Reflow Oven Belt Speed: Verification and Process Control

Reflow oven belt speed sets how long a board spends in every zone, so it is the variable that converts a temperature profile into a time at temperature. The zones decide the shape of the curve and the belt speed decides the scale, which is why a small drift in speed changes the whole process.

Speed is also the easiest variable to get wrong, because the controller displays the set value rather than the actual one. A belt speed that has drifted will still show the number that was typed in, and the boards will still be accepted until a profile check is run. That is why the speed is verified with a mark on the belt rather than read from the screen.

Why Belt Speed Decides the Profile

The oven zones are fixed lengths, so the time a board spends in each one is the zone length divided by the belt speed. A ten per cent change in speed is a ten per cent change in the dwell time in every zone at once, including the soak and the peak.

That is different from changing a zone temperature, which alters the shape of the curve locally. Speed changes the whole profile, which is why it should be treated as the master setting and verified rather than adjusted to trim a result. The conveyor speed is the setting that scales the reflow profile in time.

Measuring Actual Speed

Actual speed should be measured on the belt with a timing mark, not read from the controller. A mark of known length and a stopwatch gives a number that includes the belt, the drive and the tension, and it costs nothing. The mark should be long enough that the timing error is small compared with the drift being looked for.

The measurement should be repeated at the settings the production uses, because a worn drive can behave differently at low speed. The techniques used in reflow oven zone temperature verification work assume the belt speed is known, so the two checks belong together. The same visit can cover the zones, the belt and the cooling rate in one pass.

Speed Drift and Its Causes

Drift comes from the drive rather than from the setting. Belt tension, roller condition, gearbox wear and a slipping coupling all change the relationship between the motor and the belt, and none of them changes the display. A slipping coupling is the hardest to find, because it recovers as soon as the load is removed. A coupling that slips under a heavy load is invisible in an idle oven test.

A belt that has stretched over time is a common cause, because the drive then has to turn further for the same distance. Tension should be checked on a schedule, and the belt should be replaced when adjustment runs out. The replacement interval should be taken from the tension measurements rather than from a calendar.

Load Effects and Board Spacing

The belt moves at a constant speed, but the heat the boards take depends on how closely they are spaced. A dense load pulls the zone temperatures down, and the recovery that follows changes the profile for the boards behind it. A load that varies through the shift is the hardest case to profile and the one that needs the most margin.

The loading rules in reflow oven loading rules work cover that effect. Where the load varies, the sensible approach is to verify the profile in the worst case rather than to assume the average case applies. The densest load the line is expected to run should be the one that is profiled.

Dwell Time in the Soak and Peak

The soak and the peak are where the process window is tightest, and both are set by speed. A longer soak lets the flux activate and equalise the board, while a shorter one leaves a cold joint that the peak cannot rescue.

The time above liquidus also follows the speed, and too much of it grows the intermetallic layer while too little leaves an incomplete joint. The soak setting described in soak time reflow setting work assumes the belt speed is stable. A soak time that drifts is usually the first sign that the belt has changed. The soak and the peak should be checked together, because a speed change moves both at once.

Profile Verification Practice

A profile should be run with a thermocoupled board that represents the product, and the run should record the belt speed alongside the curve. A profile without a speed is a description of one run rather than a process that can be repeated. The speed should be written onto the profile chart at the same time as the curve is captured.

The profile should be re run after any change of speed, belt or board loading. The practice described in thermal profiling for thick boards work is the same method applied to a heavier load.

Interaction With Zone Temperature

Speed and zone temperature are often adjusted together to move a profile, and the combination is what makes a change hard to trace. A change should be made to one variable at a time, and the reason should be recorded.

Where a profile has to be shortened, increasing the speed is often better than lowering the peak, because it reduces the time above liquidus without reducing the peak temperature. The ramp questions covered in preheat ramp rate reflow work follow the same logic.

Records and Change Control

The record should carry the set speed, the measured speed, the belt tension, the profile reference and the loading pattern. With those fields, a shift in a joint result can be traced to a mechanical change rather than to the paste or the components. A speed trend plotted over a month is enough to catch a belt before it affects the product.

Where the process follows a published standard, such as the soldering documents from IPC, the acceptance criteria should come from that source. A new belt or a new drive is a change to the process and should be followed by a full profile rather than by a visual check of the boards. The profile record and the speed record should be filed together, since one is meaningless without the other.

Belt Drive and Roller Condition

The drive train is what turns a motor setting into belt movement, and every part of it can change that relationship. Gearbox wear, a slack belt, a seized roller or a slipping coupling all produce a belt that moves more slowly than the setting implies.

Rollers should turn freely and be checked for flat spots, because a roller that drags adds a periodic variation rather than a constant offset. The variation is harder to detect, and it can heat boards differently within the same load. A drive that is serviced on a schedule keeps the speed number meaningful.

FAQ

Why measure belt speed rather than trust the display? Because the display shows the set value. Belt tension, roller wear and the drive all change the actual speed without changing the number on the screen.

How much does a small speed change matter? More than most people expect, because it scales the dwell time in every zone at once. A ten per cent change moves the whole profile, not just one part of it.

Does board loading change the profile? It does. A dense load pulls the zone temperatures down and changes the recovery, so the profile should be verified in the worst case the line will run.