Small Reflow Ovens: Why Do PCBs Keep Getting Solder-Coated? 5 Common Misconceptions
Introduction
For companies just starting SMT production, small reflow ovens are often among the easiest pieces of equipment on the entire SMT production line to operate—yet they are also the most prone to operator error.
The reflow process is influenced by temperature, time, conveyor speed, PCB structure, and solder paste characteristics. The NeoDen IN6 user manual also clearly states that different PCBs have varying rates of heat conduction and heat absorption, thus requiring different heating times and heat input levels.
Therefore, when encountering solder paste smearing issues, rather than simply lowering the temperature, it is better to first identify which process parameter is causing the problem.
This article summarizes the five most common misconceptions regarding small reflow ovens based on the NeoDen IN6 user manual and provides corresponding troubleshooting methods.
Misconception 1: Higher Reflow Temperatures Lead to Better Soldering Results
When some operators notice that certain solder joints have not fully melted, they may immediately increase the temperature in that zone. In reality, reflow soldering oven is not simply about achieving high temperatures; rather, it involves ensuring the PCB follows an appropriate temperature profile to complete the preheating, hold, reflow, and cooling phases.
The NeoDen IN6 user manual divides the entire process into heating, hold, soldering, and cooling stages. Once the PCB enters the heating zone, the solvents and gases in the solder paste gradually evaporate; it then enters the hold zone for thorough preheating, followed by the reflow zone to melt the solder paste, and finally the cooling zone to solidify the solder joints.
Therefore, simply raising the temperature in a single zone will not resolve all soldering issues.
1. Why can excessively high temperatures cause PCB damage?
If the temperature rises too quickly, or if the peak temperature exceeds the tolerance range of the PCB and components, thermal stress may increase. To determine whether the reflow process is “too hot,” one cannot rely solely on a single temperature reading; instead, the following factors must be considered together:
Rate of temperature rise + Peak temperature + Reflow time.
2. Correct Approach: Establish a Process Window Based on the Solder Paste’s Recommended Profile
Recommended temperature profiles may vary depending on the solder paste. A more reasonable approach is to first refer to the temperature curve provided by the solder paste supplier, then measure and adjust the temperature based on the actual PCB, rather than blindly increasing the temperature whenever cold solder joints occur.
The NeoDen IN6 features a 6-zone design—3 upper zones and 3 lower zones—with a temperature range from room temperature to 300°C, allowing for adjustments to the heating process at different stages.
Misconception 2: Focusing Only on Temperature, Not Conveyor Speed
If temperature determines “how hot” a PCB gets, then conveyor speed determines “how long” the PCB remains in that high-temperature environment.
For example, when a PCB has insufficient solder joints, some operators may keep raising the temperature while overlooking the fact that conveyor speed also affects the PCB’s heat exposure.
1. What happens if the conveyor speed is too slow?
The longer a PCB remains inside the reflow oven, the longer it is exposed to heat.
The NeoDen IN6 user manual clearly states that the PCB’s heating temperature can be adjusted by modifying the conveyor chain speed: reducing the conveyor chain speed increases the PCB’s heating temperature, while increasing the speed lowers it.
Therefore, if a PCB shows signs of significant overheating, discoloration, or even localized damage, you should check the conveyor speed rather than focusing solely on the temperature zones.
2. How should temperature and speed be coordinated?
A relatively reliable approach is:
First, determine the initial temperature and speed → preheat the equipment → test the PCB → review the temperature curve → then make adjustments.
If the temperature curve indicates that the PCB is not heating evenly across its surface, you can appropriately reduce the conveyor speed or adjust the temperature zones.
If the temperature is too high, you can increase the conveyor speed or lower the setpoint for the corresponding temperature zone.
The NeoDen IN6 user manual also recommends that when adjusting parameters, you should not change too many factors at once; instead, make adjustments gradually and retest after each change.
Misconception 3: All PCBs Use the Same Reflow Soldering Parameters
This is a particularly common issue in small-batch SMT production.
Many factories may produce control boards one day, communication modules the next, and yet another type of PCB the day after that. If they simply reuse the temperature zone and conveyor speed parameters from the previous product every time, problems may arise.
