B-stage epoxy films are partially cured versions of a traditional epoxy resin. From a high level, b-stage films provide the same function as traditional heat-curing epoxies. Films are positioned at the bond interface, and heat is used to complete the polymerization reaction and adhere the bond.
However, there are several ways that epoxy films are superior to liquid alternatives:
- Epoxy films allow for a more efficient and controlled application process. Films can be precut to specific dimensions (supplied on wax paper or coated polyester release liners in either rolls or sheets) and pick-and-place machines can be used to perfectly position components. Not only does this process provide higher levels of accuracy, but the use of films also relieves other difficulties normally associated with liquid dispensing systems (cleaning, pot life, material waste, etc.). We should additionally note that the elimination of liquid resins and dilution or cleaning solvents in the bonding process leads to a safer and environmentally friendly work floor.
- Epoxy films provide an incredibly precise and accurate bond-line thickness. During production, films can be designed to meet custom thickness requirements. This is of particular importance to electrical applications where precise dielectric or thermal properties are necessary.
- B-stage films are able to achieve higher filler loadings than liquid alternatives. Thermally and electrically conductive epoxies require high levels of filler materials – the higher the levels the more substantial the resulting performance properties. At high filler loadings, liquid systems become highly viscous and thixotropic, making them difficult to work with. B-stage films, on the other hand, are solid materials and thus the ultimate processing capabilities are unaffected by high filler loadings.
How Are B-Stage Films Produced?
B-Stage epoxies have a two-step curing process.
- The resin is partially heat-cured (up to 10%) and then cooled (quenched) to prevent complete polymerization in the first step. At this stage, the resin still retains much of its bonding ability, however, is cured enough to be handled as a solid material at room temperature.
- At a later point, this partially cured resin can be applied to the intended substrates. Heat activation is then used to restart the polymerization process and the resin can be fully cured.
We should note that b-stage epoxies are also occasionally formed via a solvent process. Uncured resin is mixed with solvent to form a slurry. The slurry is then coated on a release surface and excess solvent is removed at a temperature lower than the activation threshold of the resin. This form of epoxy resin is also known as the ‘pre-dried’ form. The pre-dried form is not partially cured and some may argue that it is technically not a b-stage epoxy. The form and use of the pre-dried film however closely resembles that of the partially cured form. An advantage of this approach is that the quenching process is avoided which reduces variability in film properties. Material formulators may also utilize solvents to reduce the viscosity of highly loaded resins.
Typical wet layer thickness is between 25-250µm, and the cured layer thickness may vary from 10-200µm. In most cases the cure temperature is between 125-150°C. Specialized products such as TechFilm I2701F may cure at temperatures as low as 90°C. Some b-stage systems are also designed for reactivation and curing with exposure to UV light.
Pre-form epoxy film patterns can be produced via screen printing, die cutting or laser cutting to a high degree of precision and produce very uniform bond line thickness.
Thermal And Electrical Properties
As we have mentioned B-stage epoxies can also be engineered to provide thermal or electrical conductivity across the bond, and fill limits for B-stage epoxy films are generally higher than liquids. The following Resin Designs B-stage films are examples of high performance thermal and electrical films.
TechFilm T2222F is an alumina filled, thermally conductive B-stage film that is specially formulated for bonding gold, nickel, and other hard to bond substrates.
TechFilm T2781F is an aluminum powder filled B-stage film that provides high thermally conductive and electrically insulating properties. T2781F will cure at temperatures above 100°C.
TechFilm E2213F is a silver filled, electrically conductive B-stage epoxy that is specially formulated to provide excellent bond strength to gold or gold-plated substrates. E2213F is especially suited for EMI/RF grounding applications.
Filler materials are not only used to achieve thermal and electrical properties. Epoxy film adhesives are also available with built-in fiberglass or carbon fiber support for improved mechanical properties.
B-Stage Epoxy Applications
B-stage epoxy films find application in microelectronics, robotics, and composites. Process assembly considerations usually drive the choice of these film adhesives rather than performance or technical reasons. Examples of these applications include adhering lids over air cavity, hermetic packages and adhering substrates into package housing in hybrid microelectronics. Semiconductor packaging also makes use of b-staged adhesive films for flip chip processes; wafer passivation and wafer back side adhesion. Optical applications include sealing of LCD glass and flat panel displays, window mounting in optical sensors and fiber optic bonding in ferrules. Heat sinks on PCB’s may also be pre-applied with an adhesive epoxy film.
B-stage epoxy films should be considered when deposition of epoxy resin is not immediately followed by bonding of parts or if such an arrangement causes production bottlenecks. These films may also be useful when subcontractors or value-added providers are required to supply parts with pre-applied epoxy resin for smoother work flow at the assembly line. Even if application of the film adhesive and final curing are carried out at the same location, b-stage epoxy films allow the deposition and curing processes to be spaced in time. This might be necessary due to mass-production considerations or to increase flexibility of the manufacturing process.