Surface modification of polyimide to improve its adhesion to deposited copper layer - Polyimides and Other High Temperature Polymers - page 314

and copper. Therefore, the range of the operating temperature is limited because
of mechanical stress created.
Successful attempts have been made to develop adhesiveless laminates. Most
of these techniques include coating the polyimide surface with a partially cured
polyimide and laminating a copper foil to the coated polyimide under elevated
pressure and temperature. During this lamination process the partially cured poly-
imide film is fully cured and an adhesiveless copper-polyimide laminate is
formed. Additionally, attempts have been made by KANEKA Corp. to match the
coefficient of thermal expansion of polyimide to that of copper, and thus me-
chanical stress is reduced. This has led to the development of the new polyimide
compositions, e.g., the Kapton® E, H films by DuPont [1].
Adhesiveless copper-polyimide laminates show improved thermal stability, but
suffer from relatively low peel strength and high cost. An increase of the peel
strength has been achieved through the application of thin tie-coats of reactive
metals, e.g. chromium or monel (a nickel-copper alloy), between copper and poly-
imide [2]. Typical thicknesses of such tie-coats are between 75 nm and 200 nm. A
further drawback of tie-coats is that they complicate the subsequent etching proc-
ess of the conductor pattern. In many cases an additional etching step is necessary
to remove the tie-coat completely between the conductor lines to ensure the re-
quired insulation resistance.
Therefore, a copper-polyimide laminate which is based on an adhesiveless
bonding technology without applying tie-coats is needed to enable easy proc-
essability. A promising approach to improve adhesion is the surface modification
of the polyimide foil. The surfaces of polymer foils may be modified physically
(i.e. mechanical roughening) or chemically (e.g. by introduction of reactive func-
tional groups onto polymer surfaces) [3, 4]. Both approaches are widely used by
laminate manufacturers.
This paper presents a new approach to manufacture adhesiveless laminates with
improved peel strength at lower cost. It is based on an innovative Ion-Track
Technology developed by FRACTAL AG to create the so-called “surface-depth
relief” on polyimide films. This technique can be applied not only to the manufac-
ture of flexible circuits, but also to any purposes where polyimide surfaces with
well-defined profiles are needed. It can also be applied to other polymers.
2. BASICS OF ION-TRACK TECHNOLOGY
The surface modification by ion-track technology comprises two steps. First, the
irradiation of the polyimide foil with high-energy ions to produce latent ion tracks
and second, the etching of the irradiated surface to obtain the required surface
morphology (the so-called “surface-depth relief”).