Beginner-friendly! Basics of temperature measurement in MLCC manufacturing process and key points for reducing defects - Part 4

Typical failure example due to temperature anomaly

If temperature is not properly controlled during the MLCC manufacturing process, the following defects may occur:

crack

• Thermal shock cracks

  Ceramic materials can develop thermal stress and cracks during loading into or removing from the firing furnace, or due to rapid temperature changes within the furnace. This is mainly caused by rapid cooling or heating before and after firing.

• Cracks caused by differences in sintering shrinkage.

  If the sintering shrinkage rates of the dielectric layer and the internal electrode layer differ, an improper firing profile can cause internal distortion and lead to crack formation.

Delamination

This phenomenon occurs when laminated green sheets fail to properly adhere to each other during the debinding or firing process, causing them to peel apart. In particular, if organic matter is not sufficiently removed during the debinding process, or if the heating rate is too fast, gas generation can easily create gaps between the layers.

insulation failure and increased leakage current

• Under-firing/Over-firing

  If the firing temperature is too low, the ceramic will not densify sufficiently, and if it is too high, the crystal grains may grow abnormally, potentially leading to a decrease insulation properties of the dielectric layer or an increase in leakage current.

• Foreign object contamination

  Foreign matter can be mixed in during raw material blending or sheet molding, and when it burns out during firing, it can result in pinholes or voids (air pockets), leading to insulation failure.

• Electrode oxidation/reduction abnormalities

  If the firing atmosphere is not properly control, the internal electrodes may be oxidized or reduced, which can impair conductivity or cause defects at the interface with the dielectric.

How to use temperature measurement to reduce defects

To reduce these defects, it is important to accurately measure the temperature at each process and take appropriate action.

Real-time monitoring and anomaly detection

Using radiation thermometer and thermocouples, the ambient temperature of each heating furnace and the surface temperature of the products are monitored in real time. Deviations from the set temperature profile are automatically detected and alarms are issued, allowing for early detection and response to abnormalities.

Confirmation of uniformity of temperature distribution

Especially in large firing furnaces, it is crucial to check for any unevenness in the temperature distribution within the furnace. By placing sensors at multiple measurement points or visualizing the overall temperature distribution using thermography, hot spots and cold spots can be identified, which helps in adjusting the furnace and improving transport methods.

Data logging and traceability

By monitoring changes in temperature sensors over time and temperature rise trends in specific parts, we can detect signs of equipment malfunction and implement predictive maintenance, which allows for planned maintenance.

Application to predictive maintenance

If the firing atmosphere is not properly control, the internal electrodes may be oxidized or reduced, which can impair conductivity or cause defects at the interface with the dielectric.

Concepts for optimizing temperature profiles

To maximize the quality and productivity of MLCCs, it is necessary to continuously optimize the temperature profile of each process.

Understanding material characteristic

The starting point is a deep understanding of the material characteristic (thermal decomposition behavior, sintering start and end temperatures, thermal expansion coefficient, etc.) of the dielectric ceramics, internal electrodes, and binders used. Based on this data, an ideal temperature profile is designed.

Experiments and data analyzing

Through prototyping and small-batch production, we actually test various temperature profiles and analyzing the resulting product quality data (capacitance, voltage resistance, appearance, structure defects, etc.) and temperature data in detail. This allows us to find the optimal temperature conditions.

Adjustment of heating and cooling rates

In binder-free sintering process, a gradual heating rate is set to match the decomposition behavior of organic materials, thereby preventing delamination caused by gas generation. During cooling after sintering, a cooling rate is set that takes into account the material’s heat resistance shock to avoid cracks caused by thermal stress.

Integration with atmosphere control

In the firing process, not only the temperature profile but also the atmosphere, such as the oxygen concentration and moisture content inside the furnace, greatly influences the product characteristic. For example, as the internal electrodes become increasingly base metal, firing in a reducing atmosphere is common, but precise control of temperature and atmosphere in conjunction with the subsequent re-oxidation process is required.

