Nitrogen: Preparation, Placement and Process Control
A reflow oven filled with nitrogen reduces the oxygen available to the joint while it is molten. The consequence is better wetting and less oxidation, and the size of the benefit depends on the paste and on the joint being formed.
What the Oxygen Concentration Changes
The oxidation rate of molten solder falls as the oxygen concentration falls, and the surface tension of the molten alloy also changes. Both act on the same thing: how far the solder spreads before it freezes.
The relationship is not linear, and most of the benefit arrives below a few hundred parts per million. Our joint criteria notes describe the wetting that is required.
Which Defects It Affects
Insufficient wetting, solder balls and the appearance of a matte surface all improve. Bridging and tombstoning are mechanical or thermal problems and are largely unaffected.
The defect should be identified before the atmosphere is changed. Our defect troubleshooting notes describe the mapping.
How the Atmosphere Is Measured
The oxygen concentration is measured with an analyser at the zone of interest, not at the supply. A leaking door seal, an open port or a high conveyor load all raise the concentration at the board.
The measurement should be taken under production conditions. Our profile verification notes describe the procedure it accompanies.
Profile Interaction
A nitrogen atmosphere changes the surface energy, which changes the way the paste wets and therefore the appearance of the joint. A profile that was tuned in air may need its soak adjusted.
The profile and the atmosphere should be qualified together. Our soak profile notes describe the parameter that matters most.
Cost and Consumption
The gas consumption depends on the tunnel volume, the sealing and the number of open ports. A well sealed oven with a narrow entrance consumes far less than an open one.
The cost should be compared with the yield it buys rather than against the gas price alone. Our cost benefit notes describe the comparison.
Flux Chemistry and Residue
A reduced oxygen atmosphere leaves a paler and often more soluble residue, which can change whether cleaning is required. The change should be assessed before the decision is made.
Our flux residue notes describe the assessment.
Verification
The verification is an oxygen concentration measured at the board under production load, a profile qualified with that atmosphere rather than in air, and a defect comparison before and after the change.
Our first pass yield notes describe how the comparison is recorded.
Additional Considerations for This Build
Practical attention to dross 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 dross 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, wetting is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
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.
Process Control and Verification
On a design of this kind, wetting is the item that decides how the rest of the board is arranged. 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. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
Process Control and Verification
On a design of this kind, wetting is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, wetting is the item that decides how the rest of the board is arranged. 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.
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 nitrogen necessary for lead free assembly? No. It widens the process window, and a well controlled process without it can meet the same criteria.
Does lower oxygen always improve the joint? It improves wetting up to a point, and below that the benefit becomes small while the cost continues.
What does gopcb provide for reflow atmosphere control? We provide an oxygen concentration measured at the board under production load rather than at the supply, a profile qualified in that atmosphere, a defect comparison before and after any change, and an assessment of whether the residue still allows the same cleaning decision.
