Polyimides are high-temperature engineering polymers. Popular with manufacturers, they can replace conventional materials, like glass, metal, and steel, in many applications. They are also popular because of their ability to fulfill the increasing need for materials that perform well under harsh conditions. Used as plastics, films, laminating resins, insulating coatings, and high-temperature structural adhesives, polyimides exist in two formats—thermosetting and thermoplastic. They’re ideal for coating optical fibers for medical or high-temperature applications.
Polyimide use continues to grow steadily. The Global Polyimides Market will grow at a CAGR of 6.9%, says the “Polyimides – Global Market Outlook (2017-2026)” report. Propelling this growth is the rising use of polyimide in automotive and aerospace and innovation and technological expansion in semiconductor and electronics manufacturing. Industrial ovens designed specifically for baking and curing polyimides, like Despatch’s PCO2-14™, are critical for squeezing the most out of polyimides.
What is a Polyimide?
Polyimides offer an exceptional combination of thermal stability (>500°C), chemical resistance, and. high mechanical strength, which makes them ideal for applications demanding tough organic materials. Polyimides also have excellent dielectric properties and inherently low coefficient of thermal expansion. Some polyimides are photo imageable. Others have wet and dry etch characteristics for building up microelectronic structures on silicon wafers. Polyimides are a critical class of industrial synthesized polymers.
Polyimides are well suited for flexible printed circuits and semiconductors, micronized circuitry, and high-temperature adhesives. Plus, they’re well-suited for insulating film in electronic cables, varnishes and wire enamels in electric motors, protective garments in firefighting equipment, hot gas filtration bags in power plants, and cement kilns in construction. In aerospace, Polyimides are used in as aerospace adhesives at temperatures in the range of 300°C., and in polyimide foams for lightweight acoustic and thermal insulation in the marine industry.
Popular polyimides are Apical®, UPILEX®, VTEC™PI, Norton® TH, and Kaptrex. Kapton® is a classic polyimide used in manufacturing. Semi-transparent, flexible, and thin, this polyimide features low thermal mass, excellent electrical insulating properties, and superior resistance to most chemicals. Kapton also allows for high power densities with fast and efficient thermal transfer. This material is used in flexible electronics, etch foil heater mats, and space blankets and space instruments, among other applications.
Applying and Processing Polyimides
Manufacturers apply polyimide and solvents in liquid form using spin coating and other production methods. A low-temperature “soft bake” or “alpha cure” is typically used for the initial removal of solvent at temperatures typically in the range of 100-to-150°C. “Beta cure”, post-exposure-bake (PEB), and “hard bake” follow this initial cure, which removes residual solvent and finalizes the desired surface properties.
Precise temperature uniformity eliminates cracks in the polyimide layers and color variations, boosting product quality. This step often comes after UV exposure in photolithography at temperatures from 350-to-400°C, depending upon the application and temperature limit of components in the “stack.”
Polyimide curing is often done at low oxygen concentrations by either nitrogen purging at atmospheric pressure or combining nitrogen backfilling and vacuum. This step employs condensate traps, nitrogen purging at varying rates during the process, and vacuum and HEPA filtration to avoid solvent condensation on products and oven exhaust lines. This step also ensures that the polyimide layer retains uniform characteristics that are defect free.
Three Key Features of Polyimide Industrial Ovens
Polyimides are challenging resist materials to process, thanks largely to their high viscosities and use of strong solvents. It takes a clean process oven designed specifically for polyimide baking and curing applications, like Despatch’s PCO2-14™, to process polyimides effectively and efficiently. Despatch’s electrically heated oven is a high-performance, clean process oven (ISO Class 5/Class 100 recirculated airflow) with many unique components.
Three key features to look for when buying an industrial oven for polyimide baking and curing include:
• Pressure Relief System
Manufacturers must remove residual solvents and finalize the desired surface properties in the “hard bake” polyimide cure process. Removing solvents is a challenging and hazardous process. It requires an oven with equipment designed to help prevent and collect solvent condensation, which a pressure relief system can do. The system also includes a removable “cold trap,” an easy-to-clean condensate trap that prevents polyimide buildup in the oven’s exhaust.
• Oxygen Monitor and Control System
Processing a polyimide requires an oven with an inert atmosphere in the “hard bake” cure process. This feature lets users maintain the oxygen level at 20 ppm or less, which prevents the polyimides being cured from oxidizing. Supporting this system is an O2 monitoring system wired to the purge valve. That turns the nitrogen purge on whenever oxygen levels are above the O2 monitor set point.
Once the unit completes the purging, the oxygen monitoring and control system maintains the O2 level at a set point with help from a controller that operates a modulating valve during the curing process. This system also minimizes nitrogen usage but allows for consistent and repeatable product and component curing.
• Process Monitoring System
The oven’s PC features should include software that allows users to communicate between the PC and the oven, an Ethernet connection, and a flat panel display screen. The software needs to link with the oven’s controller, O2 controller, O2 monitor, and integrated PC for observing and logging entry cycles. A monitoring system provides the user with real-time information on set points, actual chamber temperatures, and O2 levels throughout the process.
These features are critical because they allow these ovens to achieve the strict oxygen level and atmospheric requirements involved in polyimide curing. Polyimides are widely used materials thanks largely to their excellent thermal stability, chemical resistance, and mechanical strength. Known for their high-temperature performance in the 400-500°C range, they fulfill the increasing need for tough, high-performance materials in a wide variety of industries. But polyimides are challenging materials to process because of their high viscosities and use of strong solvents. Manufacturers need special industrial ovens, like Despatch’s PCO2-14™, to squeeze out the most from polyimides.
If you still have questions about lab ovens that can help your company process polyimides, you can consult our website. Or, you can call our skilled customer service team to assist you. Please call 952-900-6635.
