No customs, but the connection is a bit on the short side. Was this booked as one ticket? If so, you check in your bags at Krakow all the way to San Francisco. You will not see it in Berlin at all. TXL is a very small airport, and you'll likely arrive at the C gates with AB and fly from an A gate with AA, a walk of about 5 minutes. You will need to go through an additional passport check in TXL for US flights, but this is relatively quick. Just a friendly reminder...I assume you have your ESTA sorted out.

If you are on separate tickets, then it's a different story. You may get AB to check bags all the way through, but if you miss you AA flight you may be stuck in Berlin on your own, with limited protection for a flight the next day.