Reflow Profile Setup And Thermocouple Measurement

The numbers displayed on a reflow oven are not the profile. They are the temperatures of the heating zones and the speed of the conveyor, and the profile is what the board actually experiences as it passes through them. The difference between the two depends on the thermal mass of the assembly, on the components that shield the surface, and on the position of the board in the oven, which is why the profile has to be measured on a real board with thermocouples attached to it.

This article explains what the profile has to achieve, how the measurement is made and where the sensors belong, what makes the reading wrong, and how the result is documented.

What The Profile Has To Achieve

The profile is divided into stages, and each has a purpose. The preheat and soak raise the assembly to a temperature at which the flux becomes active and the volatiles are driven off, while keeping the ramp slow enough that the flux does not spatter and that small and large components reach a similar temperature. The reflow stage takes the solder above its liquidus, holds it long enough for the alloy to wet and to form a fillet, and then the cooling stage solidifies the joint at a rate that produces a sound grain structure without thermal shock.

The paste datasheet gives the limits for each of those stages, and the alloy sets the liquidus temperature that the profile is built around. A ramp below about two or three degrees per second is typical, a soak between about one hundred and fifty and one hundred and eighty degrees for a minute or so is common, the time above liquidus is usually between forty five and ninety seconds, and the cooling rate is usually limited to a few degrees per second. The exact values are properties of the material, and the profile is developed against them rather than against a general rule.

How The Measurement Is Made

A profile is taken with fine gauge thermocouples attached to the board. The attachment matters more than the instrument: a bead held down with tape is lifted by the air flow and reads low, while a bead secured with a small amount of high temperature adhesive and covered with a spot of cement reads the surface it is touching. The wire itself conducts heat away from the junction, so a thin wire and a short run along the board reduce the error.

The sensors belong where the process is most difficult. That usually means under the largest thermal mass, under a package that shields its own solder joints, at the corner of the board, and on a small component that heats quickly. The difference between the hottest and the coldest location on the board is the figure that decides whether the profile is acceptable, because the solder at each location has to see enough heat to wet and not so much that a component is damaged.

Why The Board Differs From The Oven Setting

The board absorbs heat and the components absorb it at different rates, so the temperature of the surface lags the air around it. A large metal connector or a stiffener takes longer to reach the setpoint than a thin area of laminate, and a component body shades the joints beside it from the convection of the oven. The result is a spread of temperatures across the board at any instant, and a profile that satisfies the coldest joint may overheat the hottest one.

The conveyor also matters. A board that is placed on the belt in a different orientation sees a different air flow, and a board that is preceded by a large panel can be affected by the thermal load that was added by that panel. The profile is therefore taken with a board that represents the product, in the same orientation, and ideally with the same load on the conveyor.

Criteria And Compromises

The profile has to satisfy two conditions at the same time. The lowest temperature on the assembly has to be high enough that the solder at that point reaches a proper reflow, and the highest temperature has to stay below the limit of the most sensitive component and of the laminate. Where those two constraints cannot both be met, the answer is usually to reduce the mass, to change the oven’s air flow, or to modify the assembly rather than to accept a profile that damages a component.

A common practice is to profile the worst case assembly: the version with the largest thermal mass and with the component that has the lowest temperature rating. If the profile passes that case, the smaller and lighter assemblies will also be inside the window, although they may be closer to the upper limit than expected, which is why a light assembly is sometimes profiled separately.

Sources Of Error

The most common error is the attachment. A thermocouple that is not in direct contact with the surface reports the temperature of the air between them, and the reading can be several degrees low, with a corresponding error in the peak temperature and in the time above liquidus. A second error is the calibration of the recorder and of the thermocouples themselves, which drifts with use and with the number of thermal cycles the probe has seen.

A third error is the sample. A profile taken on a bare board predicts nothing about a populated assembly, and one taken on a board that is not the current revision may reflect a geometry that no longer exists. A fourth is the number of channels: a profile taken at a single location describes one point and cannot reveal the spread across the assembly, which is often the parameter that fails.

Documentation And Control

Each product should have a recorded profile with the sensor positions marked on a drawing, the acceptance limits stated in terms of peak, time above liquidus, and spread, and the oven settings that produce it. The profile is re-verified when anything changes: a new paste, a new component, a heavier or lighter board, a new oven, or a maintenance event, and the verification is part of the setup rather than a periodic exercise.

The oven itself is monitored separately. Zone temperatures, conveyor speed, and the atmosphere are checked against the settings, and the machine is calibrated at an interval that the process requires. The two records together, the profile of the board and the condition of the oven, are what make the process reproducible rather than merely repeatable. The alloy context is described under lead free and leaded soldering, the assembly consequences under component shift during reflow, and the fabrication context under PCB design and fabrication.

FAQ

Why not simply use the oven settings? Because the board temperature lags the air and varies across the assembly. The settings describe the oven, while the profile describes what the solder actually experiences.

How are thermocouples attached? With a small amount of high temperature adhesive and a spot of cement over the junction, so the bead touches the surface. Tape alone lifts the bead and produces a reading that is too low.

How often should a profile be verified? At the start of a production run, after any change to the product or the process, and after maintenance on the oven. A profile is a setup parameter rather than a periodic inspection.