This is not an STD forum. Condoms generally fail in a tearing apart rupturing way, exposing the crown of your member. I am unable to figure out failure in your case when tip of your tool was still covered. If so, as rightly advised by ECN, you are safe.
Condoms provide excellent protection against HIV and other STD's as long as they remain intact. Once they fail, you are exposed to a risk, equivalent to that from unprotected prenetration, because in most of the cases failure takes place while yours/his member is inside. They must be used carefully and correctly to ensure positive protection.
I dont know about other STD, but from HIV if your uretra and the foreskin remain covered and didnt got in contact with any vaginal fluid then you didnt had a risk.