Substrates for the New Generation of SMT

For military applications the use of LCCCs is generally accepted and welcomed because of their availability, reliability and relative ease of assembly. Moreover, the problems of TCE mismatch, substrate flatness and thermal management have been overcome by the use of copper‐clad Invar as a substrate material. The cost and availability of such substrates lead to a continuing search for more suitable materials or refinements in the present‐day manufacturing processes. The emergence of the new epoxy based laminates such as BT and High Temperature is encouraging designers to move away from the more expensive‐to‐make polyimide materials. However, polyimide has superior physical properties that make it more suited to SMT and should not be discarded without fully understanding the penalties that may occur with alternative polymers. The further development of VHSIC and gallium arsenide integrated circuits means that future substrates will not only have to provide solutions to present problems but will also need to accommodate the increase in signal speeds, control of impedance and other properties. Thus substrate design will see a move towards finer line widths, low‐loss dielectric material, stripline construction and special copper foils.