Nitrogen Reflow: Oxygen Concentration, Dew Point and Wetting

Nitrogen in a reflow oven does one thing: it keeps oxygen away from the alloy while the flux is trying to clean it. Whether that is worth the gas, the plumbing and the analyser depends on the joint that has to be made, on the flux chemistry and on the humidity of the gas being delivered.

What Nitrogen Changes in the Oven

In a nitrogen reflow oven the copper and the solder oxidise during the ramp less, because the oxygen that would form the oxide has been displaced; in air they oxidise at a rate that competes with the flux’s ability to reduce the oxide. Nitrogen lowers the partial pressure of oxygen, so less oxide forms and the flux has less work to do, which shows as a faster wetting time and a smoother fillet.

The gas does not change the thermal profile, and it does not repair a poor profile. Its effect is on the surface chemistry of the joint, and that effect is largest where the joint is small, the alloy is lead free and the paste is at the end of its working life.

Oxygen Concentration and the Working Window

The oxygen concentration in the heating zones is usually held below 1000 parts per million for general lead free work, and below 500 parts per million where fine pitch or a marginal paste demands it. The measurement is taken with an analyser sampling from the zone, and the probe position is part of the specification.

Lower is not automatically better. The cost of gas rises steeply as the target falls, and the wetting improvement between 1000 and 500 parts per million is small for a joint that is already sound, so the target is set from a measured comparison rather than from habit. The analyser is sampled continuously from one zone and cross checked at the others on a schedule, because a single point represents the tunnel only while the curtains are set correctly.

Gas Purity, Delivery and Curtains

Nitrogen is usually supplied at 99.999 percent purity, and the delivery system matters as much as the purity, because a leak in a hose or a badly set curtain lets air into the tunnel. The curtains at the entry and exit are adjusted together with the exhaust so that the oven stays slightly positive without blowing the flux vapour out of the ends.

A flow that is too high cools the first zone and disturbs the profile, and a flow that is too low allows air to be drawn in by the movement of the boards. The balance is found once with an analyser and then verified per shift, because a curtain that has been nudged changes both the oxygen level and the profile.

Dew Point and the Moisture in the Gas

The dew point of the gas at the oven is normally held below about minus forty degrees Celsius, and a rising dew point is the first sign of a problem in the supply or in the lines. Moisture in the gas works against the flux by re-oxidising the surface it has just cleaned, and it also changes the heat transfer slightly.

Liquid nitrogen supplied in bulk usually holds the specification, while a generator or a long unlagged line can carry moisture through. The measurement is taken at the point of use rather than at the source, because that is the gas that reaches the joint.

Flux Activation and Wetting

A flux is designed to activate at a particular temperature and to remain active long enough to reduce the oxide and protect the surface until the alloy flows. Nitrogen extends the useful life of that activity by removing the oxygen that would otherwise consume it, which is why flux activation and gas quality are treated as one parameter.

The benefit is visible in a wetting balance measurement or in the spread of a paste deposit on a coupon, and it is larger for a no-clean flux than for an active one, which is where our board failure notes are useful in separating a wetting shortfall from a profile fault. A highly active flux performs well in air, so the gas buys less for it and the justification for nitrogen has to come from somewhere else.

Where Nitrogen Does Not Help

Where the joints are large, the flux is active and the profile is well matched, nitrogen reflow makes almost no difference that can be seen in the finished joint. In that case the gas is paying for a margin that the process does not need, and the cost is better spent on stencil or placement improvements.

There are also cases where the process depends on some oxygen, such as certain water soluble chemistries, and where the appearance of the joint changes enough that the inspection programme has to be re-tuned. Neither is a reason to avoid nitrogen, but both are reasons to make the decision with measurements rather than by default.

Effect on Joint Appearance and Inspection

Nitrogen produces a brighter, smoother joint with less oxide on the surface, and that changes what the inspection equipment sees. An AOI programme that was built on air-reflowed joints may raise false calls on the new appearance, and the thresholds have to be re-tuned with the same defect set used originally, so that the comparison is between two programmes rather than between two opinions.

The change is also visible in the fillet angle, which is generally smaller with nitrogen because the alloy wets further. Where a specification quotes an angle, the figure is taken from a section rather than from a photograph, since the appearance varies with the finish and the gas together, and our profile verification notes cover the measurement side of that evidence.

Measuring and Monitoring

The instruments are an oxygen analyser, a dew point sensor, flow meters on each zone and a profile recorder. Each has its own calibration interval, and the analyser is the one that drifts unnoticed, because a reading of 1200 parts per million still looks plausible on a display.

The analyser is calibrated against a certified gas at a fixed interval, and the calibration is recorded with the shift data. Where the oven logs the oxygen level, the log is reviewed against the alarm limit rather than only against the alarms that were raised. A drift check against a certified gas at the start of a shift takes a few minutes and catches the failure mode that produces a plausible but wrong number.

Cost, Records and Process Control

The cost of nitrogen is a flow rate multiplied by a price, and the flow rate is set by the curtains and the target level. That makes the gas a controllable cost rather than a fixed one, and a line that measures its consumption per thousand boards can see a leak as a change in the figure.

The record should carry the target and measured oxygen concentration, the dew point, the flow settings and the profile, together with the analysis that justified the target. Our reflow oven zone settings notes describe the profile side of that record, which nitrogen changes not at all.

FAQ

Is nitrogen reflow needed for every lead free assembly? No. It is justified where the joints are small, the flux is weakly active or the paste is near the end of its working life. For a robust process with an active flux, the measured benefit is often too small to pay for the gas.

Can oxygen be measured at the oven exit instead of in the zone? It can, and the reading is useful as a trend, but the figure at the joint is the one that matters. The sampling point is therefore stated with the target so that two measurements can be compared.

Why does the dew point drift upwards? Usually because of moisture entering the line, either through a joint that is not purged or through a section of hose that is exposed to a cold surface. The sensor is fitted at the point of use so that the drift is seen before it reaches the joints.