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.

Process Control and Verification

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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

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.