Reflow oven: Difference between revisions
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{{Short description|Machine used in circuit board production}} |
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In commercial high-volume use, reflow ovens take the form of a long tunnel containing a [[conveyor belt]] along which PCBs travel. For prototyping or hobbyist use PCBs can be placed in a small oven with a door. |
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[[Image:RSS Components of a Profile1.svg|thumb|300px|Example of reflow soldering [[thermal profile]].]] |
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Commercial conveyorised reflow ovens contain multiple individually heated zones, which can be individually controlled for temperature. PCBs being processed travel through the oven and through each zone at a controlled rate. Technicians adjust the conveyor speed and zone temperatures to achieve a known time and temperature [[thermal profile|profile]]. The profile in use may vary depending on the requirements of the PCBs being processed at the time. |
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==Types of reflow ovens== |
==Types of reflow ovens== |
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=== Infrared and convection ovens === |
=== Infrared and convection ovens === |
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<ref>{{cite web|last=Girouard|first=Roland|title=Mark5 Reflow Oven|url=http://hellerindustries.com/reflow-1826.php|work=Heller Industries Website|publisher=Heller Industries Inc|accessdate=28 September 2012}}</ref> |
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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 belt]], and is therefore subjected to a controlled time-temperature profile. |
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In [[infrared]] reflow ovens, the [[heat]] source is normally ceramic infrared heaters above and below the conveyor, which transfer heat to the PCBs by means of [[radiation]]. |
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[http://www.hellerindustries.com/reflow-1826.php Convection Reflow Oven Detailed Description] |
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Convection ovens heat air in chambers, using that air to transfer heat to the PCBs by means of [[convection]] and [[Thermal conduction|conduction]]. They may be fan assisted to control the airflow within the oven. This indirect heating using air allows more accurate temperature control than directly heating PCBs by infrared radiation, as PCBs and components vary in infrared [[absorptance]]. |
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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''. |
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Ovens may use a combination of infrared radiative heating and convection heating, and would then be known as 'infrared convection' ovens. |
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Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. [[Nitrogen]] (N<sub>2</sub>) is a common gas used for this purpose. This minimizes [[oxidation]] of the surfaces to be soldered. |
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Some ovens are designed to reflow PCBs in an oxygen-free atmosphere. [[Nitrogen]] (N<sub>2</sub>) is a common gas used for this purpose. This minimizes [[oxidation]] of the surfaces to be soldered. The nitrogen reflow oven takes a few minutes to reduce Oxygen concentration to acceptable levels within the chamber. Thus nitrogen ovens typically have nitrogen injection in at all times which decreases defect rates.<ref>{{cite web|last=Girouard|first=Roland|title=Mark5 Reflow Oven|url=http://hellerindustries.com/reflow-1826.php |work=Heller Industries Website|publisher=Heller Industries Inc|accessdate=28 September 2012}}</ref> |
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=== Vapour phase oven === |
=== Vapour phase oven === |
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The heating of the PCBs is sourced by thermal energy emitted by the [[phase transition]] 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. |
The heating of the PCBs is sourced by thermal energy emitted by the [[phase transition]] of a [[heat transfer]] liquid (e. g. [[krytox|PFPE]]) condensing on the PCBs. The liquid used is chosen with a desired [[boiling point]] in mind to suit the solder alloy to be reflowed. |
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Some advantages of vapour phase soldering are: |
Some advantages of vapour phase soldering are: |
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* High energy efficiency due to the high heat transfer coefficient of vapour phase media |
* High [[Efficient energy use|energy efficiency]] due to the high heat transfer coefficient of vapour phase media |
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* Soldering is oxygen-free. There is no need for any protective gas (e.g. [[nitrogen]]) |
* Soldering is oxygen-free. There is no need for any protective gas (e.g. [[nitrogen]]) |
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* No overheating of assemblies. The maximum temperature assemblies can reach is limited by the [[boiling point]] of the medium. |
* No overheating of assemblies. The maximum temperature assemblies can reach is limited by the [[boiling point]] of the medium. |
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This is also known as condensation soldering. |
This is also known as condensation soldering. |
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=== Pressure Curing Ovens === |
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Pressure curing ovens, or Autoclave, is widely utilized to minimize voiding and improve adhesion strength in bonding processes. Pressure cure ovens are typically employed in die attach and underfill applications. Increasing pressure during the curing process removes voids. |
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{{main|Process Window Index}} |
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[[Image:Process Window Index (bullseye).svg|thumb|A graphical representation of the Process Window Index for a thermal profile.]] |
