Reflow Soldering: How Thermal Profiles Form SMT Solder Joints
Reflow soldering is the thermal process used to transform printed solder paste into permanent solder joints between surface-mount components and PCB pads.
After solder paste application and component placement, the populated PCB is heated according to a controlled temperature-time profile. The solder alloy passes through several thermal stages before melting, wetting the component terminations and PCB pads, and finally cooling into solid solder joints.
In Brief
Reflow soldering is not simply a matter of heating a PCB to one temperature. The board follows a thermal profile that controls how quickly it heats, how long different temperature regions are maintained, how the solder passes through its liquidus temperature, and how the assembly cools afterwards.
The appropriate profile depends on the solder paste, PCB design, component population, thermal mass and the recommendations applicable to the materials and components being assembled.
What Is Reflow Soldering?
Before reflow, solder paste is deposited onto PCB pads and surface-mount components are placed into those deposits.
The populated PCB then enters a controlled heating process. As the board temperature increases, the solder paste changes physically and chemically until the solder alloy melts and wets the metallic surfaces of the PCB pads and component terminations.
During subsequent cooling, the molten solder solidifies and forms the final electrical and mechanical connections.
What Is a Reflow Profile?
A reflow profile is a representation of assembly temperature versus time during the soldering process.
It describes how the PCB heats and cools throughout the reflow cycle. A programmed oven recipe and the actual temperature experienced by the assembly are related, but they are not necessarily identical.
Board dimensions, copper distribution, component size and other thermal characteristics influence how the real PCB responds to the heat delivered by the oven.
Oven Temperature Is Not the Same as PCB Temperature
The programmed oven settings describe how the heating system is controlled. A thermocouple attached to the PCB measures the temperature actually experienced at a particular location on the assembly.
This distinction is important when developing and validating a reflow process.
The Main Stages of a Reflow Profile
Preheat
The assembly temperature rises in a controlled manner from its initial temperature toward the higher process regions.
Soak
The assembly spends time in an elevated temperature range, allowing different parts of the PCB to approach a more balanced thermal condition.
Reflow
The solder alloy passes through its liquidus region and becomes molten, allowing wetting and solder joint formation.
Peak
The assembly reaches its maximum temperature during the programmed thermal cycle.
Cooling
The board temperature decreases and the molten solder solidifies into the final joints.
Inspection
The completed PCB can be inspected after cooling for solder joint quality and assembly defects.
Why Is the Preheat Stage Important?
The preheat stage raises the temperature of the PCB and its components before the solder reaches the liquidus region.
Heating too aggressively can increase thermal stress across the assembly. At the same time, an excessively slow process can expose components and materials to elevated temperatures longer than necessary.
The required heating behaviour therefore needs to follow the solder paste recommendations and the thermal characteristics of the assembly.
What Happens During the Soak Region?
A PCB is not thermally uniform. Large copper areas, connectors, small chip components and large packages can all heat at different rates.
The soak region can help reduce temperature differences across the assembly before the solder enters the reflow region. It also forms part of the activation behaviour of the solder paste flux system.
The appropriate soak strategy depends on the paste formulation and assembly being processed; it should not be treated as one universal temperature and time specification.
What Is Time Above Liquidus?
The liquidus temperature is the temperature above which the particular solder alloy is fully molten.
Time Above Liquidus, often abbreviated as TAL, describes the amount of time the solder joint area remains above that temperature during reflow.
This interval needs to be sufficient for the soldering process to occur, while avoiding unnecessary thermal exposure to the PCB and components.
What Is Peak Temperature?
Peak temperature is the highest temperature reached by the assembly during the reflow profile.
There is no single correct peak temperature for every PCB. The appropriate value depends on the solder alloy and paste, component limitations, PCB construction and the recommendations supplied by the relevant manufacturers.
Do Not Copy a Generic Profile Blindly
A profile that works for one PCB may not be suitable for another. Changes in board size, copper distribution, component population, solder paste or package type can change the thermal behaviour of the assembly.
Why Does Cooling Matter?
The soldering process does not end when peak temperature has been reached. The assembly must subsequently cool and the molten solder must solidify.
Cooling behaviour is therefore part of the complete thermal profile. As with heating, appropriate cooling conditions depend on the materials, components and solder paste process being used.
Why Use Thermocouples?
A thermocouple allows temperature to be measured directly at a selected location on the PCB during the reflow cycle.
Measuring several points can help reveal differences between thermally light and thermally heavy areas of the same board.
Actual Board Temperature
Measures temperature at the assembly rather than relying only on the programmed oven settings.
Thermal Differences
Multiple measurement points can show whether different areas of the PCB are heating differently.
Profile Development
Measurement data can be used when developing or checking a process for a particular PCB assembly.
Why Can Different Areas of a PCB Heat Differently?
