Thermal Process of Low-temperature Cofired Ceramics
12/01/2003
HOT PRODUCT IN CONTROL
BY MARYBETH ALLEN
Low-temperature cofired ceramics (LTCC) are finding increasing application in standard components such as multilayer chip capacitors, which are manufactured in high volume today. Other popular LTCC parts include modules for high-frequency radio frequency and wireless components, as well as Bluetooth, wireless fidelity and local area networks. The parts are also finding growing use in biomedical and automotive applications.
The majority of the world's production of LTCC is done in box furnaces. When firing in a box furnace, there may be several processes for one batch that provide the flexibility for a slow ramp rate and a fast sintering process. However, this cycle can be quite long, often requiring several days, and limits the throughput of larger substrates. One way to increase volume is to move the thermal process from batch to conveyor furnaces, which is especially desirable for larger substrates but can result in extremely lengthy furnace time.
A high level of precision is required during the entire manufacturing process of LTCC, including the thermal process. In the initial stages of ramp up, the thermal process window is tight. Process controls during burn out are critical. A slow ramp for green-tape systems must be maintained so that organics and binders burn out before the sintering stage. Depending on the system used, this binder burnout occurs at 200 to 450°C. Improper burnout prior to sintering causes severe problems, including bubbles, blisters and delamination.
The sintering process usually requires less time than the initial ramp stage. In this process, the temperature is increased from 450°C to as much as 875°C, where it is held for 20-30 minutes. While the product is sintering, it is shrinking and this shrinkage must be controlled. Uneven and inconsistent shrinkage between materials or within the part causes movement and misalignment — resulting in a trapezoidal or deformed part. The cooling stage is critical with parts larger than 8 inches. A 10-inch-long part running at a belt speed of 0.5-inch per minute at 2°C/minute can have a 40°C variance from front to back. The same part, running at the same belt speed with a ramp of 5°C/minute will have a 100°C variance from front to back.
To address shrinkage problems associated with firing larger parts, one company* developed a near-zero x-y shrinkage material that only shrinks in the z-axis. This enables higher volume manufacturing of larger parts in a conveyor-type furnace (Figure 1).
 Figure 1. The 11 x 7-inch part was cut in half on the diagonal, and one half was fired. The fired lower half shows no shrinkage when compared to the green, upper half. |
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High quality must be achieved among and within batches. The first step is to develop a process with an optimized profile. Within this batch there must be tight consistency, with all parts seeing the identical profile. The second step, process control, ensures tight consistency among the batches with the ability to address and fix problems.
Process Development
Defining the process window. The profile for LTCC must be precise.
Measuring results. Critical times and temperatures across the belt must be measured on the parts at various locations to determine if the products are being manufactured within the specific process window. It is necessary to send a trailing wire through the furnace, which is possible during production of a full load with a multi-channel thermocouple harness and a real-time thermal (Figure 2). The resulting profile accurately shows the actual product results of time vs. temperature, and this method also allows back-to-back profiles.
Improving the process. LTCC's small process window makes optimizing the setup critical, allowing for any furnace shifts or changes. By keeping cross belt uniformity down, a narrow bell curve is achievable. Finding this narrow bell curve may be achieved with an automated profile prediction tool — sophisticated software capable of generating and evaluating thousands of potential furnace recipes to determine the optimal profile in a matter of minutes. With critical thermal process specifications entered, the automated prediction tool ranks recipes on the basis of how much of the process window is used. This allows users to choose an appropriate profile. A profile that will process product without exceeding the critical process statistics is defined as being inside the process window.
 Figure 2. Multi-channel thermocouple harness and real-time thermal profiler. |
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Process Control
LTCC requires substrate-to-substrate monitoring and documentation. Technology for high-temperature applications like thick film and LTCC has improved, but there are still differences from lot to lot. Sampling and documentation data from lot to lot is time-consuming and expensive. Good sampling requires many samples and only affords an after-the-fact reaction. Continuous monitoring automates this process.
Continuous monitoring is a simple and effective way to reduce the number of costly and time-consuming confirmation profiles significantly. A real-time thermal manager has 30 thermocouples permanently mounted in two probes, each containing 15 internal thermocouples at the conveyor level in the furnace. These sensors continuously monitor process temperatures along the full length of the conveyor, taking readings at rates of one thermocouple every 5 seconds. Temperatures are continuously displayed as "process profiles" on a PC running a Windows-based software package, while data is permanently recorded on the hard disk. During production, any temperature drift and its location are instantly visible. The thermal manager is outside the oven control loop, so it can reveal critical process temperature deviations that are often hidden from the oven control.
The continuous supply of data is correlated to the original baseline profile and a relationship is calculated between process temperature as read by the probes and product temperature — establishing a virtual profile. During production, the software continuously calculates how the virtual profile has changed on measured changes in process temperature from the probes permanently located in the oven. A virtual profile is automatically updated every 30 seconds and recorded so that it can be viewed live or provide data history. If the virtual profile ever falls outside user-defined limits, an alarm will show on the screen. If an alarm relay option is used, the alarm feature sounds an audible alarm, activates a light or shuts down the conveyor feeder to ensure that product never enters the furnace unless the product profile meets specifications. As process temperature drifts over hours, days or weeks, the virtual profile calculates how the changes will affect the product thermal profile. Virtual profiling, in theory, is as close as you can get to attaching a thermocouple to every product manufactured in the furnace.
The LTCC process in a conveyor-type furnace, while potentially shorter than in a box furnace, is still a lengthy process. Data collected during the process can be used to recognize furnace changes that may cause the product to fail to meet specifications. This preventive ability is more critical when accomplished by alarming on warning conditions or changes that are close to the edges of process windows, indicating that continuing in this mode may result in defective products. The use of live data can also be delivered via an intranet or the Internet to offsite locations for remote process monitoring.
Conclusion
LTCC in a continuous furnace can significantly increase production throughput by using a recipe to achieve a profile within an extremely small process window. A product being processed can be monitored continuously to ensure that any drift in the process chamber is immediately identified — allowing preventive measures to ensure no defects in this process. This type of monitoring operation can be enhanced with a thermal management system and virtual profiling.
MARYBETH ALLEN, North America sales manager, may be contacted at KIC, 15950 Bernardo Center Dr., #E, San Diego, CA 92127; (858) 673-6050; e-mail: [email protected].
*Heraeus