When does BLAST fail to align 2 DNA sequences?
BLAST works by finding a perfect match between sequences of a length equal to this "word length" and then enlarging it in a standard way -- yet there will be no alignment without this perfectly matched word.

So in your case, you must look for two 20bp sequences with no common 5bp sub-sequence; for instance:

AAAAAAAAAAAAAAAAAAAA


and

AAAACAAAACAAAACAAAAC