A Study on Transmission Loss of Build-up Film Applied to IC Substrate

With the commercial application of the fifth-generation communication technology and the development of electronic technology, electronic devices evolve towards higher integration, multi-function, higher signal transmission speed and higher frequency, and as a result the signal loss becomes more critical issue. The integrated circuit (IC) packaging substrate is an important part of the semiconductor package, providing electrical connection, protection, mechanical support and heat dissipation for the chip. The build-up film (BF) is one type of thermosetting material with low thermal expansion coefficient (CTE) and low dissipation factor (Df). Because of its advantages in easy processing of fine lines, BF trend to be the insulating medium for IC substrates. However, in order to meet high-frequency and high-speed usage scenarios, the signal integrity problem needs to be considered when designing or manufacturing IC substrates based on BF.In this paper, signal transmission loss is systematically analyzed by electrical simulation in a single-layer microstrip line structure composed of BF, copper foil and air. The main sources of signal transmission loss are conductor loss and dielectric loss. Firstly, it is found that conductor loss is larger than dielectric loss at low frequency, while dielectric loss will be larger at high frequency. The increase of both dielectric constant (Dk) and Df of BF will increase the dielectric loss, while the influence of Df on dielectric loss is more obvious than that of Dk. Secondly, the influence of rough surface between copper foil and BF on signal transmission loss is also considered. It is found that the roughness has little effect at low frequency, while the conductor loss increases with the increase of copper foil roughness at high frequency. Thirdly, the parameters of resistance (R), inductance (L), conductance (G) and capacitance (C) of single-layer microstrip line are also analyzed based on the transmission line theory. The mechanism of signal transmission loss variation with frequency is explicated, based on the effect of frequency on the equivalent R, L, G and C parameters of single-layer microstrip line structure. This study may provide theoretical guidance for understanding the influence of BF material properties on signal transmission loss and its mechanism at different frequency.