Surface modification of polyimide to improve its adhesion to deposited copper layer - Polyimides and Other High Temperature Polymers - page 313

Surface modification of polyimide to improve its
adhesion to deposited copper layer
MANFRED DANZIGER ∗ and WINFRIED VOITUS
FRACTAL AG, Vor dem Gröperntor 20, 06484 Quedlinburg, Germany
Abstract—Flexible copper-polyimide laminates with improved stability under rapidly changing
thermal and mechanical stresses have been obtained by a pre-treatment of the polyimide surface
comprising irradiation of the polyimide foil with heavy ions and a subsequent chemical etching of
the latent ion tracks. During the etching step a special “surface-depth relief” is created on the surface
of the polyimide. The metallisation of this polyimide (so-called “ion track foil”) by chemical elec-
troless deposition and electrodeposition of copper leads to a copper-polyimide laminate with im-
proved peel strength and high resistance to both thermal and mechanical stresses.
Keywords: Polyimide; copper; peel strength; adhesion; surface modification; ion irradiation.
1. INTRODUCTION
Commercially available copper-polyimide laminates have been used for several
decades as base materials for flexible printed circuit boards. Laminates with prop-
erties customised to different applications have been developed. However, it is
well known that the copper-polyimide interface has some severe weaknesses, es-
pecially if the laminate is used under harsh operating conditions. Such conditions
include high temperature, high humidity, thermal stress, vibrational stress or
combinations thereof. Especially, the automobile industry needs a flexible base
material which is able to withstand the high temperatures and vibration loads
which are present close to the engine.
Most of the commercially available laminates are adhesive laminates, i.e. an
adhesive is applied between the polyimide and copper foil to ensure a sufficiently
high peel strength. The thermal resistance of these adhesives is generally lower
than that of polyimides. Polyimides can withstand continuous operating tempera-
tures of about 200°C and short-term soldering processes of more than 300°C. The
coefficient of thermal expansion of the adhesive differs from that of polyimide