One of the lures of using an Arduino programmed Atmel Microprocessor is that at the slightest whim, you can attach a two wire serial cable connection to the RX/TX pins and from the Arduino IDE, reprogram the chip for its upgraded function within the circuit designed around it.
A BoArduino has a 6 pin header connector for the FTDI TTL-232R USB to 5.0v TTL serial cable. This takes up a bit of real estate, plus it's 3 more pins than we really need. In our circuit design around the Atmel microprocessor, a three pin header with connections for RX Data, TX Data and signal ground is all that's really necessary.
What if you wanted to eliminate the need for a header all together? Enter stage left, the spring probe. For obvious reasons, Everett Charles Technologies (ECT) trademarked their contact test probes under the name "Pogo Pin" which has also become somewhat a generic term for these handy devices.
The brass body is just the right size to slip through the holes in any 0.040" dia. drilled board to be soldered into place for making circuit test adapters, programming jigs, or more permanent connections between stacked proto-boards. The spring-loaded contact head is 0.055 dia. and shaped so it will center in 0.040 drilled holes, making contact with the copper trace and gets best contact when used in plated through-holes.
Two samples below out of the many head configurations used in spring probes.
The top is a chisel tip for probing plated through-holes.
The bottom is a ball tip for use on flat pads or holes.
Another tip design used for pads is the "Potato Masher" tip with a crosscut diamond raised contact surface. Cup tipped probes can be used for automated test jig contact with header pins, the cup quickly centering on the pointed top of the header pin.
Example of creating a test fixture using spring probes.