Reflow Conveyor Speed Calibration and Belt Maintenance

The profile that was qualified on a machine describes the product only while the conveyor delivers it at the speed the profile assumed. Belt wear, rail alignment and drive calibration all change that speed, and none of them announces itself with an alarm. This article covers how conveyor speed is measured, how it drifts, and the maintenance that keeps the thermal profile repeatable from one shift to the next.

Why Conveyor Speed Sets the Profile

A reflow profile is a record of temperature against time, and time on the machine is produced by conveyor speed. Changing the speed by five percent moves every point of the profile by the same proportion, which is enough to shift the peak and the time above liquidus at the same time.

Because the relationship is direct, the speed is verified whenever the profile is verified. A profile taken without the measured speed describes an oven at an unknown rate rather than a process, and it cannot be compared with the next measurement.

How Speed Is Measured

The most reliable measurement is a timed run: a board with a marker travels a measured length of the conveyor and the elapsed time is recorded. The speed is the distance divided by the time, and the measurement is taken over the full heated length rather than over a short section.

A tachometer reading at the drive pulley is a useful cross-check, but it measures the belt at the pulley rather than the board on the rail, and the two differ when the belt slips or the rail is misaligned. Where the two disagree, the timed run is the figure that describes the process.

A timed run is simple enough to be repeated by an operator, which is what makes it a control rather than a calibration laboratory exercise.

Belt and Chain Wear

A mesh belt stretches with use, and a chain drive develops slack that shows as a periodic change in speed rather than a steady error. The symptom is a profile that looks correct on average while individual boards see a slightly different thermal history.

Wear is checked by measuring the pitch over a fixed number of links or by observing the belt tracking at the idler. A belt that has stretched beyond its limit is replaced rather than re-tensioned indefinitely, because the tension that removes the slack also loads the bearings.

Rail Alignment, Width and Board Support

The rails must be parallel and level along the heated length, because a board that is pinched on one side moves at a slightly different rate from one that is free. Rail width is set with a gauge rather than by eye, and it is re-checked after any maintenance that disturbs the rails.

Boards are supported by the rails at their edges only, so a thin board sags in the middle and a board that is too narrow for the rail spacing may drop or tilt. Where the product mix includes small boards, a carrier or a pallet keeps them on the same path as the rest.

Vibration and Its Sources

Vibration moves components before the alloy solidifies, and in reflow it appears as tombstoning, shifted parts or disturbed joints rather than as a visible machine fault. Its sources are the blower, the drive train and the building services rather than the conveyor alone.

Vibration is measured on the rail with an accelerometer while the oven is at temperature, because fans and blowers change their balance as they warm. A reading that changes after a blower service points to the blower rather than to the conveyor.

A measurement taken with the oven cold is not comparable with one taken in production, because the drive and the fans behave differently at operating temperature.

Drive Calibration and Control

The drive is a motor with a controller, and the controller’s displayed speed is only as accurate as the calibration behind it. The calibration is checked against the timed run, and any correction is recorded so that the displayed value and the measured value agree.

Where the motor is a stepper under open loop control, a mechanical drag that increases with temperature produces a slow loss of speed that no display will show. Closed loop control with an encoder removes that class of error, and it is worth specifying on a machine that runs fine pitch work.

Verification Frequency and Record

Conveyor speed is verified when the profile is verified, and additionally after any maintenance that touches the rails, the belt or the drive. The record carries the measured distance, the elapsed time, the calculated speed and the name of the person who took the measurement, following the practice in our profile verification guide.

Where the speed is corrected, the correction and the resulting profile are recorded together, because a profile taken before the correction does not describe the machine afterwards. The two records are what allow a change in the process to be separated from a change in the product.

Effect on the Process Window

A slower conveyor lengthens every segment of the profile, which increases the soak and the time above liquidus, while a faster one shortens them. The zone temperatures can be adjusted to compensate for part of the change, but the time above liquidus is set by the speed and cannot be recovered by raising a zone.

That is why the speed is treated as a fixed parameter of the recipe rather than as a throughput control. Where throughput has to change, the profile is re-qualified at the new speed instead of the speed being adjusted until the boards look acceptable. The zone settings behind that shape are discussed in our zone settings guide.

Maintenance That Keeps It Repeatable

Routine maintenance covers belt tension and tracking, rail alignment, lubrication of the drive, cleaning of the blower and a check of the drive calibration. Each item is recorded with a date and a result, because an unrecorded adjustment cannot be correlated with a later change in the profile.

Where the machine runs continuously, the maintenance interval is set by hours rather than by calendar, and the interval is shortened after any event that disturbs the mechanics. The aim is a machine whose thermal behaviour changes only when something is deliberately changed, which is what makes the profile a useful document.

The drive is a motor with a controller, and the controller’s displayed speed is only as accurate as the calibration behind it. The calibration is checked against the timed run, and any correction is recorded so that the displayed value and the measured value agree.

Rail spacing is checked with a gauge at several points along the heated length, because a rail that is parallel at the entrance can be out by a millimetre at the exit once the machine is hot. The check is repeated at operating temperature, since that is the condition the product sees.

Where the machine has more than one lane, each lane is measured separately. A lane that carries less traffic is often neglected during maintenance, and the difference between lanes then appears as a difference between the products assigned to them.

Board loading is part of the same picture, because a board placed slightly across the rails takes a different path from one placed square. Loading guides or a loader with a reference edge keep the position repeatable, and the effect is visible in the profile.

The drive is checked for backlash where a chain or a gearbox is used, since backlash shows as a small periodic variation in speed rather than a steady error. That variation is invisible in a profile averaged over the whole oven.

FAQ

Can the conveyor speed be adjusted to fix a profile? Only with a re-qualification. The speed sets the times in the profile, so changing it invalidates the profile that was approved.

Why measure speed with a timed run? Because the tachometer reads the pulley rather than the board, and the two differ when the belt slips or the rails are misaligned.

How does vibration show up? As tombstoning, shifted components and disturbed joints, because the movement happens while the alloy is still molten.