The you are working with (e.g., FPGA, custom ASIC, or embedded memory).
Adds silicon area near arrays; introduces minor timing delays. Board-level interconnect and pin testing via JTAG.
A Test Pattern Generator (TPG), often using a Linear Feedback Shift Register (LFSR), sends pseudorandom patterns through the logic. A Signature Analyzer then compresses the output responses.
The "Solution" in Testable Design is proactive. You don't just build a circuit and hope it's testable; you design it to be tested.