Hi,
Google never reveal that but yes it is a ranking signal. I'm quoting from Moz
"Usability and user experience are second order influences on search engine ranking success. They provide an indirect but measurable benefit to a site's external popularity, which the engines can then interpret as a signal of higher quality. This is called the "no one likes to link to a crummy site" phenomenon."
You can read full post here @ https://moz.com/beginners-guide-to-seo/how-usability-experience-and-content-affect-search-engine-rankings
Hope this helps.
Thanks