NeoDen IN6 Internal Sensors vs. External Sensors: How to Obtain the Most Accurate PCB Temperature?
Introduction
When debugging reflow soldering processes, many engineers simply observe the temperature displayed on the equipment. For example, if a temperature zone is set to 200°C, they assume the PCB is already in a 200°C environment. In reality, however, from the moment a PCB enters the reflow oven, it goes through various stages, including preheating, isothermal holding, reflow, and cooling. The actual temperature of the PCB is also influenced by factors such as board thickness, copper area, component density, conveyor speed, and hot air circulation. Therefore, if the goal is to determine whether the solder paste has completed the reflow process as required, the focus should truly be on the actual temperature profile of the PCB.
The NeoDen IN6 features a 6-zone, full hot-air convection design and supports measuring the PCB’s actual temperature via temperature sensors. Understanding the difference between the internal temperature sensors of the equipment and external temperature measurements on the PCB is a crucial step in correctly setting reflow soldering parameters.
Why does the temperature displayed by the reflow soldering machine not equal the PCB’s actual temperature?
First, it is necessary to distinguish between three concepts: heating unit temperature, chamber temperature, and the PCB’s actual temperature.
The heating system inside the reflow soldering machine is responsible for generating heat, and the temperature control system monitors the heating process via sensors to maintain each temperature zone as close as possible to the setpoint. However, once a PCB enters the oven, it does not instantly reach the temperature displayed by the equipment.
The PCB itself needs to absorb heat, and the rate of heat absorption is influenced by factors such as material, thickness, copper area, and component distribution.
For example, a PCB with a larger copper area may absorb more heat than a PCB with a simpler structure. Even when using exactly the same zone settings, the actual temperature profiles of two PCBs after passing through the reflow oven may differ.
The NeoDen IN6 user manual explicitly states that different PCBs have varying rates of heat conduction and heat absorption capabilities, thus requiring different heating times and heat inputs. Therefore, while equipment temperature parameters can serve as the basis for process control, they cannot be simply equated with the actual temperature of the PCB.
Why Do Different PCBs Require Different Reflow Soldering Temperature Profiles?
1. PCB thickness, copper area, and component density all affect the actual temperature.
A set of temperature zone parameters that works well on one PCB may result in the following issues when applied to a different board:
- Insufficient solder paste melting.
- Excessively high peak temperatures.
- Locally low temperatures on the PCB.
- Inconsistent solder joint quality.
- Thermal damage to the PCB.
The cause does not necessarily lie with the equipment itself.
The PCB’s dimensions, thickness, copper area, number of components, and layout all alter the heat transfer process. Therefore, parameters from one PCB cannot simply be copied verbatim to another.
The NeoDen IN6 features a total of 6 heating zones (3+3, top and bottom), with a temperature range from room temperature to 300°C, and adjustable conveyor speed. For R&D and small-batch SMT production that require frequent product changes, this multi-zone temperature control capability allows for the establishment of appropriate process windows for different PCBs.
What is the difference between internal temperature sensors and external PCB sensors?
1. What is the primary function of internal sensors?
The temperature sensors inside a reflow soldering machine primarily serve the equipment’s own temperature control. They help the system understand the current temperature status of the heating zones and control the heating process based on preset values.
Internal sensors are crucial for the normal operation of the equipment. Without stable temperature feedback, the temperature zones cannot be controlled according to the set parameters. However, if an engineer wants to verify the actual temperature experienced by the PCB, further measurement of the PCB’s actual temperature is required.
2. How Do External Temperature Sensors Measure the Actual Temperature of a PCB?
The concept behind external temperature sensors is straightforward: install the temperature sensor at a designated location on the PCB, then allow the PCB to pass through the reflow oven as normal. The data recorded in this way represents the temperature changes experienced by that PCB during the actual production process.
Temperature measurement points can be selected based on the PCB’s structure and process requirements, such as:
- The center of the PCB.
- The edge of the PCB.
- Large copper areas.
- Near components with high thermal mass.
- Solder joints requiring special verification.
The NeoDen IN6 user manual supports connecting a temperature sensor to the PCB to measure its actual temperature, thereby helping to establish a reflow soldering temperature profile.
What should you focus on when examining a PCB reflow soldering temperature profile?
A complete temperature profile should not be evaluated based solely on a single peak temperature.
