Roller-Hearth Furnace for Ceramic Capacitors
Today’s electronic equipment would not exist without the presence of multilayer ceramic capacitors (MLCC). For example, a personal computer utilizes approximately 700 MLCC, a liquid crystal TV has approximately 500 and a car navigation system uses approximately 1,000.
MLCC are a key circuit component of electronic devices, and hundreds of MLCC are required for these devices to function. With the growth of mobile devices such as smart phones, MLCC are required to be smaller, have higher capacitance and be less expensive.
As shown in Fig. 1, a typical structure of MLCC consists of many alternate layers of BaTiO3, (a dielectric substrate) and Ni, which is a metallic electrode layer. This gives MLCC high reliability and a compact size when compared to an electrolytic capacitor. In the 1990s, the material of the inner electrode was changed from Ag-Pd to Ni. This utilization of Ni required a rapid heat up in the sintering process to compensate for the differences in shrinkage between the dielectric and the nickel. The ability of a furnace to achieve this high rate of temperature rise has now become an important parameter of MLCC manufacturing technology.
MLCC manufacturing has traditionally used pusher-type tunnel kilns and elevator-type furnaces (Fig. 2), which are both commonly used around the world for sintering. With the incorporation of Ni as the electrode layer, however, use of these furnaces to fire MLCC tends to result in delamination between the dielectric and electrode layers due to their slower heat-up rates. To compound the problem, MLCC are becoming smaller to satisfy the demands of the mobile market. The smaller sizes make the delamination problem even worse. In response, Tokai Konetsu Kogyo Co. Ltd. (TKK) has developed a new type of roller-hearth furnace that will be presented in this article.
The typical roller-hearth furnace (Fig. 3) is built with roller conveyers placed regularly from the entrance to the exit. Rollers are made of metal or ceramic with the driving system set outside of the furnace. Each roller is turned by a belt or chain and moves the product being sintered from entrance to exit at a constant speed. Inside, heating elements are positioned above and below the rollers. The hottest elements will be at the center of the furnace and cooler ones toward the ends. The heating rate is controlled by the speed of the rollers, and the time at soak is determined by the number of elements at peak temperature.
Rapid Heat Up
A conventional furnace such as a pusher-type tunnel kiln is shown in Fig. 4. These furnaces require boats to carry the product (stacked on multiple setters) pushed up against each other to force the product through the kiln. These boats are usually structural ceramics with a wall thickness of 30 mm. To heat up the stacked products on aboat to 1200°C (2192°F), approximately 25 KWH of energy is required in a conventional furnace. The roller-hearth furnace does not require such boats, so the products are placed in a single layer on the rollers. The consequence is that the same heat-up rate of the product to 1200°C requires only 12 KWH of energy. This lower thermal mass also enables a roller-hearth furnace to achieve heat-up rates of more than 100°C per minute. This is one of the major advantages of the roller-hearth furnace and is critical in processing MLCC.
Figure 5 shows the temperature comparison between the roller-hearth furnace setpoint and the actual temperature of the product at a heating rate of 70°C per minute. The temperatures of the product were measured by thermocouples that traveled with the product. They followed the furnace setpoint very closely due to the high heat-transfer ratio of the roller-hearth furnace.
As mentioned above, new MLCC are thinner with more layers to achieve better performance, which in turn presents a higher risk of delamination during the sintering process. Figure 6 shows the shrinkage pattern of the dielectric substrate (BaTiO3) and inner electrode (Ni) in the sintering-temperature range. Because of the difference in material, there is a difference in shrinkage at each temperature between the layers while the entire piece of MLCC is being sintered.
The Ni layer begins sintering first at the lower temperature range, with rapid shrinkage starting at 400°C (752°F). The BaTiO3 layer doesn’t begin to shrink until the temperature reaches more than 800°C (1472°F). If too much time is spent below 800°C, only Ni shrinks, which will cause delamination and/or cracks between the Ni layer and the dielectric layer. Figure 6 also shows cross sections of MLCC that were sintered with a 45°C/minute heat-up rate by the conventional batch furnace and MLCC sintered at a 130°C/minute heat-up rate in a TKK roller-hearth furnace.
