\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  y(t)=\int_0^{\boldmath t} H'(t-\tau)u(\tau)d\tau.
\end{equation*}
\end{document}