Nitrogen Reflow And Oxygen Concentration Control
A reflow oven is normally filled with air, and the oxygen in that air reacts with the solder, the pads and the flux while the joint is being formed. Replacing the atmosphere with nitrogen reduces that reaction, and the result is better wetting, fewer defects and a wider process window. The cost is the gas, the equipment and the consumption, and the decision of whether to use it is a balance rather than an obvious improvement.
This article explains what an inert atmosphere changes, how the oxygen level is set and monitored, what it does and does not fix, and how the decision is made.
What Nitrogen Changes
Oxidation is a competing reaction. While the flux is trying to reduce the oxide on the surfaces to be joined, the oxygen in the oven is rebuilding oxide on the same surfaces. Reducing the oxygen concentration slows the rebuilding enough that the flux can win, and the solder then wets more completely and spreads further in the same time and at the same temperature.
The visible consequences are a brighter joint, a smaller contact angle and a fillet that extends further up a lead. The less visible ones are equally important: a paste that is at the end of its working life behaves more like a fresh one, a reflow that is run with a shorter time above liquidus still forms a sound joint, and the number of defects that depend on marginal wetting falls. The effect is largest on a lead free alloy, whose higher melting point gives the oxidation more time to act.
Setting And Measuring The Oxygen Level
The parameter that matters is the oxygen concentration inside the tunnel, not the flow rate of the gas. It is measured with an oxygen analyser at a probe placed near the zone of interest, and it is normally quoted as parts per million. Air contains about 209,000 parts per million, so any useful inerting is a reduction of two or three orders of magnitude.
The usual targets are between 1000 and 3000 parts per million for general assembly, and below 1000 for the most demanding work. Achieving the lower figure needs a well sealed tunnel, a balanced flow in each zone and a gas distribution that matches the profile, since a poorly balanced tunnel will show a low reading at the probe and a high one at the board. The measurement is therefore taken on the board or at several points, not only at the analyser.
What It Improves
Three categories of defect respond to inerting. The first is the wetting related defect, including incomplete fillets, dewetting and the poor spread that appears on an oxidised pad finish. The second is the surface appearance, which matters where an automatic optical inspection system is trained on the colour and texture of a joint. The third is the process window, which widens because the profile no longer has to be long enough to overcome the oxide.
The widening of the window is often the most valuable effect. A board with a large thermal mass and a small one on the same panel is difficult to profile in air, because the small parts are at the peak temperature while the large ones are still catching up. In nitrogen the same profile produces sound joints on both, which is a real gain for a mixed assembly.
What It Does Not Fix
Nitrogen does not fix a bad profile. A joint that does not reach the melting point, a soak that is too short or a peak that is too high are all unaffected by the atmosphere. Nor does it remove the need for a flux that works: the flux still has to reduce the oxide that is already present, and the atmosphere only slows the formation of new oxide.
It does not fix a poor print, a contaminated pad or a component with an unwettable termination. Those are upstream problems, and the improvement that nitrogen produces can hide them by making a marginal joint look acceptable. That is a real risk, because a process that is only sound in nitrogen cannot be run anywhere else and cannot be diagnosed by appearance. The related alloy effects are described under lead free versus leaded solder.
Cost, Consumption And Balance
The cost of inerting is the gas, the equipment and the maintenance. Consumption depends on the tunnel volume, the sealing, the conveyor openings and the flow setting, and a well designed tunnel with entrance and exit curtains uses far less gas than an open one. The gas is normally supplied from a tank or generated on site, and the choice affects the running cost more than the capital cost.
The balance is different for different products. A high reliability assembly with fine pitch parts and a high value per board justifies the gas, while a simple single sided board with generous pads does not. The measurement that supports the decision is the defect rate with and without, recorded over enough boards to be meaningful, and the cost of the defects that were removed. The assembly process as a whole is described under PCBA development process.
Interaction With The Profile
Moving to nitrogen is a process change, and the profile has to be re-established rather than carried over. The improved wetting means that the same profile produces a different joint, and a peak that was marginal in air may be more than sufficient in nitrogen. The usual result is that the profile can be shortened slightly, which partly offsets the cost of the gas by increasing throughput.
The flux chemistry also interacts with the atmosphere. A flux designed for air has to work in the presence of oxygen, and the activators it contains are often more aggressive than necessary in nitrogen. The residue that remains may be different, which matters if the board will be coated or if the residue has to be cleaned. The interactions with the rest of the fabrication and assembly flow are described under PCB design and fabrication.
Verification
Verification is by measurement of the oxygen level at the board and by inspection of the joints. The analyser is calibrated on a schedule, and the reading is recorded for the profile. The joints are inspected for the wetting angle and the fillet, and a comparison is made with a reference board that was run in air, so that the effect of the atmosphere is visible rather than assumed.
The electrical and reliability checks are the same as for any process: a sample of boards is thermally cycled and the joints are examined, and the defect rate is tracked over a period. The value of inerting is established by that comparison and not by the appearance of the joint on its own, which is the reason the measurement should be recorded before and after the change.
FAQ
What oxygen level should be used? Between 1000 and 3000 parts per million is enough for most assemblies, and below 1000 is used for demanding work. The level is measured at the board rather than only at the analyser.
Does nitrogen remove the need for cleaning? No. It reduces the oxidation of the surfaces and the darkening of the residue, but the flux residue is still present and the cleaning requirement depends on the flux and on the product rather than on the atmosphere.
Can a nitrogen process be run in air as a backup? It can, with a re-established profile and a check that the defect rate is still acceptable. A process that was tuned in nitrogen often needs a longer profile in air to reach the same wetting.