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In a pressure cure process, air is pressurized in a rigid vessel or chamber while heating or cooling with forced convection. Heaters, heat exchangers, and blowers are mounted internal to the pressure vessel, continuously circulating air across the pressure chamber providing consistent heat transfer to the product. Upon completion of the curing process, pressure is relieved and the product cools. |
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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.<ref name="lead">{{cite web|url=http://www.leadfreemagazine.com/pages/pdf/pain_out_of_reflow.pdf |title=Taking the Pain Out of Pb-free Reflow|last=Houston|first=Paul N|coauthors=Brian J. Louis, Daniel F. Baldwin, Philip Kasmierowicz |publisher=Lead-Free Magazine|pages=12|page=3|accessdate=2008-12-10}}</ref> |
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Pressure cure ovens can utilize air or nitrogen as the pressurizing agent. |
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Each thermal profile is ranked on how it "fits" in a process window (the specification or tolerance limit).<ref name="KIC">{{cite web|url=http://www.kicthermal.com/products/pwi-ds.html|title=A Method for Quantifying Thermal Profile Performance|publisher=[http://www.kicthermal.com KIC Thermal]|accessdate=2008-12-10}} {{Dead link|date=September 2010|bot=H3llBot}}</ref> The center of the process window is defined as zero, and the extreme edge of the process window as 99%.<ref name="KIC" /> 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.<ref name="KIC" /> 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.<ref name="lead" /><ref name="KIC" /> |
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=== Vacuum Reflow Ovens === |
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Traditional reflow can be augmented by the addition of a late-stage vacuum chamber. Introducing a vacuum chamber to the reflow process allows voids and bubbles to escape, significantly reducing voiding in solder joints and interfaces. |
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During the vacuum process, the circuit board is stationary, assuring no shifting parts. Smooth travel into and out of the vacuum chamber, minimizing vibration, is critical. |
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Employing heat inside the vacuum chamber allows peak temperatures to be achieved during vacuum, assuring shorter time above liquidous and greater process flexibility. High vacuum chamber temperatures also prevent flux buildup inside the chamber. |
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Advanced vacuum reflow ovens employ multiple conveyor systems allowing higher throughput by optimizing transfer time into the vacuum chamber, dual rail processing, and closed-loop vacuum pumps to prevent solder and flux spatter. |
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=== Formic Acid Reflow Ovens === |
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Formic acid reflow ovens operate similarly to a traditionally reflow oven, with the addition of formic acid vapor injection into the key soak zones for flux-free reflow and vapor soldering. Upon injection, the formic acid removes any oxides present on the metal prior to reflow. |
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Formic acid concentration is maintained by a bubbler system that is monitored in real time to provide stable and consistent formic concentrations to within 0.5% in the process chamber. |
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Formic acid reflow ovens employ sets of double doors at the oven’s entrance and exit to dramatically reduce process gas consumption. During production, only one door opens at a time, thus isolating the process chamber and lowering nitrogen and formic acid consumption. |
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Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. |
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In the electronics manufacturing industry, SPC (statistical process control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements. <ref>{{cite web|url=http://www.ipc.org/TOC/IPC-7530.pdf |title=Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave) |format=PDF |date= |accessdate=2019-07-01}}</ref> |
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<ref>{{cite web|title=Modern thermal profiling device|url=https://www.solderstar.com/en/solderstar-solutions/solutions-reflow/solderstar-pro/|work=Solderstar Website|publisher=Solderstar|accessdate=28 September 2018}}</ref> |
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[[File:Example thermal profiler.png|thumb|Example of a modern thermal profiler]] |
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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.<ref name="lead" /> |
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==See also== |
==See also== |
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{{Reflist}} |
{{Reflist}} |
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===General references=== |
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{{refbegin}} |
{{refbegin}} |
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* {{cite web | title=T.Bazouni: Reflow Soldering | work= | url=http://www.solarfreaks.com/ftopic58_reflow-soldering-.html | accessdate=2008-04-11 |archiveurl= |
* {{cite web | title=T.Bazouni: Reflow Soldering | work= | url=http://www.solarfreaks.com/ftopic58_reflow-soldering-.html | accessdate=2008-04-11 |archiveurl=https://web.archive.org/web/20080618163704/http://www.solarfreaks.com/ftopic58_reflow-soldering-.html |archivedate=2008-06-18}} |
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{{refend}} |
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[[Category: |
[[Category:Printed circuit board manufacturing]] |
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{{Industry-stub}} |
{{Industry-stub}} |
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Latest revision as of 15:05, 18 August 2024
A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards (PCBs).
