Conveyor Speed: Reflow Conveyor and Rail Maintenance: Holding the Profile
A reflow oven is calibrated as a system, and the profile it delivers depends on the conveyor speed being what the profile assumes. Rails wear, drive belts slip and edge supports bend, and the resulting change is a slow drift in the time the board spends in each zone. The oven still reaches its set points, which is why the drift is easy to miss.
Why the Conveyor Is Part of the Profile
The profile is defined by the temperature the board reaches and the time it spends at each temperature, and both are functions of conveyor speed. A belt that delivers the board 5 percent slower than the set value adds 5 percent to every dwell time, which lengthens the soak, raises the peak slightly and changes the time above liquidus.
Because the change is systematic and gradual, the joints still look acceptable at the surface. The effect appears in the things that depend on time rather than on temperature: flux activation, intermetallic growth and the void content under a large pad, each of which shifts without any obvious change on the board.
Measuring and Verifying Conveyor Speed
Speed is verified by running a board or a fixture through the oven and measuring the transit time against the length, or with a tachometer on the drive, and the two should agree. The measurement is taken at the operating speed rather than at a single reference value, because a drive that is accurate at one speed can be off at another.
Frequency follows the process: a monthly check is typical, with a re-check after any drive repair, belt replacement or controller change. Recording the value each time produces a trend, and a speed that moves consistently in one direction points to wear or to a slipping drive before it becomes a defect.
Rail Wear and Edge Support
Boards travel on the rails or on the edge conveyor, and both wear. Rails develop burrs and flat spots that change the friction the board sees, while the chain or belt stretches and changes the effective length of the path. Wear also produces the opposite problem, a board that is not driven evenly on both sides and skews in the tunnel.
Inspection is visual and tactile: run a hand along the rail for burrs, check the chain tension and look for slack on the return path. Where a board skews, the cause is usually a differential between the two sides, and correcting it is a mechanical adjustment rather than a profile change, although the profile will need re-verification afterward.
Board Support and Edge Clearance
Many ovens carry the board on the edges, which means the edges are cooler than the centre and the assembly is heated unevenly. Center support, mesh belts and carriers all change how the board is held, and each affects the profile the board actually sees. Where the process was qualified with one support method, changing it invalidates the qualification.
The support also has to accommodate board thickness and warpage. A thin board on edge supports can sag in the middle, and a warped panel may catch on a rail. Checking the support method is part of the same inspection as the rails themselves, because the two interact.
Zone Temperature Versus Delivered Heat
Zone set points are a machine condition, and the board temperature is a process condition. Airflow, impeller condition and the sealing of the tunnel all influence how much of the set point reaches the assembly, which is why a profile with a thermocouple on the board is the only valid check after a mechanical change.
Where a zone cannot hold its set point, the cause is usually airflow rather than the heater. Restricted filters, a failing blower or a door that does not seal will each produce a zone that reads correctly to the controller while delivering less heat to the board, and the profile verification is what reveals it.
Nitrogen, Airflow and Mechanical Interaction
Where nitrogen is used, the tunnel has a slight positive pressure and the curtains at each end restrict the escape of gas. Those curtains also affect airflow and therefore heat transfer, and a damaged curtain changes both the oxygen level and the profile. The two effects are worth checking together after any mechanical work.
Flow balancing between top and bottom is another mechanical setting with a thermal consequence. A board that is heated more from below than from above warps upward, and the resulting profile differs between the two faces, even though the zone temperatures are unchanged.
<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pcb走线.png" alt="Conveyor speed calibration check on a reflow oven entrance” />
Scheduled Maintenance That Protects the Profile
A workable schedule covers rail inspection, chain tension, speed verification, filter condition, blower and impeller inspection, and door sealing, with the interval set by throughput. The items are mechanical, but each one has a thermal consequence, so the maintenance is a process activity rather than a facilities activity.
Every item that is adjusted should be followed by a profile verification, because the mechanical setting and the thermal result are linked. Adjusting a chain tension or replacing a filter and then running production without a profile check is how a mechanically correct oven produces an out-of-specification assembly.
Keeping Records That Show the Drift
Record speed, rail condition, filter change dates and each profile verification together. Read as a set, they show whether the oven is stable or whether a mechanical element is progressively changing the process, and they allow a defect found on a specific date to be compared against the oven condition at that time.
A profile that was qualified once and never re-verified has no protection against this drift. Treating verification as a scheduled activity, with the record kept beside the profile, is the difference between a qualified process and a process that was qualified once.
When to Re-Qualify the Oven
A profile needs to be re-established after any change that could alter heat transfer: a new belt or rail set, a blower replacement, a different board support, a curtain change or a relocated oven. Where the change is mechanical and large, a full profile development is warranted rather than a check that the old profile is still met.
Where the oven is unchanged but the product is, the profile is also re-established, because the thermal mass and the layout are different. Re-using a profile across products is only valid when the boards are thermally equivalent, and that equivalence should be demonstrated rather than assumed.
Points to Confirm at First Article
Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.
Related reading: our fabrication notes, board quality and design release notes cover the same ground.
FAQ
Why does conveyor speed affect the reflow profile? Dwell time in each zone is set by speed. A belt running 5 percent slow lengthens the soak and the time above liquidus even though the zone temperatures are unchanged.
How often should conveyor speed be verified? Monthly is typical, plus after any drive, belt or controller change. Record the value so a gradual drift becomes visible.
What happens if board support is changed? The heat the board receives changes, so the qualification no longer applies. Any change to rails, mesh or carriers should be followed by a profile verification.
