Convection Versus Vapour Phase Reflow

Two Ways to Heat

Every reflow oven has to deliver heat to a board and its joints, and there are only two mechanisms available: convection, where the heat is carried by a moving gas, and condensation, where the heat is released when a vapour turns to liquid on the surface. Convection is the dominant method in production, and vapour phase is a niche process used where the thermal uniformity of condensation outweighs its cost and its handling difficulties. The two produce different temperature profiles and different behaviour on boards with mixed thermal mass.

Convection Ovens

A convection oven moves heated air or nitrogen through zones, each with its own set point, and the board is carried through at a controlled speed. The heat transfer depends on the gas velocity, the temperature difference and the geometry, so a large, massive component heats more slowly than a small one and a part in the shadow of another heats more slowly still. This is the origin of the mixed-assembly problem: the profile has to be set so that the slowest joint reaches temperature without the fastest one exceeding its excursion budget. Modern ovens use high velocity gas and multiple zones to reduce the spread, but the spread never disappears. The advantages of convection are the wide availability, the ability to run nitrogen for oxidation control, the relatively low cost per board, and the fact that the board is dry and can be handled immediately after the oven.

Vapour Phase

Vapour phase heating uses a liquid that boils at a temperature above the solder melting point, so that the vapour condenses on the board and releases its latent heat directly. Because condensation occurs at a fixed temperature, every surface reaches that temperature and no part can exceed it, which gives an inherently uniform and self-limiting profile. The board cannot overheat, which is attractive for assemblies with components of limited temperature tolerance, and complex boards with uneven thermal mass are heated more evenly than in a convection oven. The difficulties are the cost of the fluid, which is consumed and has to be managed, the handling, since the board emerges wet with fluid and has to be cleaned or the residue dealt with, and the throughput, which is generally lower.

Profile Differences

A convection profile is a curve that can be shaped by the zone temperatures and the conveyor speed, which gives the flexibility to run a soak, a ramp-spike or something in between. A vapour phase profile is much less flexible: the board rises to the fluid’s boiling point and stays there, so the reflow dwell is set by the immersion time and the peak is set by the fluid rather than by the recipe. Where a design needs a specific time above liquidus or a specific soak, convection gives the freedom; where a design only needs to reach a uniform temperature without exceeding it, vapour phase is the simpler answer.

Large and Heavy Assemblies

The larger and heavier the assembly, the more attractive vapour phase becomes, because the uniformity of condensation removes the shadowing and thermal mass problems that a convection profile struggles with. A thick board with large connectors and a mix of small and large parts is the case where the difference is greatest. Against that, a board with a heavy thermal mass takes longer to heat in vapour phase as well, since the heat has to flow into the material, but it approaches the fluid temperature uniformly rather than differentially, which is what the profile needs.

Process Control and Monitoring

Both processes need a profile measured with thermocouples on the board, and both need the profile rechecked when the board or the assembly changes. Convection ovens need the zone temperatures and the conveyor speed monitored, and they drift slowly as the heating elements age. Vapour phase needs the fluid condition and the immersion time monitored, and the fluid degrades with use, which changes the boiling point. The monitoring requirement is therefore different in kind: a convection oven is monitored against a profile curve, while a vapour phase system is monitored against a fluid property.

Cost and Throughput

Convection is cheaper per board and better suited to volume production, and nitrogen consumption is the main recurring cost where it is used. Vapour phase has a higher cost per board because of the fluid, and its throughput is lower, which makes it attractive mainly for high-value assemblies where the yield benefit or the temperature limits justify it. The decision is usually driven by the assembly rather than by the process: where a board cannot be profiled reliably in convection because of its thermal mass, or where a component cannot tolerate an overshoot, vapour phase earns its cost.

Reflow and the Assembly Materials

The oven is only one part of the thermal system, and the materials that go into it with the board change the result. A paste with a high metal load needs more heat to coalesce; a board that has absorbed moisture will release it during the ramp; a component with a moisture-sensitive body will popcorn if it is not dried. This is why the reflow step cannot be considered separately from the storage and handling of the board and the parts. A part that has been out of its dry pack for too long, a board that has been stored in a humid workshop, and a paste that has been open on the stencil for a shift are all thermal factors, and the profile has to be robust to them rather than tuned to a perfect input. Where the process is marginal, the improvements usually come from the material handling rather than from a further adjustment of the oven.

FAQ

What is the main difference? Convection transfers heat from a moving gas and needs a shaped profile; vapour phase transfers heat by condensation at a fixed temperature and is self-limiting.

Can a board overheat in vapour phase? No. The board cannot exceed the boiling point of the fluid, which is why the process is used for temperature-sensitive assemblies.

Which gives a better profile for a mixed assembly? Vapour phase is more uniform, which helps where thermal mass varies widely, but convection offers more freedom to shape the profile.

Why is vapour phase not used everywhere? Because of the fluid cost, the handling and cleaning of the wet board, and lower throughput.

Does nitrogen matter? In convection it reduces oxidation and improves wetting on marginal finishes. A vapour phase system operates in a vapour atmosphere, so the situation is different.

Conclusion

Convection and vapour phase differ in how heat reaches the joint, and the choice follows from the assembly rather than from a preference: convection for volume and flexibility, vapour phase for thermal uniformity and self-limiting peak temperature. Whatever is used, the profile has to be measured on the board. The reflow process is described in SMT PCB assembly, the joint quality that results is verified through PCBA testing, and the thermal layouts that make a board easy to profile belong to PCB design and layout. Profiles are normally established during prototype PCB assembly in 2026.