In commercial high-volume use, reflow ovens take the form of a long tunnel containing a conveyor belt along which PCBs travel. For prototyping or hobbyist use PCBs can be placed in a small oven with a door.
Commercial conveyorised reflow ovens contain multiple individually heated zones, which can be individually controlled for temperature. PCBs being processed travel through the oven and through each zone at a controlled rate. Technicians adjust the conveyor speed and zone temperatures to achieve a known time and temperature profile. The profile in use may vary depending on the requirements of the PCBs being processed at the time.
Types of reflow ovens
[edit]Infrared and convection ovens
[edit]In infrared reflow ovens, the heat source is normally ceramic infrared heaters above and below the conveyor, which transfer heat to the PCBs by means of radiation.
Convection ovens heat air in chambers, using that air to transfer heat to the PCBs by means of convection and conduction. They may be fan assisted to control the airflow within the oven. This indirect heating using air allows more accurate temperature control than directly heating PCBs by infrared radiation, as PCBs and components vary in infrared absorptance.
Ovens may use a combination of infrared radiative heating and convection heating, and would then be known as '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. The nitrogen reflow oven takes a few minutes to reduce Oxygen concentration to acceptable levels within the chamber. Thus nitrogen ovens typically have nitrogen injection in at all times which decreases defect rates.[1]
Vapour phase oven
[edit]The heating of the PCBs is sourced by thermal energy emitted by the phase transition of a heat transfer liquid (e. g. PFPE) 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.
Pressure Curing Ovens
[edit]Pressure curing ovens, or Autoclave, is widely utilized to minimize voiding and improve adhesion strength in bonding processes. Pressure cure ovens are typically employed in die attach and underfill applications. Increasing pressure during the curing process removes voids.
In a pressure cure process, air is pressurized in a rigid vessel or chamber while heating or cooling with forced convection. Heaters, heat exchangers, and blowers are mounted internal to the pressure vessel, continuously circulating air across the pressure chamber providing consistent heat transfer to the product. Upon completion of the curing process, pressure is relieved and the product cools.
Pressure cure ovens can utilize air or nitrogen as the pressurizing agent.
Vacuum Reflow Ovens
[edit]Traditional reflow can be augmented by the addition of a late-stage vacuum chamber. Introducing a vacuum chamber to the reflow process allows voids and bubbles to escape, significantly reducing voiding in solder joints and interfaces.
During the vacuum process, the circuit board is stationary, assuring no shifting parts. Smooth travel into and out of the vacuum chamber, minimizing vibration, is critical.
Employing heat inside the vacuum chamber allows peak temperatures to be achieved during vacuum, assuring shorter time above liquidous and greater process flexibility. High vacuum chamber temperatures also prevent flux buildup inside the chamber.
Advanced vacuum reflow ovens employ multiple conveyor systems allowing higher throughput by optimizing transfer time into the vacuum chamber, dual rail processing, and closed-loop vacuum pumps to prevent solder and flux spatter.
Formic Acid Reflow Ovens
[edit]Formic acid reflow ovens operate similarly to a traditionally reflow oven, with the addition of formic acid vapor injection into the key soak zones for flux-free reflow and vapor soldering. Upon injection, the formic acid removes any oxides present on the metal prior to reflow.
Formic acid concentration is maintained by a bubbler system that is monitored in real time to provide stable and consistent formic concentrations to within 0.5% in the process chamber.
Formic acid reflow ovens employ sets of double doors at the oven’s entrance and exit to dramatically reduce process gas consumption. During production, only one door opens at a time, thus isolating the process chamber and lowering nitrogen and formic acid consumption.
Thermal profiling
[edit]Thermal profiling is the act of measuring several points on a circuit board to determine the thermal excursion it takes through the soldering process. In the electronics manufacturing industry, SPC (statistical process control) helps determine if the process is in control, measured against the reflow parameters defined by the soldering technologies and component requirements. [2] [3]
See also
[edit]References and further reading
[edit]- ^ Girouard, Roland. "Mark5 Reflow Oven". Heller Industries Website. Heller Industries Inc. Retrieved 28 September 2012.
- ^ "Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave)" (PDF). Retrieved 2019-07-01.
- ^ "Modern thermal profiling device". Solderstar Website. Solderstar. Retrieved 28 September 2018.
General references
[edit]- "T.Bazouni: Reflow Soldering". Archived from the original on 2008-06-18. Retrieved 2008-04-11.
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