I think the source of the error `Underfull \hbox (badness 10000) in paragraph' in the MWE is insufficiency of text.
\documentclass{llncs}
\usepackage{graphicx}
\begin{document}
\begin{figure}[t]
\begin{minipage}[b]{0.25\textwidth}
\centering
\caption{Interface among Processor, RAM and TPM in a Hybrid System}
\end{minipage}
\end{figure}
\end{document}
\raggedrighttells it to give up trying to justify the right margin so that white space is allowed to stretch on the right, so then the problem, goes away and it doesn't have to stretch inter-word space – David Carlisle May 02 '15 at 10:41