Inside an align environment I have a \lim with two variables which should be displayed in two lines:

\begin{align} ... f & = \lim _ {a \rightarrow -\infty \linebreak b \rightarrow \infty} ... \\ ... \end{align} 

Currently there is no line break where I want to have one; it's displayed in one line. How can I force a line break there? \linebreak is ignored and \\ breaks the align environment.