System-in-Package (SiP) has been facing the problem of excessive radiation leakage. In this article, a novel technique of absorptive sheet is proposed for suppressing unwanted microwave radiated emission in an advanced high-speed SiP module backed with a heat sink. Different from the conventional periodic resonant structure or absorptive film, the electromagnetic energy is consumed based on the mechanism of spoof surface plasmon polaritons (SSPP) in our design. By adjusting the length of the metallic strip to control its k-dispersion characteristics, the SSPP modes could be excited around one frequency point. Therefore, a relatively wide absorption band is created by arranging stepped strips in arrays on a dielectric medium. According to the electromagnetic distribution in the SiP module, SSPP-based absorptive units are circularly arranged to constitute the complete proposed absorptive sheet with adequate absorption efficiency. From the simulated results, the proposed design can provide 90

$\%$

absorption performance covering a wide frequency range of 26.3–31.6 GHz, with a substrate thickness of only

$\lambda _{L}$

/228 and a total thickness of

$\lambda _{L}$

/134, where

$\lambda _{L}$

is the wavelength at the lowest operating frequency. Finally, the proposed absorptive sheet is fabricated and measured, where measurement results agree well with the simulated ones, which corroborates the feasibility and effectiveness of applying the SSPP-based absorbing techniques for suppressing the radiated emission in microwave and millimeter-wave frequency range in SiP modules.

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