Lightweight, Dimensionally Stable Printed-Wiring Boards

In comparison with traditional PWBs, these offer better heat dissipation and CTE matching.

Printed-wiring boards (PWBs) that are especially suitable as substrates for highly reliable, lightweight electronic circuits for aircraft and spacecraft have been developed. Like traditional PWBs, these PWBs are laminated composites that include dielectric inner layers plus copper outer layers that can be etched to form signal and power conductors. Going beyond the designs of traditional PWBs, these PWBs include multiple copper layers separated by dielectric (e.g., polyimide) layers, plus inner cores that contain carbon cloth.

These PWBs are intended to accommodate high densities of electronic components and of interconnections among them. The designs of these PWBs can be optimized to satisfy several requirements, including removal of heat generated in electronic components, maximization of stiffness with minimization of weight, and minimization of mismatches between the coefficients of thermal expansion (CTEs) of the PWBs and the components (e.g., leadless chip carriers) mounted on them. Previous solutions to the heat-dissipation and CTE problems have included the use of copper/Invar/copper (CIC) cores. While CIC cores contribute to reduced CTEs and increased thermal conductivities, they also contribute to increases in weights and costs.

Carbon cloth is used in the core layers of the present PWBs because it affords a combination of properties (low CTE, high thermal conductivity, low mass density, and high stiffness) that help to satisfy the requirements mentioned above. [The use of carbon cloth in the core layers of PWBs for this reason is reported in the preceding article, “Mounting Flip Chips on Heat-Dissipating, Matched-CTE Boards” (LEW- 16890).] The present carbon-core PWBs offer the advantages, but not the disadvantages, of PWBs with CIC cores; that is, unlike those with CIC cores, these are lightweight and relatively inexpensive. Because of its superior heat-dissipating and CTE characteristics, a PWB of this type (see figure) can accommodate about 40 percent more electronic components than can a traditional PWB.

This work was done by Richard A. Bohner and William E. Davis of Applied Materials and Technologies, Inc., for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-16648.