PCB assemblies contain structures with very different thermal masses. A small passive component and a large connector do not necessarily respond to heating at the same rate.
Copper planes, board thickness, component density and component dimensions also influence heat absorption and distribution.
This is one reason why thermal profiling should be performed on the assembly rather than assuming that every point on the board follows the same temperature curve.
Air Reflow and Nitrogen Reflow
Reflow soldering can be performed in normal air or in a controlled atmosphere with reduced oxygen content, commonly using nitrogen.
Reflow in Air
Standard air reflow is widely used and avoids the additional nitrogen supply required for an inert process.
For many PCB prototypes and conventional SMT assemblies, air reflow can provide a practical process when the paste, board finish and profile are suitable.
Nitrogen Reflow
Introducing nitrogen reduces the oxygen concentration around the assembly during heating.
Reduced oxidation can improve wetting conditions and provide a wider process window for some solder pastes, finishes and demanding assemblies.
What Does Nitrogen Change?
At reflow temperatures, oxygen contributes to oxidation of metallic surfaces. Replacing much of the chamber atmosphere with nitrogen reduces the amount of oxygen available for these reactions.
This can improve wetting behaviour and make nitrogen useful for certain fine-pitch, difficult-finish or process-sensitive assemblies. Nitrogen, however, does not automatically make every solder joint better and should not be considered a substitute for correct solder paste printing, component placement and thermal profiling.
Nitrogen Has a Process Cost
Nitrogen reflow requires a suitable gas supply and creates an additional operating cost. Whether it is useful should therefore be evaluated according to the PCB, solder paste, surface finish and process requirements rather than specified automatically for every assembly.
Typical Reflow Problems
Reflow defects are not always caused by the oven alone. Solder paste printing, deposit volume, component placement, PCB design, component termination condition and the thermal profile all interact.
Insufficient Wetting
Can be influenced by surface condition, oxidation, flux activity and the thermal process.
Bridging
Excess paste, component alignment, pad geometry and solder behaviour can contribute to unwanted connections.
Tombstoning
Uneven wetting forces and thermal conditions can contribute to small passive components lifting onto one end.
Solder Beads
Paste volume, component geometry, placement and process conditions can all influence solder bead formation.
Incomplete Reflow
Thermally heavy areas may fail to experience the intended soldering conditions if the profile is not suitable.
Thermal Stress
Excessive temperatures or unsuitable heating and cooling rates can expose the PCB and components to unnecessary stress.
Why One Reflow Profile Does Not Fit Every PCB
Even two PCBs using the same solder paste can have very different thermal behaviour.
A small thin PCB with lightweight components may heat rapidly, while a large board containing heavy connectors and extensive copper planes may require substantially different oven settings to produce an appropriate assembly profile.
Profile development should therefore consider the actual PCB assembly rather than treating the solder paste datasheet profile as an oven program that can simply be copied unchanged.
Batch Reflow for PCB Prototyping
Laboratory and prototype reflow ovens often process the complete PCB in a single chamber rather than moving the board through a long multi-zone conveyor oven.
This format can be well suited to R&D, educational environments and low-volume assembly where compact equipment, flexible recipes and direct thermal profiling are more important than high production throughput.
MIPEC Reflow Oven Options
MIPEC offers several batch reflow systems for different PCB sizes and process requirements.
HR-11
Compact IR and forced-convection reflow oven for PCBs up to 300 × 200 mm. Designed for air reflow with programmable profiles, direct thermocouple measurement and automated cooling.
HR-11-N2
Compact version with both standard air and nitrogen reflow capability for applications where a reduced-oxygen atmosphere is required.
HR-30
Large-format reflow system for PCBs up to 600 × 410 mm, including nitrogen capability for oversized PCB assemblies and applications such as large LED panels.
Where Does Reflow Fit Into PCB Assembly?
Reflow is the final thermal stage of the SMT assembly sequence. Solder paste has already been applied and the surface-mount components have been positioned on the PCB.
During reflow, the solder paste forms the permanent electrical and mechanical joints that connect those components to the board.
Sources & Further Reading
- IPC-7801 – Reflow Oven Process Control Standard: terminology and process guidance for reflow recipes, thermal profiles, ramp rate, soak, time above liquidus, peak temperature and process control.
- Solder paste manufacturer technical documentation: recommended reflow profiles, alloy liquidus temperature, process window and paste-specific thermal requirements.
- Component manufacturer soldering recommendations: applicable peak temperature, thermal exposure and assembly process limitations for individual component packages.
Technical Note
Reflow parameters depend on the solder paste, solder alloy, PCB construction, component population, surface finishes, thermal mass and component temperature limits. The information above describes general reflow principles and should not replace the process recommendations supplied by the solder paste, PCB or component manufacturers.