Thermal management of 3D RF PoP based on ceramic substrate

In this paper, a new high performance three dimensional radio frequency package on package (3D RF PoP) based on ceramic substrate is designed for micro base station which is able to cover the multicasting of critical messages to as many mobile users as possible even under communications network failure events, such as the failure of macro base stations. The RF PoP integrates receiver (RX) module, transmitter (TX) and digital predistortion (DPD) module, and analog-to-digital/digital-to-analog (AD/DA) and clock (CLK) module vertically, has better signal integrity and faster data-rate transfer due to shorter signal paths among the three modules. Additionally, the ceramic substrate has higher thermo-mechanical reliability and better heat dissipation performance compared with organic substrate. The paper firstly studies the thermal performance of the RF PoP without external heat sinks using commercial software ANSYS Icepak. At the ambient temperature of 25 °C, the highest junction temperature of the RF PoP is 239.8 °C, which is above the acceptable baseline for a silicon chip. Secondly, in order to improve the heat dissipation capability of the RF PoP, a large copper bottom heat dissipation plate is employed. The impact of bottom heat dissipation plate on the thermal performance of the RF PoP is investigated. It is found that a bottom heat dissipation plate of 200×200×2 mm³ is reasonable, and the highest junction temperature is about 47.0 °C. Thirdly, we study the thermal performance of the entire and internal package structure of the RF PoP mounted on the bottom heat dissipation plate. The temperature distributions of the top and middle substrates are almost uniform. Heat generated from active devices on the top and middle packages is mainly transferred to the edges, conducted to the bottom heat dissipation plate through the edges and bottom substrate and heat slug, heat generated from active devices on the bottom pack- ge is conducted to the bottom heat dissipation plate through the bottom substrate and heat slug, and then dissipated into the ambient by natural convection, so that the temperature at the edge of the bottom substrate is higher than the other parts. Lastly, the effect of different ambient temperatures on the thermal performance of the RF PoP is investigated. When the bottom heat dissipation plate size is 200×200×2 mm³ and the ambient temperature reaches 100 °C, the highest junction temperature is about 121 °C. In order to further improve heat dissipation capability of the RF PoP, a copper top heat spreader is employed. The simulation result shows that the junction temperature drops to about 115.6 °C.