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A new type of 3D multichip module (3D-MCM) for wireless sensor network was developed based on a kind of embedded FR-4 substrate for the wireless sensor network, in which FCOB (flip-chip on board), COB (chip on board), BGA (ball grid array) technologies, wirebonding and flip-chip interconnection technologies were combined together. The PBGA device and bare die were hybrid-integrated on the embedded multi-layer FR-4 substrate. By solder ball placement and reflow the BGA was formed at the bottom of 3D-MCM, and solder balls with different melting points were used for initial and final vertical interconnections for the sake of compatibility of all levels interconnections of BGA by reflow soldering. The application of embedded substrate solved the problem that the top surface of the encapsulated chip overtops the solder balls in the condition that the chip was assembled in the same side of substrate with BGA. The thermal management was conducted and the thermal related reliability of 3D-MCM were simulated and evaluated respectively. This kind of packaging structure satisfies the electrical performance and thermal requirement, and meets the challenge of minimization, high reliability and low cost of the package design for the wireless network.