Vapor Phase: Design Rules and Process Limits

Vapor phase soldering heats a board by immersing it in a saturated vapour whose temperature is fixed by the boiling point of the fluid. The vapour condenses on the board and releases its latent heat, which gives an extremely uniform transfer that does not depend on the shape or the colour of the assembly. It is an older process that survives because some assemblies cannot be heated evenly by any other method.

How the Heat Is Transferred

The fluid boils at a defined temperature, and the vapour above it is saturated. When a cooler board enters the vapour, the fluid condenses on every exposed surface and gives up its latent heat, which is a large quantity compared with the sensible heat of air.

The condensation continues until the surface reaches the vapour temperature, so the whole board rises toward the same temperature at a rate that is set by its own thermal mass rather than by its position in an oven. Shadowing, which plagues infrared heating, does not occur.

This is the defining property of vapor phase: heat transfer is by condensation rather than by convection, and it is uniform over a complex assembly with tall components, cavities and a wide range of thermal masses.

Fluids and Temperature Steps

The working temperature is set by the fluid. A single fluid gives a single saturated temperature, and the common fluids cover the range used by lead free solders. Where a specific profile is required, two fluids with different boiling points can be used in sequence.

A two stage process immerses the board first in a vapour at an intermediate temperature for the soak and then in a second vapour at the peak. The result is a profile that resembles a conventional soak and spike, produced without a conveyor and without zone control.

The fluid is expensive and it is not consumed only by the process. It is lost through the openings, it is carried out on the boards and it degrades with use, so the consumption and the quality of the fluid are part of the operating cost.

Voids and Entrapment

The uniform heating is an advantage for void content. A board that heats evenly reflows at nearly the same time across its area, so the flux volatilises gradually and the gas has an easier path out of the joint than in a process where one area melts long before another.

Void levels in vapor phase are usually lower than in convection reflow for comparable profiles, which is the reason the process is still chosen for power devices and for assemblies where the thermal interface is critical.

Voids are not eliminated. A paste with a high solvent content, a pad with a large aperture or a profile with too fast a ramp will still produce them, and the mechanism is the same as in any other process.

Comparing with Convection

A convection oven controls the profile by zone temperature and conveyor speed, which gives the designer a wide range of shapes to work with and a process that is easy to scale. Vapor phase gives a fixed temperature and a shape that is set by the fluid, which is a narrower offer.

The convection oven exposes the assembly to moving gas, which can disturb light components and can carry heat unevenly around a shadowed part. The vapour chamber has no gas flow in the same sense, so a delicate part stays where it was placed.

The convection oven is easier to integrate into a line. A vapour chamber is usually a batch process, which limits the throughput and makes it suitable for high value assemblies rather than for volume production.

Process Control

The parameters are the fluid temperature, the immersion time and the position of the board in the chamber. The temperature is set by the fluid, so the control is really about the time and about the rate at which the board is introduced.

A board that is lowered too quickly into the vapour can be heated faster than the flux can cope with, which produces spatter and solder balls. A board that is raised too quickly at the end of the cycle leaves liquid fluid on the surface and can disturb the joints before they solidify.

The vapour zone height has to be maintained, because the loss of fluid changes the level and therefore the position of the board relative to the saturated zone. The level should be checked as part of the daily routine rather than when a defect appears.

Materials and Compatibility

The fluid attacks some materials. Labels, marking inks, certain plastics and some adhesives are affected, and a component that is not compatible with the fluid may be damaged even though the soldering temperature is acceptable.

Components that have absorbed moisture behave in the same way as in any reflow process. The rapid heating releases the water and the resulting pressure can crack the package, which is why the bake and the moisture sensitivity level rules apply here as well.

The residue left by the fluid is minimal compared with a flux residue, and the process is often described as leaving a clean board. The flux in the paste is still present, so the cleaning requirement depends on the paste rather than on the soldering medium.

Where It Still Wins

The process is chosen for assemblies with a very large thermal mass difference, for boards with tall components that shadow in a convection oven, for small batches of high value product and for prototypes where the profile would otherwise be difficult to develop.

It is also used where the void requirement is strict. A power module with a thermal interface to a heat sink benefits from the even heating, and the improvement in void content translates directly into a lower junction temperature.

The decision should compare the total cost rather than the soldering step. The fluid consumption, the batch nature of the process and the extra handling all belong in the comparison against a convection line that is already available.

Practical Rules

Control the immersion rate and the withdrawal rate, maintain the fluid level and check the compatibility of the components and the markings. Use the even heating to reduce voids, but do not expect it to correct a bad paste or a bad aperture.

Record the fluid batch, the cycle and the profile with the build records and the defect history, and compare the results with the convection profile data and the underfill process when voids are the reason for the choice.

Process Control and Verification

On a design of this kind, heat transfer is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

What makes vapor phase heating uniform? The heat comes from condensation of a saturated vapour rather than from convection. The board rises toward the fluid boiling point regardless of its shape or thermal mass.

Why are void levels lower? The assembly reaches reflow temperature almost uniformly, so the flux volatilises gradually and the gas has an easier path out of the joint.

Why is the process not used everywhere? It is usually a batch process with a fixed temperature set by the fluid, and the fluid is expensive and consumed. It suits high value assemblies rather than high volume.