In order to improve the surface mount reliability of electronic equipments for space use, we investigated of a design method of thermal expansion coefficient (TEC) for a Multilayer Printed Wiring Board (M-PWB) . The method is to use the equation of composite thermal expansion coefficient (C-TEC) derived from the Hooke's rule and the control metal which is mainly made of a low thermal expansion alloy, “Invar”. The M-PWB considered here has four layers of power supply and ground, two signal layers, two surface wiring layers and two control metal layers. Its C-TEC was designed to be 9. 8×10【u-6】/°C. To avoid warping, we arranged two control metal layers to the neighborhood of the M-PWB's surfaces. We then fabricated such designed M-PWBs and measured their TEC, and found their TEC agreed with the designed C-TEC within an accuracy of ± 2×10【u-6】/°C. Since the TEC of the substrate materials varied by ± 3-4 ×10【u-6】/°C , this good accuracy is supposed to be resulting from high elasticity of the control metals. We also found the surface TEC of M-PWB was lower than the inner parts, which is due to an effect of the arrangement of the control metals. This difference in TEC between surfaces and inner layers developed the inner stress of M-PWB, giving rise to the lowering of the through-holes connecting reliability. The through-holes connecting life, however, was found to have 250 cycles in the heat-shock test of -65°C←→125°C, which is good enough for the practical use.
