Nitrogen in Reflow: Oxygen Level, Wetting and Oven Maintenance

Nitrogen in a reflow oven displaces the oxygen that would otherwise oxidise the solder and the pads during the critical seconds when the alloy is molten. It is a real benefit and it is not always worth the cost, and the deciding variable is the rest of the process.

What Oxygen Does at Reflow

At soldering temperature the surface of the molten solder oxidises quickly, and the oxide raises the surface energy that the solder must overcome to spread. Metal surfaces that have been exposed to air carry their own oxide, which the flux removes.

A lower oxygen concentration slows both processes, so the flux has more time to work and the solder spreads more readily. The measured effect is a lower wetting time and a larger spread on a wettability test.

The benefit is largest where the process window is tight: fine pitch apertures, small paste deposits on oxidised finishes, and alloys with a high melting point where the time at temperature is short.

The Oxygen Concentration Question

The relevant figure is the oxygen concentration at the board, measured in parts per million. Ambient air is about two hundred and nine thousand parts per million, and typical nitrogen reflow targets are between five hundred and a few thousand.

The improvement from two thousand to five hundred parts per million is smaller than the improvement from ambient to two thousand, and going below a few hundred yields little additional benefit for most assemblies.

The concentration is measured with an analyser at a point that represents the board, since the reading at the inlet is not the reading at the joint. The measurement is the only way to know whether the specification is being met.

Where Nitrogen Helps Most

Fine pitch assemblies benefit because a small deposit has a high surface area to volume ratio and oxidises faster. The same applies to small chip components whose joints are tiny.

Boards with a marginal surface finish benefit, because the finish condition determines how much oxide the flux must remove. A board stored near the end of its shelf life is the case where nitrogen most often rescues a marginal process.

Lead free alloys with a narrow process window benefit because the time available to spread is short and the driving force for oxidation is higher. Our solderability notes describe how the finish and the alloy interact.

Where It Is Not Worth the Cost

A coarse assembly with generous paste deposits and a good finish will reflow acceptably in air. The cost of the nitrogen, the consumption rate and the monitoring is not recovered.

Where the flux is aggressive enough to remove the oxide that forms, the nitrogen changes little. The measurement that decides is a comparison of the defect rate with and without, on the actual product.

The gas consumption is not trivial, because the oven must be purged continuously and the tunnels are open at both ends. The consumption depends on the inlet design and on the exhaust balance, which is why two ovens with the same specification can have very different operating costs.

Oven Maintenance and the Atmosphere

An oven that has lost its atmosphere may have a blocked nozzle, a leaking door seal or a fouled thermocouple. The periodic checks are the same as for an air oven, with the addition of the oxygen analyser calibration.

The flux management system also matters. An oven that accumulates flux condensate in the exhaust produces deposits that fall onto boards and profile drift as the heat transfer characteristics change.

Zone temperature accuracy should be verified with a calibrated probe rather than with the controller display. Our quality notes describe how the verification records are kept.

Interaction With Flux Chemistry

A flux that is designed for air reflow is formulated with activators that work in the presence of oxygen. In nitrogen the same flux may leave a different residue, and the residue may be more or less acceptable depending on the chemistry.

A no-clean flux that leaves a light residue in air may leave a sticky deposit in nitrogen, because the lower oxygen concentration changes the reaction products. This is a reason to qualify the combination rather than to change the atmosphere on an existing process without testing.

Our solder defects notes describe the residue related failures and how they appear at inspection.

Deciding on Measurement

The decision should be made on data from the product. The comparison is the defect rate and the profile margin with and without nitrogen, measured on the same board with the same paste and stencil.

A useful intermediate step is to run nitrogen only on the products that need it. Many ovens allow the atmosphere to be controlled per program, which allows the cost to be applied selectively.

The records should include the oxygen concentration during production and not only during a qualification run. A process that was qualified at one concentration and runs at another is not the process that was qualified. Our plating notes describe how the copper surface condition affects the oxide that forms before soldering.

Additional Considerations for This Build

Practical attention to flux residue 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 flux residue 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, flux residue 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, flux residue 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.

FAQ

Does nitrogen eliminate defects? It reduces defects caused by oxidation. It does nothing for defects caused by paste volume, placement or profile.

What concentration should be used? Choose the concentration that meets the defect requirement on the actual product, which is usually between five hundred and two thousand parts per million.

What does gopcb provide for nitrogen reflow? We provide profile development with and without nitrogen on the customer’s assembly, measured oxygen concentration during production, oven maintenance and verification records, and a comparison of the resulting defect rates so that the decision is based on data.