The reason is simple:
PCBs are not entirely identical “heating objects.”
1. Why do PCB results differ even with the same temperature settings?
The following PCB characteristics:
- Dimensions.
- Thickness.
- Material.
- Copper area.
- Component density.
- Component distribution.
all affect heat transfer.
The NeoDen IN6 user manual explicitly states that different PCBs have different heat conduction rates and heat absorption capacities, and therefore require different heating times and heat input levels.
2. What should be done when introducing a new PCB?
We recommend the following process:
Initial parameters → Actual PCB temperature measurement → Temperature curve analysis → Parameter fine-tuning → Temperature measurement again
The NeoDen IN6 supports saving conveyor speed and temperature settings for each zone, and previously saved parameters can be recalled using the LOAD function.
For small-batch, multi-model production, the practical value of this feature goes beyond mere “convenience”. It allows for the gradual establishment of specific process parameters for different products.
For example:
PCB A → Profile A
PCB B → Profile B
PCB C → Profile C
When producing the same products again in the future, you can retrieve the corresponding parameters and fine-tune them based on actual conditions.
Misconception 4: Relying Solely on the Reflow Oven’s Displayed Temperature Without Measuring the Actual PCB Temperature
Many operators assume that because the equipment displays 200°C, the PCB is operating in a 200°C environment.
In fact, the NeoDen IN6 user manual clearly distinguishes between several different temperature concepts.
- Heating Unit Temperature refers to the real-time temperature of the heating plate.
- Furnace Temperature refers to the temperature of the air between the conveyor belt surface and the heating plate.
- PCB Surface Temperature refers to the temperature at the component lead locations during the PCB soldering process.
The temperature displayed on the equipment panel primarily indicates the actual temperature of the heating unit, which is not equivalent to the furnace chamber temperature or the actual PCB temperature. The manual also notes that the displayed temperature may be approximately 20–40°C higher than the air temperature inside the furnace, and the actual temperature is further influenced by conveyor speed, PCB size, thickness, material, and component density.
1. How can you determine if a PCB is actually overheating?
The most direct method is to perform an actual PCB temperature profile test.
The IN6 provides a TEMP SENSOR interface.
Following the instructions in the manual, connect the temperature sensor to the interface and secure it to the PCB. Then, run the PCB through the reflow process as normal to obtain the temperature curve on the Graph interface.
This way, engineers will no longer be limited to asking:
“What temperature was the oven set to?”
but will instead be able to ask:
“What temperature changes did this PCB actually experience?”
This provides more meaningful insights for diagnosing issues such as PCB discoloration, overheating, and insufficient reflow.
Misconception 5: Neglecting Preheating and Equipment Operating Conditions
While the parameters themselves may not have any obvious issues, production begins immediately after the equipment is turned on. This can also cause the actual process conditions to differ from those during normal production.
1. Reflow Soldering Preheating Process
The NeoDen IN6 user manual further recommends in the temperature zone settings section that, after setting the initial temperature and conveyor speed, the equipment should be preheated for approximately 25 minutes before allowing the PCB to pass through the reflow system, once the temperature has stabilized.
Therefore, the correct procedure should be:
Power on → Set parameters → Preheat → Confirm temperature stability → PCB test → Full-scale production.
There is also a green indicator bar at the equipment inlet that shows whether all temperature zones have reached their set temperatures. This indicator will light up once the actual temperatures in all zones match the set values.
This serves as a simple yet practical reference for daily operations.
2. The PCB has “burned”—how should this be troubleshot?
If the PCB shows obvious discoloration or signs of overheating, it is not recommended to immediately adjust all parameters.
Instead, troubleshoot step by step in the following order.
Step 1: Check Zone Temperature Settings
Confirm whether any zone is set too high.
Pay particular attention to the temperature in the reflow zone.
Step 2: Check Conveyor Speed
Confirm whether the PCB is spending too much time in high-temperature zones due to an excessively slow conveyor speed.
Step 3: Check the actual temperature profile
Do not rely solely on the equipment’s display readings.
