Manufacture of a Multilayer Polyimide PCB Containing Integral Copper/Invar Composite Planes for Thermal Expansion Control

The thermal mismatch between leadless ceramic chip carriers and conventional PCB materials can be obviated by using integral planes of a low TCE material to constrain the thermal expansion. Using a Copper/Invar/Copper composite for ground and voltage planes, a six‐layer polyimide circuit has been manufactured which closely matches the expansion of the alumina substrates used for chip carriers. The processing techniques used in the manufacture of this circuit are described with particular reference to lamination, hole preparation and the integration of the Invar composite layers into the multilayer structure. Processing of Invar containing structures poses certain problems due to the inherent properties of the alloy which render conventional PCB techniques such as NC routing difficult to utilise. The use of alternative techniques to process Invar composites is described.