1. Heating Rate
If the heating rate is too fast, it may increase the thermal shock experienced by the PCB and components, and may also cause the flux to volatilize too quickly.
The NeoDen IN6 user manual recommends limiting the heating rate to approximately 1°C/s before reaching 160°C, and notes that excessive heating rates may cause damage to the PCB and components, PCB warping, and premature flux volatilization.
Therefore, when measuring temperature, one should not focus solely on the final peak value but should also observe whether the initial portion of the curve is too steep.
2. Peak Temperature
Peak temperature is one of the parameters engineers are most familiar with, but it is also the one most often used in isolation.
Solder paste suppliers typically provide recommended reflow profiles. Actual process settings should be based on the requirements of the solder paste being used, rather than assuming that higher temperatures are always better.
The NeoDen IN6 supports temperature settings ranging from room temperature to 300°C and allows for adjustment of the reflow process through six heating zones.
3. High-Temperature Dwell Time
If the peak temperature is normal but the PCB remains in the high-temperature zone for too long, this can also increase the thermal load.
Therefore, when analyzing an actual temperature profile, both temperature and time should be considered together.
4. Cooling Process
The reflow process is not complete once the peak temperature is reached. The PCB then enters the cooling phase, and a complete temperature profile should include this process as well.
Therefore, when establishing a process profile, the entire process should be considered, rather than just a segment of it.
What are the common causes of unstable reflow oven temperatures?
1. Why do different PCBs exhibit different profiles even with the same zone settings?
First, check for differences in the PCBs themselves.
If the board thickness, copper area, or component layout has changed, you cannot assume that the original parameters still apply.
Solution: Re-measure the actual temperature of the PCB.
The NeoDen IN6 provides ample room for process adjustment through its 6-zone temperature structure.
2. Why do temperatures differ between the center and edges of a PCB?
Different areas of a PCB absorb heat at different rates, and combined with variations in the PCB’s structure itself, temperature differences between the center and edges are not uncommon.
In such cases, the issue must be assessed based on actual temperature measurement points, rather than relying solely on the temperature displayed on the equipment panel.
The NeoDen IN6 uses full hot-air convection, the temperature distribution deviation specified in the product documentation is ±1°C, with a lateral temperature difference of less than 2°C.
3. Why do solder joints still have issues even though the equipment displays normal temperatures?
If the equipment temperature display is normal but the actual soldering results are abnormal, we recommend further inspection of the PCB temperature profile.
You may need to re-evaluate:
- Heating rate.
- Peak temperature.
- Dwell time at high temperature.
- Temperatures at different locations on the PCB.
This is because the equipment displays its own status, whereas the solder joints are ultimately affected by the actual thermal process of the PCB.
Internal Sensors vs. External Sensors: Which Should You Choose?
| Comparison Items | Internal Sensor | External PCB Temperature Sensor |
| Primary Measurement Target | Equipment/Oven Temperature Status | Actual PCB Temperature |
| Primary Purpose | Equipment temperature control | PCB process validation |
| Does it directly measure the PCB? | No | Yes |
| Is it suitable for establishing a PCB temperature profile? | Limited | More direct |
| Is it suitable for equipment temperature control? | Yes | Not primary purpose |
| Can they be used interchangeably? | No | No |
Therefore, the truly correct answer is not “which is better: internal or external sensors?” Rather, the two serve different purposes. Internal sensors ensure the reflow oven operates according to settings, while external sensors help engineers verify the actual temperature profile the PCB has undergone.
Conclusion
The most common misconception in reflow soldering temperature management is confusing equipment temperature with PCB temperature. What truly affects solder paste reflow and joint formation is the actual temperature profile experienced by the PCB:
heating rate, temperature hold period, peak temperature, dwell time at high temperature, and cooling process.
Therefore, when introducing a new PCB, changing solder paste, or encountering abnormal soldering results, simply observing the temperature displayed by the equipment is insufficient. Only by directly measuring the PCB with temperature sensors and establishing an actual temperature profile can reliable data be provided for subsequent process adjustments.
The NeoDen IN6 features a 6-zone hot-air convection design, supports temperature settings from room temperature to 300°C, conveyor speeds of 5–30 cm/min, and enables testing of the PCB’s actual temperature profile via temperature sensors.
For PCB prototypes, R&D validation, and small-batch SMT production, this means engineers can not only control the reflow soldering equipment but also further verify the actual thermal process the PCB undergoes.
