Vapour Phase Reflow Compared with Convection
Vapour phase soldering heats a board by condensing a saturated vapour onto it, and forced convection heats it by moving hot gas past it. The two transfer heat in different ways, and the difference shows up on boards that carry components of very different mass.
How Each Transfers Heat
Condensation delivers the latent heat of the fluid directly to the surface, so the heat transfer coefficient is high and it is the same on every surface that the vapour can reach. Convection depends on the gas velocity and on the local geometry.
The consequence is that vapour phase heats a heavy and a light component more evenly. Our mixed thermal mass notes describe the problem it solves.
The Profile Shape
The vapour stops heating at the boiling point of the fluid, so the peak is set by the chemistry rather than by the oven. That makes the peak reproducible, and it makes it difficult to exceed.
The ramp into the boil is steep unless the board is held in the vapour zone above the work. Our profile notes describe the measurement.
Where Convection Wins
Convection allows a soak, a controlled ramp and a choice of atmosphere, and it handles a wide range of board sizes without a fluid inventory. For most boards it is the more flexible process.
The profile shape can be tuned to the paste chemistry. Our soak notes describe the effect on voids.
Where Vapour Phase Wins
A very heavy assembly, a board with a large metal core, or a stack that cannot be brought to temperature uniformly in an oven is where the condensation process earns its place. It also limits the peak temperature, which protects components with a low tolerance.
Our metal core notes describe the assemblies that fall into this class.
Fluid Loss and Maintenance
The fluid is consumed and it has to be monitored, because a change in its composition moves the boiling point and therefore the peak. Contamination from flux residue also accumulates in the fluid.
The fluid condition should be part of the process record. Our maintenance notes describe the routine discipline that applies here as well.
Selecting the Process
The decision follows the assembly rather than the equipment: a uniform board with a modern paste suits convection, and an assembly with extreme mass differences or a tight ceiling on peak temperature suits vapour phase.
Where both are available, the choice can be made per product. Our design release notes list the points to confirm.
Verification
The verification is a profile recorded on the board itself, a fluid condition check at a stated interval, and a joint inspection that covers the heaviest and the lightest component on the assembly.
Our joint criteria notes describe the acceptance that applies.
Additional Considerations for This Build
Practical attention to mixed assembly pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating mixed assembly explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, profile is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
Process Control and Verification
On a design of this kind, profile is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.
Process Control and Verification
On a design of this kind, profile is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
Process Control and Verification
On a design of this kind, profile is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Is vapour phase obsolete? It is not widely used for general assembly, but it remains in service where a uniform peak on a heavy assembly matters more than flexibility.
Does vapour phase need a specific paste? It works with the same pastes, though the profile shape means the flux chemistry has to tolerate a rapid ramp.
What does gopcb provide for reflow process selection? We provide a process chosen against the assembly rather than the equipment, a profile measured on the board at the heaviest and lightest components, a fluid condition recorded where vapour phase is used, and a joint inspection that covers the extremes of the assembly.
