(An article by Clive Knowles of Goldhosts Februrary 2015)
Does using an SSL Certificate have any SEO implications? Yes - and they are mainly good ones...
With an SSL Certificate, communications between the browser and the website will be secure. So anyone listening in on traffic across a home or office network, or across the internet will not be able to see the content. As this may include client data, order details, addresses, passwords etc. this is a good thing.
We can also add code to force the requests for insecure pages to be secure, so the original links should work fine. These are proper "301 Permanent Redirects" so that the search engine know to update their index.
The search engines like sites which are secure a little more than non-secure pages, Google started this late in 2014. And we saw major improvment in the initial test-pages which were converted.
There is a minor down-side to converting to SSL use, in that Search Engines can decide that the content is new (because it comes from a different URL). This is normally resolved quite quickly though.
Overall, it is very much worth using an SSL Certificate for your website, and this is best done from the start.