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Advanced Semiconductor Packaging & Circuit Materials

Science of Semiconductor Packaging & Circuit Materials

Flexible Printed Circuit Technology

DuPont™ Kapton® polyimide film has been used in the electronics industry for over 40 years. Kapton® has excellent dielectric strength, thermal stability, chemical resistance, flexibility, and dimensional stability. While Kapton® is widely used in electrical applications such as insulation for field coils, motor and generator liners, as well as wire and cable insulation, it is particularly well suited for use as a dielectric substrate for flexible copper clad laminate. In fact, flexible circuit laminate is the largest single end use for Kapton® today. Kapton® HN is the primary dielectric core for adhesive based flexible circuits.

Adhesive Based Flex Circuit Laminate

Adhesive based flexible laminates are comprised of Kapton® film coated with adhesive on one or both sides and subsequently laminated to copper. There are two kinds of adhesive used in flex, epoxy and acrylic. They each have positive and negative attributes that may make one or the other more suitable for a specific end use. In some applications like actuator circuits for hard disk drives, the adhesive actually performs the critical function of keeping the copper in the "neutral axis". This enables the flex to bend many millions of cycles without failure. However, both acrylic and epoxy adhesives have inferior mechanical and electrical properties as compared to the Kapton® core. In certain applications, the presence of an adhesive layer actually inhibits the use of flexible circuits. Therefore, a solution was needed to eliminate the adhesive layer from the substrate.

Adhesiveless Flex Circuit Laminate

In response to this challenge, DuPont developed an all-polyimide laminate that incorporates a bondable polyimide adhesive at the surface to secure the copper foil. This laminate, named DuPont™ Pyralux® AP, opened the door to many new applications for flexible circuits. One example is high layer count rigid-flex boards. In this application, the adhesives’ high coefficient of thermal expansion (CTE) in the Z-axis created reliability problems when exposing the board to thermal cycling. The introduction of Pyralux® AP allowed the design and fabrication of rigid flex and multi-layer flex circuits with much higher layer counts and hole densities.

Also, with a maximum operating temperature (MOT) of 180C, compared to 105C for typical adhesive based laminates, many higher temperature applications became feasible. Its superior chemical resistance enabled the use of flex circuits in much harsher environments, such as that found in "under-the-hood" automotive applications. Probably most important, though, is the improvement in electrical properties. Adhesive based laminate systems have higher loss tangents and dielectric constants (Dk) and consequently, very high signal attenuation, especially at high frequencies. Pyralux® AP has a lower Dk that remains stable well into the Gigahertz range. Further, because it is available in a thickness of up to 20 mils, Pyralux® AP is ideal for circuitry requiring impedance control.

As the need for adhesiveless-based laminates began to emerge in the consumer market, DuPont developed an all-polyimide cast-on-copper laminate called DuPont™ Pyralux® AC. This laminate is produced in roll form, thus allowing roll-to-roll flex fabrication for high volume manufacturing. Additionally, this material is available in dielectrics as low as 20 um, allowing for increased flexibility, thinner form factors and lighter weight, all of which are key to the consumer electronics market.

Non Flex Circuit Polyimide Laminate

Polyimide can also be used as a laminate core in non-flex circuit applications. Recently DuPont leveraged its extensive dispersion technology to fill polyimides with organic and inorganic materials in order to achieve enhanced and/or unique properties. DuPont™  Interra™ HK laminates use this technology for the purpose of embedding planar capacitance in printed circuit boards. Because polyimide film does not contain any woven fabric reinforcement, there is no concern for conductive anodic filaments (CAF), no matter how thin the dielectric.

Another product that takes advantage of dispersion technology in polyimides is DuPont TC. For this material, thermally conductive fillers are used to enhance the transfer of heat through the laminate without significant loss in dielectric strength. Applications include heating and cooling elements for thermoelectric modules as well as insulated metal and direct bond substrates.

Polyimide laminate technology has been the foundation of the flex circuit industry for many years. With the introduction of adhesiveless laminates, the possible uses for flex circuits have greatly expanded. With the development of filled polyimide technology, DuPont is continuing to lay the groundwork for the continued use of polyimide laminates well into the future in new and exciting applications in a broad range of markets.