Reflow oven
Encyclopedia
A reflow oven is a machine used primarily for reflow soldering
of surface mount electronic components to printed circuit board
s (PCB).
, 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 infrared 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.
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:
This is also known as condensation soldering.
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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.
Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit). The center of the process window is defined as zero, and the extreme edge of the process window as 99%. 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. 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.
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.
Reflow soldering
Reflow soldering is a process in which a solder paste is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint...
of surface mount electronic components to printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
s (PCB).
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 PCB moves through the oven on a conveyor beltConveyor belt
A conveyor belt consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley...
, and is therefore subjected to a controlled time-temperature profile.
The heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
source is normally from ceramic infrared heaters, which transfers the heat to the assemblies by means of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. Ovens which also use fans to force heated air towards the assemblies (which are usually used in combination with ceramic infrared heaters) are called infrared convection ovens.
Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. Nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
(N2) is a common gas used for this purpose. This minimizes oxidation of the surfaces to be soldered.
Vapour phase oven
The heating of the PCBs is sourced by thermal energy emitted by the phase change of a heat transferHeat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...
liquid condensing on the PCBs. The liquid used is chosen with a desired boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
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. nitrogenNitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
) - No overheating of assemblies. The maximum temperature assemblies can reach is limited by the boiling pointBoiling pointThe boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of the medium.
This is also known as condensation soldering.
Thermal profiling
Process Window Index
Process Window Index is a statistical measure that quantifies the robustness of a manufacturing process which involves heating and cooling, known as a thermal process...
(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.
Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit). The center of the process window is defined as zero, and the extreme edge of the process window as 99%. 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. 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.
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
Thermal shock
Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...
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.