Furnace and Muffle Designs Enhance Continuous Furnace Performance
Traditional continuous or conveyor-type thermal processing furnaces, such as hot-box, infrared (IR) and radiant heat designs, are widely used in industry because they offer flexibility to rapidly respond to different process requirements. Manufacturers continue to push for improved operating efficiencies including better atmosphere control and lower power consumption as the need to support a variety of processes involving different atmospheres and material loads increases. There are some limitations associated with thermal processing in traditional furnaces that need to be addressed to achieve better operating efficiency including:
- Use of inefficient light-gauge coiled wire heating elements in precast ceramic plates
- Use of insulation layers typically cut from preformed sheets and bonded together using ceramic adhesives, which have limited durability and have the potential to leak heat
- Use of lay-up construction, which requires disassembling a large section of the hot box to replace a failed heating element
- Use of a conventional crowned muffle having a large internal volume and predetermined gas injection and exhaust ports, which require high gas volume to maintain the required inert atmosphere and limit the functionality of the system to a specific process or profile
- Use of rotameters to manage gas flow, which do not have the capability of the precise control needed for consistent process results (leading to a higher overall gas consumption), and do not provide the process traceability required for data capture
- Use of copper tubing typically potted around a cooling section or secured to the outside by means of aluminum heat sinks; copper tubing can be a source of water leaks at solder joints, as well as oxidation that can cause a reduction in cooling capacity resulting in a change in the cooling profile of the furnace
Newer continuous process machines (CPMs) provide superior atmosphere control similar to that of a sealed batch system, 360 degree uniformity around the belt without the need for buffer or balancing zones, greater durability through the use of heavier gauge wire heating elements rated to 1300 C (2370 F) and more flexibility to accommodate multiple processes using less equipment. In addition, they can data log and track all the process parameters.
Atmosphere control in a muffle-type furnace is a significant issue. Typically, these muffles have a large, square or a semirectangular throat opening at both the entrance and exit ends, essentially creating a simple straight through tunnel. This configuration, while necessary to provide enough clearance for the product, has a large volume requiring a large volume of process gas (nitrogen, hydrogen, argon or forming gas) to maintain atmospheric control.
Incorporating a D-style muffle in a CPM counters this by reducing the volume, preventing outside air from entering the tunnel area, which could cause unwanted, harmful oxide layers on the products. In addition to reducing gas consumption, optimal atmospheric distribution is achieved in the D-style muffle using adjustable distribution injectors with a programmable gas control panel and mass flow control (MFC), which can maintain a gas source of 5 to 7 ppm in the muffle. As much as 25 to 30% reduction in gas consumption can be achieved in a CPM compared with conventional systems incorporating square or semi-rectangular muffles. A muffle volume comparison that supports the gas consumption savings described above is shown in Fig. 1.
360 degree uniformity
Across-the-belt uniformity is always a critical process parameter. To achieve this in conventional hot-box designs with lay-up construction could require from three to five zones across a belt, plus a need to balance the heat from the side zones (which need to be adjusted) to achieve +/-2 C across the belt width. This is vital when processing wide parts or components, or when using the full width of the belt for the material.
In a CPM with a cylindrical body, the heating element can be sized accordingly to the relationship of the muffle, resulting in superior temperature uniformity without having to have buffer or balance zones. A sealed cylinder arrangement provides 360 degree heating compared with a top and bottom type of arrangement with flat panel heaters.
Furnace durability is a concern with the push for greater throughput and extending process limits. Light gauge wire heating elements and ceramic plates used in conventional style conveyor furnaces do not hold up to today's heavier loads and higher process temperatures.
For example, an annealing process requiring temperatures in the 1000 C (1830 F) range stretches the limits of a light gauge element to its maximum, increasing its chance for failure. By comparison, the typical operating temperature of a medium or heavier gauge element in a CPM is well within the maximum range of efficiency, and it also can accommodate temperatures at or above the typical 1100 C (2010 F) maximum operating range, with sufficient available power and temperature capability.
Multiple process flexibility
Earlier furnace systems were specifically designed to handle one type of process and one type of product. For example, an IR furnace works well for fast ramp up where surfaces are heated instead of bulk materials, or for solder pastes. They are typically not used for processes like metal annealing. Also, IR lamps can be expensive and eventually have maintenance issues. Radiant or conventional hot plate technology ovens and furnaces heat via convection, so it is limited to processes that require heated gas or air.
Today, manufacturers need to run multiple processes using less equipment and furnaces, so they cannot afford to have a furnace dedicated to a certain product type or size, such as a circuit board or wafer. With a modular design, a CPM can be scaled up or down (without having to change all the frames as with conventional systems) to accommodate changes in belt width and frame length. This provides the flexibility to modify the furnace to support multiple processes or changes in throughput requirements. An example would be a furnace designed to handle a metal seal process and a solder reflow process in a single system, with temperatures ranging from 250 to 1000 C (480 to 1830 F).
Recent test results performed for a major solar panel supplier of its critical, oxygen-free atmospheric process system confirmed the CPM's ability to achieve extremely low levels of O2 (3 to 5 ppm over source) throughout the entire length of the muffle and cooling section. The furnace also maintained a temperature uniformity of +/-2 C across the belt at a peak annealing temperature of 800 C (Fig. 2).
The CPM's load specific entrance- and exit-curtain enabled the system to be continuously loaded with product to meet throughput requirements of 1,500 wafers per hour. In addition, gas consumption was reduced from 535 SLPM (batch system) to 287 SLPM (CPM), and power required was lowered from 86 kW (batch system) to 33 kW (CPM) for the same overall wafer throughput. (Note: Consumption numbers for the example are averaged over the entire process cycle including ramp-up and cool-down cycles for both types of systems.)
A major advantage of the CPM is its ability to data log and track all critical process parameters using MFCs (for gas control) and PLCs. Earlier systems used a manually set rotameter to accomplish this, which can be changed by an operator. The CPM's MFC tracks and can report at any time what the product or lot experienced, such as ramp rates, time at temperature, gas flows, belt speed, cooling rates and profile thermocouple junctions.
Because material is introduced into an existing heated chamber, there is no need to ramp up the heat. The patented element design allows for sufficient thermal storage, which provides a very rapid load response as well as the across-the-belt temperature uniformity. Further economies are gained with the CPM during cooling, which is handled in an area apart from the heating zone unlike batch systems. So, the heated chamber maintains the process temperature. This helps with throughput, fuel consumption, energy consumption, durability, element longevity and uniformity.
The cylindrical CPM cuts maintenance time by providing easy access to all the components through the use of hinged panels (Fig. 3). Conventional systems typically have a full frame assembly with individual panels that need to be unscrewed and removed, making it more difficult to access thermocouples, elements, etc. From a safety standpoint, the hinged doors of the CPM are interlocked to restrict access to designated operators and maintenance personnel. IH