f90-break-delimiters

f90-break-delimiters is a variable defined in `f90.el'.
Its value is

"[-+\\*/><=,% 	]"

• This variable is safe as a file local variable if its value
  satisfies the predicate `stringp'.

Documentation:
Regexp matching delimiter characters at which lines may be broken.
There are some common two-character tokens where one or more of
the members matches this regexp. Although Fortran allows breaks
within lexical tokens (provided the next line has a beginning ampersand),
the constant `f90-no-break-re' ensures that such tokens are not split.

You can customize this variable.