I assume you are flying with Croatia Airways or Lufthansa to Frankfurt? If so, you can check your luggage in Split right through to Boston -- no need to collect your luggage in Frankfurt. Since you are coming from Croatia, which is not a member of the Schengen agreement, and you are directly transferring to a flight to the US, you will stay in the non-Schengen part of the Frankfurt airport -- that means, you won't have to clear either immigration or customs in Germany. (However, you will have to clear Security again.) As to whether the connection is too tight for baggage to make the connection: tell us what your flights are and we'll give you a prediction!