helps measure how well a process "fits" into a user-defined process limit known as the Specification Limit.[1]
Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit).[2] The center of the process window is defined as zero, and the extreme edge of the process window as 99%.[2] A PWI greater than or equal to 100% indicates that the profile does not process the product within specification. A PWI of 99% indicates that the profile processes the product within specification, but runs at the edge of the process window.[2] A PWI of 60% indicates a profile uses 60% of the process specification. By using PWI values, manufacturers can determine how much of the process window a particular thermal profile uses. A lower PWI value indicates a more robust profile.[1][2]
For maximum efficiency, separate PWI values are computed for peak, slope, reflow, and soak processes of a thermal profile. To avoid the possibility of thermal shock affecting the output, the steepest slope in the thermal profile must be determined and leveled. Manufacturers use custom-built software to accurately determine and decrease the steepness of the slope. In addition, the software also automatically recalibrates the PWI values for the peak, slope, reflow, and soak processes. By setting PWI values, engineers can ensure that the reflow soldering work does not overheat or cool too quickly.[1]
[edit] See also
[edit] References and further reading
- ^ a b c Houston, Paul N; Brian J. Louis, Daniel F. Baldwin, Philip Kasmierowicz. "Taking the Pain Out of Pb-free Reflow". Lead-Free Magazine. p. 3. http://www.leadfreemagazine.com/pages/pdf/pain_out_of_reflow.pdf. Retrieved 2008-12-10.
- ^ a b c d "A Method for Quantifying Thermal Profile Performance". KIC Thermal. http://www.kicthermal.com/products/pwi-ds.html. Retrieved 2008-12-10.
- General references
- "T.Bazouni: Reflow Soldering (Reflow Ovens)". http://www.solarfreaks.com/ftopic58_reflow-soldering-.html. Retrieved 2008-04-11.
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