Reflow Atmosphere Oxygen Level Control
Reflowing in nitrogen reduces the oxygen around the joint, which reduces the oxidation of the alloy and the pad during the melting phase. The result is better wetting, a wider process window and, in some cases, a lower defect rate. The oxygen level required depends on the alloy, the flux and the geometry, and the level achieved depends on the oven, the sealing and the consumption. Measuring the level and controlling it is a separate discipline from profiling.
What Nitrogen Changes
In air, the alloy and the pad oxidise while the flux is active, and the two processes compete. The flux has to remove the oxide that forms as well as the oxide that was present, and the competition is what limits the process window. Reducing the oxygen slows the formation of new oxide, so the flux has more capacity for the oxide it was intended to remove and the wetting improves.
The improvement is largest where the flux is weak or the surfaces are difficult. A no-clean flux with a low activity benefits far more than a water-soluble flux with a strong activator, and a joint on a nickel or a palladium finish benefits more than one on copper. Where the process is already comfortable in air, nitrogen may improve the appearance without changing the yield, and the cost is then hard to justify on quality alone.
Oxygen Measurement and Calibration
The oxygen level is measured with a sensor in the tunnel, usually a zirconia cell in the exhaust or a probe inside a zone. The measurement is local, so a single probe describes the zone it is in rather than the whole tunnel, and the level at the entry is usually higher than at the peak. The probe position should be recorded, because a reading taken at the entry and one taken at the peak are different numbers describing the same atmosphere.
The sensor drifts and it ages. A zirconia cell reads low as it degrades, which is the dangerous direction because it suggests a better atmosphere than exists. The sensor should be calibrated against a known gas on a defined interval, and the calibration should be recorded. Where the oven has a reading that has been stable for a long period with no calibration, the stability is more likely to be a symptom than a virtue.
Sources of Oxygen Ingress
The entry and exit openings are the largest sources, because the board has to pass through them and the opening cannot be sealed. Curtains, baffles and a slight positive pressure reduce the ingress, and their condition decides how much. A curtain that has been torn or a baffle that has been removed and not replaced is a common finding, and it raises the oxygen level in the first zone without affecting the reading at the peak.
The nitrogen supply itself is a source if its purity is lower than assumed. A supply at 99.9 % contains a thousand parts per million of other gases, which is negligible against a target of 1000 ppm oxygen but significant against a target of 100 ppm. Where a tight level is required, the purity should be measured at the point of use rather than at the generator, because a leak or a contaminated line changes it.
Oxygen Level and Defect Rate
The relationship between oxygen and defect rate is not linear. Reducing from 2000 ppm to 1000 ppm usually produces a measurable improvement in wetting; reducing from 1000 ppm to 500 ppm produces a smaller one; and below about 500 ppm the improvement is marginal for most products. The point at which the curve flattens is the point at which further nitrogen is not buying quality.
The defects most affected are those related to wetting: incomplete fillets, dewetting, and solder balls from flux spitting. Defects related to placement, paste volume and profile are not affected at all. This is why a nitrogen trial should be run with the defect pareto in hand, so that the effect on the relevant defects can be measured rather than the total yield being compared.
Consumption and Cost Control
Nitrogen consumption is proportional to the flow needed to hold the level, and the flow depends on the sealing of the oven. A well-sealed tunnel holds a low level with a modest flow; a tunnel with open curtains requires far more. The consumption should be measured rather than estimated, because the estimate is usually based on the maker’s figure for a new oven and the installed oven is rarely in that condition.
The control strategy matters as much as the flow. Supplying a constant flow regardless of the production load wastes nitrogen during idle periods; controlling the flow from the oxygen reading saves it, provided the control loop is stable. Where the reading is controlled, the response of the loop should be checked so that the level does not oscillate around the setpoint, because an oscillating level produces a varying process, which is worse than a steady level that is slightly high.
Interaction With the Profile
Nitrogen changes the heat transfer as well as the chemistry, because the gas has different properties and the flow pattern is different. A profile developed in air will not produce the same board temperature in nitrogen, and the difference is large enough on a heavy board to move the peak by several degrees. The profile should therefore be developed in the atmosphere that will be used in production.
The lower oxygen also allows a slightly lower peak or a shorter time above liquidus for the same wetting result, which can be used to reduce the thermal load on the components. That is the main quality argument for nitrogen on a product with thermally sensitive parts. The change should be made deliberately, with the profile re-measured, rather than by leaving the air profile in place and assuming the atmosphere improves everything.
Verification and Records
The record should carry the oxygen level at the measurement points, the sensor calibration date, the nitrogen flow and the profile. With those four items, a change in the atmosphere can be separated from a change in the profile, which is otherwise difficult because both affect the same defects. The level should be recorded during production rather than only at setup, because the level changes with the load.
The verification interval should follow the risk. A product with a tight wetting requirement should have the level checked at the start of each production run; a product that does not depend on the atmosphere can be checked less often. Where the oven is shared between products, the level should be confirmed before the run that depends on it, rather than assumed from the previous run. The maintenance schedule should include the sensor calibration and the inspection of the curtains and baffles.
Deciding Whether Nitrogen Is Worth It
The decision should be made on the defect data rather than on a general expectation. The trial should run the same product in air and in nitrogen, with the profile adjusted for each, and compare the relevant defect rates and the first pass yield. Where the difference is small, the cost of the gas and the equipment is not justified by quality, although it may still be justified by the wider process window on a difficult product.
The second consideration is the process window rather than the average. A process that runs comfortably in air does not need nitrogen; one that is at the edge of its window may benefit far more than the average defect rate suggests, because the benefit is in reduced variability rather than in a lower mean. The nitrogen reflow notes set out the trade-offs, and the profile record should note the atmosphere so that the comparison is valid.
Additional Considerations for This Build
Practical attention to oxygen sensor 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 oxygen sensor explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
FAQ
What oxygen level should be specified? For most assemblies a level between 500 ppm and 1000 ppm captures most of the benefit. Below 500 ppm the improvement is small for a large cost, and above 2000 ppm the benefit is marginal. The specification should be written as a range rather than a single value.
Does nitrogen remove the need for cleaning? No. It reduces oxidation and improves wetting, and it does not remove flux residue. A no-clean flux still leaves residue, and a water-soluble flux still requires a wash.
Can a nitrogen oven run in air? Most can, with a change of profile and a check of the effects on the components. Where the oven is switched between atmospheres, the profile for each should be recorded separately, because using the wrong one is a common source of unexplained variation.
