Why DFT Logic is required in VLSI flow?

DFT stands for Design for testability. This is require because of following reasons

1. To validate the functionality of the chip
2. DFT is very important in  diagnostics and testing of chip
3. DFT ensure early fault detection which saves the reputation of chip manufaturer, and ensuring functioning of chip before coming to the hand of customer

How Testing is done?

1. Test Patterns are being generated and that test patterns are given to design under test (DUT) as vector. if the result of test pattern and result of vector which is given to DUT matched then chip is manufatured without defects.

Common Method to implement DFT logic 

1. Scan Chain
2. ATPG (Automatic Test pattern Generation)