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Power consumption during at-speed scan-based testing can be significantly higher than that during normal functional mode in both shift and capture phases, which can cause circuits' reliability concerns during manufacturing test. This paper proposes a novel X-filling technique, namely “iFill”, to address the above issue, by analyzing the impact of X-bits on switching activities of the circuit nodes in the two different phases. In addition, different from prior X -filling methods for shift-power reduction that can only reduce shift-in power, our method is able to cut down power consumptions in both shift-in and shift-out processes. Experimental results on benchmark circuits show that the proposed technique can guarantee the power safety in both shift and capture phases during at-speed scan-based testing.