Some typographers suggest (i.e. they consider it a possible way to do things, not a compulsory one) that you can use en-dashes (
--) when you are not using a compound word or expression, but two different words that are in some sort of logical relationship (opposition, link). So for instance: "ion--atom collision", "London--Washington relationship", but "non-negligible".
Also, you can use an en-dash if one the words is already a compound word: "Pulitzer prize--winning", but "Selvyn-type source" or "short-lasting event" (they are expressions, not words of their own). It may not apply when the "compound" word only has a prefix in it: "non-English-speaking".
The logic behind it is that you should use a longer sign when the link between the two elements of the phrase is weaker, and a shorter one when they are very much related. For the same reason, many (more) typographers recommend using an en-dash between digits that indicate a range (see Bringhurst p. 80--81, with a dash).
Note: Few typographers openly endorse this practice, and few talk about it at all – but it does not mean it should not be used. Matthew Butterick is one that does recommend it (Typography for Lawyers, p. 49).
Some people recommend it only insofar as you do not use em-dashes (
---) in your document, e.g. for dialogues and asides.