Utilization of simulation technology

In recent years, efforts have been made to optimize processes efficiently by utilizing technologies such as thermal fluid analyzing and sintering simulation to virtually test various temperature profiles.

Practical advice for beginners

Temperature measurement in MLCC manufacturing process may seem complex to beginners. However, by understanding a few basic points, you can gain confidence in your work on-site.

Proper temperature measurement procedure

• Selection of measurement points

  • We understand the purpose of each process (e.g., removal of organic matter during binder removal, densification during firing) and select the temperature that has the greatest impact on that purpose (product surface, furnace atmosphere, conveying section, etc.) as the measurement point.
  • We focus our measurements on areas prone to defects and areas where temperature distribution is likely to be uneven.

• Selecting the right equipment

  • For non-contact, high-speed response radiation thermometer; for precise control of the furnace atmosphere, choose a thermocouple; and so on. Select the appropriate measuring instrument according to the object being measured and the purpose.
  • Check specifications such as measurement range, accuracy, and response speed.

• Equipment calibration and maintenance

  • We regularly calibrate our temperature measuring instruments to ensure that we always obtain accurate readings.
  • Dirt and deterioration of sensors can cause measurement errors, so we perform daily inspections and cleaning, and replace them as needed.

• Data recording and trend monitoring

  • Always record the measurement data, clearly indicating the date, time, location, and measurement conditions.
  • We will regularly check data trends to detect any abnormal fluctuations early on.

Useful checkpoints for the field

• Confirmation of uniformity inside the furnace

  • In firing and drying ovens, it is extremely important to ensure that the temperature distribution inside the oven is uniform. This should be checked not only at the oven entrance, center, and exit, but also considering temperature differences in the lateral direction.
  • We periodically run dummy products or test pieces through the furnace and record and evaluate their temperature profiles.

• Checking the heating and cooling curves

  • In particular, during binder removal and firing, we verify that the temperature changes according to the set heating and cooling curves. Deviating from the curve can directly cause defects.
  • Using data loggers that can record the actual temperature the product is subjected to will provide more accurate information.

• Management of atmospheric gases

  In the firing process, the type of atmospheric gas, flow rate, and dew point are also important. We verify that these factors, along with the temperature, are being properly controlled.

• Correlation between product status and temperature

  • If any abnormalities such as swelling, warping, or discoloration are observed in products during manufacturing, the temperature data from the previous process is compared with the current data to help identify the cause.
  • When a defective product occurs, we conduct a detailed analysis of the temperature data from its manufacturing history.

Common problems and how to deal with them

• Variation in measurement values

  • Causes: Sensor contamination, deterioration, unsuitable installation location, and uneven temperature distribution within the furnace.
  • Countermeasures: Regular cleaning and calibration of sensors, review of installation locations, and re-evaluation and homogenization of the temperature distribution inside the furnace.

• Temperature does not reach or exceeds the set value.

  • Causes: Malfunctions in the heater's malfunction and control systems, deterioration of the furnace insulation, and excessive exhaust volume.
  • Countermeasures: Inspect heaters and control panels, maintain the furnace, and optimize exhaust volume.

• Product crack occurrence

  • Causes: Rapid heating and cooling, thermal shock, and differences in sintering shrinkage.
  • Countermeasures: Review the heating and cooling rates, consider transport methods that minimize thermal shock, and readjust the temperature profile to match the material characteristic.

• Delamination occurs

  • Cause: Gas generation during binder removal, poor adhesion during lamination.
  • Countermeasures: Gradually increase the heating rate in the binder removal process, introduce aging treatment (to promote the diffusion and migration of organic constituent and remove poorly bonded areas), and review the lamination and compression conditions.

By incorporating these points into your daily work and improving the accuracy of temperature measurement and management, you can contribute to improving the quality of MLCCs and reducing defects.