From the photo, it is clear that delamination is occurring between the metal and dielectric layers in the batch kiln product using a 45°C/minute heating rate. If the continuity of the electrode layers is disrupted, it impacts the electrostatic capacity of the MLCC, and the desired capacitance cannot be obtained. To prevent this kind of problem, rapid heat-up sintering is critical. As MLCC become thinner and thinner, the heating rate becomes more and more important. A roller-hearth furnace can achieve almost any heating rate, so it is an ideal furnace for MLCC production.
For sintering furnaces, the temperature uniformity inside the furnace is one of the most important requirements. Minimum variation of temperature within the furnace hot zone will promote a stable sintering process and attain higher productivity. To achieve this, TKK’s roller-hearth furnace developed for MLCC sintering has 24 controlling circuits in a 3.6-meter (11.8-feet) furnace length. The 3.6-meter furnace is divided by 12 circuit controls, the upper half and lower half separate at each station. Each circuit has two to four silicon carbide (SiC) heating elements, a thermocouple and a temperature controller to monitor and adjust the furnace temperature precisely to the desired setpoint.
The products are stacked in the conventional pusher-type tunnel kiln, and the middle-level part tends to have a lower temperature. It is typical that the variation of the temperature within the furnace chamber is +/-3°C. For the roller-hearth furnace with single-layer product placement, the difference in temperature was measured to be only 1°C in a recent test. Figure 7 shows the temperatures and location at 10 points and a photo of the setup in the furnace used for this test.
With a wider roller-hearth furnace, the temperature around the middle may be higher than the two sides, which will affect the furnace chamber temperature uniformity. In this case, SDL-type* SiC heating elements are recommended because they have a non-heating zone in the center of the rod. This cool center zone will create a more uniform heat distribution within the wide furnace, which has been shown to be within 2°C variation.
In addition to the temperature-related controls, the atmosphere in the furnace chamber also influences the quality of MLCC. Organic binders are used to fabricate MLCC, and they need to be fully removed in the furnace at low temperatures. It has been found that complex atmospheres consisting of hydrogen, nitrogen and steam are required to remove the binder without oxidizing the metallic electrodes. It is critical to accurately control the amount of each gas introduced and to adjust these amounts as needed to maintain a specific atmosphere inside the furnace.
The first step in atmosphere control is to fabricate an air-tight envelope. TKK has developed highly sealed metal furnace shells for their roller-hearth furnaces. Not only is the hot zone in a sealed envelope, but the roller drive mechanism is also sealed in a secondary envelope (Fig. 8).
The second step is to control the gases being introduced. Mass-flow controllers, oxygen and hydrogen analyzers, and dew-point meters are often installed on the furnace to control and accurately adjust the gas mixture. These are then monitored through electronic sequence controllers. The result is a highly controlled atmosphere that can be continuously monitored and updated to maintain such parameters as the moisture level inside the furnace.
Successful processing of multilayer ceramic capacitors (MLCC) requires a special type of sintering furnace. The furnace must:
• Be capable of rapid heat-up rates (>100°C/minute) to the firing temperature.
• Achieve temperature uniformity of one or two degrees.
• Maintain a constant firing atmosphere by controlling at least three separate gases.
This article has shown that the roller-hearth furnace is capable of achieving and exceeding all three demands. In particular, TKK has developed a furnace with an air-tight shell and low thermal-mass rollers that can meet future, more stringent demands from the electronics industry. Multi-zone heat control, mass-flow controllers, gas analyzers and dew-point sensors all combine to optimize TKK’s roller-hearth furnace performance.
Mobile devices are always under development to be more compact/high performance, and MLCC to be used for these devices are required to be thinner with higher capacity. To satisfy more complicated requirements of newer MLCC, TKK’s sintering furnace needs to present improvements from a variety of areas of operation controls.
As a manufacturer of SiC heating elements and furnaces to support the heat-treatment industry since 1936, TKK is able to provide quality products and technical support to customers that are working on challenging tasks such as newer MLCC developments. IH
* TKK is a manufacturer of silicon-carbide heating elements under the EREMA brand, and this product is used in a variety of furnaces around the world. A standard rod-type SiC element has a “hot zone” in the center so that both sides are called “cold ends” with lower temperature for terminal connection. Type SDL has a cold zone at the center of the rod, dividing the hot zone into two parts plus normal cold ends at both sides.