Connect a temperature sensor to the PCB to obtain the actual temperature profile.
Step 4: Check for PCB differences
If previous batches were problem-free but the new PCBs are experiencing “burned-out” issues, verify whether there are significant differences between the two PCBs in terms of dimensions, thickness, material, copper area, and component density.
Step 5: Check the solder paste recommended profile
Confirm whether the current temperature settings comply with the process requirements of the solder paste being used.
Step 6: Adjust Only One Key Parameter at a Time
If you change multiple parameters—such as temperature zones and conveyor speed—simultaneously, it will be difficult to determine which one is actually effective, even if the results improve.
Therefore, a more reasonable approach is:
Adjust one parameter → Re-measure the temperature → Compare the curves → Then decide on the next step.
The manual also recommends that beginners make adjustments in small increments.
How Does the NeoDen IN6 Help Reduce the Risk of Solder Paste Clogging in Small-Batch SMT?
1. 6 Temperature Zones Make Temperature Curves Easier to Adjust
The NeoDen IN6 features 3 upper temperature zones and 3 lower temperature zones—a total of 6 heating zones—and is equipped with 1 cooling zone. The temperature range extends from room temperature to 300°C.
For small-batch production, the value of multiple temperature zones lies in the ability to adjust settings for different stages based on the PCB’s actual temperature profile. This makes it easier to establish a controllable process curve than simply relying on a single temperature zone to complete the entire heating process.
2. Full Hot-Air Convection Ensures Continuous Heat Transfer to the PCB
The IN6 employs a Full Hot-Air Convection design and is equipped with high-sensitivity temperature sensors, allowing temperatures to be stabilized within a range of ±0.2°C.
It is important to note that ±0.2°C describes the stability of the equipment’s temperature control; it does not mean that the actual temperature at every point on the PCB deviates by ±0.2°C from the set value.
The actual PCB temperature still needs to be verified using temperature sensors.
This is why “equipment temperature control” and “actual PCB temperature measurement” should be understood separately.
FAQ
Q1. Why does a small reflow soldering machine burn the PCB?
A. This may be related to excessively high peak temperatures, too rapid a temperature rise, an excessively long reflow time, or a conveyor speed that is too slow. You cannot diagnose the problem simply by lowering the temperature in a single zone; we recommend analyzing the actual PCB temperature profile.
Q2. Is a higher temperature always better for reflow soldering?
A. No. The temperature must be matched to the solder paste melting point, the recommended reflow profile, and the thermal tolerance of the PCB and components. The IN6 user manual also explicitly warns that temperature adjustments must not exceed the thermal tolerance of the PCB and components.
Q3. Can a slow conveyor speed in a reflow oven cause solder paste to stick to the board?
A. It is possible. A reduced conveyor speed extends the PCB’s dwell time in the heating zone, thereby increasing the degree of heat exposure. The NeoDen IN6’s conveyor speed range is 5–30 cm/min and can be adjusted based on the actual temperature profile.
Q4. Why does the same reflow oven require parameter readjustment for different PCBs?
A. Because different PCBs vary in size, thickness, material, copper area, and component density, resulting in differences in heat conduction and heat absorption capabilities.
Q5. What types of SMT production is the NeoDen IN6 suitable for?
A. In terms of equipment specifications and operation, the IN6 features a desktop design, a 260 mm soldering width, a conveyor speed of 5–30 cm/min, and a 6-zone temperature control system. It is best suited for PCB prototyping, R&D, and small-batch SMT applications that require flexible adjustment of the reflow process.
Conclusion
For PCB prototyping, R&D, and small-batch SMT production, the value of the equipment lies not merely in “melting the solder paste,” but in helping engineers establish a reflow process that can be tested, adjusted, and repeated.
If you are currently using a small reflow oven but are experiencing issues such as PCB discoloration, overheating, insufficient solder joints, or difficulty controlling the temperature profile, you may want to further evaluate your reflow parameters based on PCB size, thickness, solder paste type, and production requirements.
Want to know if the NeoDen IN6 is right for your PCB and SMT production needs? Feel free to contact NeoDen for product specifications, technical documentation, or a quote.
