Vapour Phase Soldering Control for PCB Assemblies: 6 Rules

Vapour phase soldering heats a board by condensing a hot vapour onto it. The vapour is produced from a fluid with a defined boiling point, and the board can never exceed the temperature of that vapour, which makes the process inherently self-limiting.

That property is the reason the method is used for assemblies with a wide range of thermal masses, for boards carrying parts that cannot tolerate an overshoot, and for processes where the peak has to be identical on every board regardless of what is placed on it.

How Vapour Phase Soldering Works

The chamber contains a fluid that boils at a temperature above the melting point of the solder being used. When the board enters the saturated vapour above the boiling liquid, vapour condenses on the cooler surfaces and releases its latent heat there.

Because the heat is delivered by condensation, it is delivered fastest where the surface is coolest, which is exactly where it is needed. The board heats uniformly and the components that lag are heated more strongly until they catch up.

The process stops when the surface reaches the vapour temperature, because condensation then ceases. That is the self-limiting behaviour, and it means the peak is set by the fluid rather than by an oven controller.

Peak Temperature and the Boiling Point

The fluid is selected so that its boiling point is above the liquidus of the solder with enough margin for the joint to form fully. Too little margin produces incomplete melting and cold joints; too much margin overheats the parts unnecessarily.

Single-fluid systems operate at one temperature, and the profile is therefore flat at the top rather than peaked. Where a profile with a soak is required, a two-fluid system is used, with a lower-boiling fluid for the first stage and a higher-boiling fluid for the second.

The boiling point of the fluid drifts as the fluid is contaminated with flux, moisture and residues from the boards. That drift is one of the reasons the bath is analysed rather than trusted, and a rising boiling point can push the process outside the component rating.

Dwell, Soak and Thermal Lag

The time the board spends in the vapour is the control variable that replaces the conveyor speed of a reflow oven. A longer dwell does not raise the peak, but it does allow the slower components to reach the vapour temperature and the joints to form fully.

Too short a dwell leaves the larger parts below the required temperature, which produces joints that look formed but have not fully reflowed. Too long a dwell adds no benefit and increases the exposure of the parts and the flux.

Where a two-stage profile is used, the dwell in each vapour stage is controlled separately, and the transition between them is part of the recipe. The thermal lag of the assembly is what determines the required times, so the recipe is qualified on the real product rather than on a coupon.

Fluid Loss, Contamination and Top-Up

The fluid is lost continuously as vapour escapes, and the loss is replaced by a top-up. The top-up maintains the volume but does not remove the contamination that has accumulated, so the concentration of flux residues and moisture rises over time.

Contamination changes the boiling behaviour, leaves residue on the board and can produce a smell that indicates the fluid is breaking down. The bath is therefore sampled on a schedule and changed when a defined limit is reached.

Moisture is a particular concern because water in the fluid causes violent boiling and can splash hot fluid onto the board. Assemblies that have absorbed moisture are baked before they enter the chamber, and the floor life records show which parts need that treatment.

Drainage, Residue and Cleaning

When the board leaves the vapour it carries a film of fluid, and the film has to drain before the board is handled. Draining is done in a defined orientation and for a defined time, because fluid trapped under a component will otherwise remain there.

The residue that remains after drainage is either a cleaning requirement or a no-clean residue, depending on the flux system. Where cleaning is needed, the fluid has to be removed completely, since a heavy fluid film can protect the residue from the wash. The no clean residue review describes the alternative case.

Where the assembly carries a flux that has to be matched to the alloy, the pairing is described in the alloy and flux pairing notes. Void behaviour in large thermal pads under vapour phase is covered in the thermal pad voiding review, and the alternative of a focused beam process is set out in the laser soldering guide.

Verification and Records

Verification uses a profiled board with thermocouples at the coldest and hottest points, run in the production orientation. Because the peak is set by the fluid, the profile confirms the dwell and the time above liquidus rather than the maximum temperature.

Records should include the fluid identity, the bath analysis, the boiling point check, the dwell times and the profile. The boiling point check is the single most useful number, because it combines the fluid condition with the contamination level in one measurement.

Where a joint fails later, the fluid history is part of the investigation, because a bath that has been topped up for months without analysis may have been running above the component rating without anyone noticing.

Rework, Repair and Process Selection

Vapour phase is often chosen for a board that mixes large thermal masses with small ones, and for a rework station where the local peak has to be controlled. The same principle applies to a hand-operated chamber that condenses vapour onto a single assembly.

For rework, the chamber has to hold the vapour blanket over the joint without disturbing nearby parts. That usually means a local shield or a nozzle that directs the vapour, and the profile is qualified on a scrap assembly before it is used on product.

Choosing between vapour phase and convection reflow is a matter of the defect being solved. Vapour phase fixes peak uniformity and removes shadowing from tall parts, while convection makes a soak profile easier to shape and usually costs less per board.

Where both are available the decision should be made on the assemblies that fail, not on the ones that already pass. A process change that helps nothing measurable adds cost and a new set of variables.

FAQ

Why is vapour phase soldering so uniform? Because heat is delivered by condensation, which is strongest where the surface is coolest. The process naturally drives the whole assembly toward the vapour temperature, so the difference between parts closes as the dwell continues.

Can the peak temperature be raised without changing the fluid? No. The vapour temperature is the boiling point of the fluid, so a different peak requires a different fluid or a two-fluid system. Contamination changes the boiling point slightly but cannot be used as a control.

Does vapour phase soldering need a different flux? It needs a flux that is compatible with the fluid and with the longer dwell at temperature. Some activators are consumed faster in a vapour phase profile, so the paste and the flux are qualified with the process rather than separately.