Reflow Conveyor Speed And Profile Balance
A reflow oven is programmed with a set of zone temperatures and a conveyor speed, and the combination produces a profile at the board. The zone temperatures are what most people adjust, and the conveyor speed is what actually determines how long the board spends in each zone. Changing the speed changes the profile more than a small change of setpoint does, and it is the setting that is most often altered for a reason that has nothing to do with the product, such as throughput.
This article explains what the conveyor speed controls, how the profile is built from the zones, and how the settings are balanced for a board with a wide range of thermal masses.
What The Conveyor Speed Controls
A convection oven transfers heat to the board at a rate that depends on the air temperature and on the airflow, and the board temperature approaches the zone temperature over time. The time available is the length of the zone divided by the conveyor speed, so slowing the conveyor increases the time in every zone and raises the temperature that the board reaches. Speeding it up does the opposite.
The effect is not linear. A board that has already reached the zone temperature gains nothing from a longer dwell, while a board that is still climbing gains a great deal. That is why the conveyor speed affects the profile most strongly in the zones where the board is changing temperature fastest, which are the ramp to soak and the ramp to peak rather than the soak itself.
Zones, Setpoints And The Board
A typical oven has four to ten zones, and the profile is the result of the whole sequence rather than of any one of them. The first zones bring the board up, the middle zones hold it at a temperature below the melting point so that the flux can activate and the volatiles can escape, and the last zones take it above the melting point and begin the cooling.
The setpoints are chosen from the profile that the paste and the components require, and the conveyor speed is then set so that the times in the profile are achieved. Where a board is so dense that the setpoints have to be raised, the speed is reduced instead if the profile can tolerate the longer dwell. Recording the setpoints and the speed together, rather than the profile alone, is what makes the recipe reproducible on another machine.
Soak And Time Above Liquidus
The soak is the period during which the board is held below the melting point. Its purpose is to bring the whole assembly to a uniform temperature, to activate the flux and to let the volatile material leave the paste. A soak that is too short leaves a temperature difference between a large component and a small one at the moment the solder melts, which is the cause of skewed joints and of tombstoning.
The time above liquidus is the period during which the solder is molten. It has to be long enough for the joint to form and for the surface tension to pull the solder into place, and short enough to limit the growth of the intermetallic layer and the exposure of the components. A typical lead free profile holds the board above the melting point for 45 to 90 seconds, and the figure is measured on the board rather than set on the oven.
Thermal Mass And Mixed Assemblies
The profile is measured with a thermocouple attached to the board, and the position of the thermocouple determines what the profile describes. A thermocouple on a small component near the edge of the board reads a different profile from one under a large connector in the middle, and the difference can be 20 degrees or more during the ramp. A profile is therefore reported as a set of curves rather than a single one.
On a board with a wide range of masses, the profile is a compromise. The large parts need a longer soak to catch up, while the small ones need the peak to be no higher than their rating allows. Setting the thermocouples on the worst cases, one on the largest thermal mass and one on the smallest component, and adjusting the profile until both are inside their windows is the practical approach.
Measuring And Adjusting
The measurement is made with a profiler that travels through the oven with the board and records the temperature at several points. The data is used to set the conveyor speed and the setpoints together, not separately: a change in speed changes every time in the profile, so the setpoints are adjusted afterwards and the profiler is run again.
Once a profile is established it becomes the recipe for the product, and it is recorded with the oven and the conveyor identified. A change of paste, of board thickness, of component or of oven requires a new profile measurement rather than an adjustment by eye. The standards that the profile has to satisfy depend on the alloy, and the differences are described under lead free versus leaded solder.
Documentation And Daily Checks
The recipe is documented as a set of zone setpoints, a conveyor speed and a measured profile with the peak temperature and the time above liquidus. Some products also specify a maximum ramp rate, because a fast ramp can damage a ceramic component through thermal shock, and a minimum ramp rate for the flux to work.
A daily check confirms that the oven is producing the profile that the recipe describes. The check may be a periodic profile measurement or a verification with a set of thermocouples on a test board, and it catches a heater that has failed, a fan that has slowed or a conveyor whose speed has drifted. The oven is a machine and it changes with use; the profile is the measurement that detects the change. The assembly flow that the profile belongs to is described under PCBA development process, and the way the board’s structure affects the heat it receives is described under PCB design and fabrication.
Cooling Rate And Belt Loading
The cooling section is as much part of the profile as the heating. A joint that cools too quickly forms a fine grain structure and can crack under the thermal shock, while one that cools too slowly spends longer above the temperature at which the intermetallic grows and produces a coarser structure. The cooling rate is controlled by the final zones and by the fan settings, and it appears in the profile as the slope after the peak.
The way the boards are loaded on the belt also changes the profile. A belt that is fully loaded transfers heat between the boards and the air differently from one that carries a single board, and the difference is large enough to move the peak temperature. Where a product is profiled on an empty belt and then run on a full one, the profile that was measured is not the profile that is produced, and the check has to be repeated with the production loading.
FAQ
Can the conveyor speed be raised to increase throughput? It can, up to the point where the profile still meets its requirements. Beyond that, the joints are not formed properly and the defects appear as insufficient wetting rather than as an obvious failure.
Why measure the profile on the board rather than the oven? Because the board temperature is what determines the joint, and the oven air temperature is only an input. Two boards with different masses in the same oven have different profiles.
How often should the profile be checked? At the start of a production run, after any change to the oven or the recipe, and periodically during a long run. A daily check on a running product is common practice.
