What is DFT and why do we need it?

A simple answer is DFT is a technique, which facilitates a design to become testable after production. Its the extra logic which we put in the normal design, during the design process, which helps its post-production testing. Post-production testing is necessary because, the process of manufacturing is not 100% error free. There are defects in silicon which ontribute towards the errors introduced in the physical device. Of course a chip will not work as per the specifications if there are any errors introduced in the production process. But the question is how to detect that. Since, to run all the unctional tests on each of say a million physical devices produced or manufactured, is very time consuming, there was a need to device some method, which can make us believe without running full exhaustive tests on the physical device, that the device has been manufactured correctly. DFT is the answer for that. It is a technique which only detects that a physical is faulty or is not faulty. After the post-production test is done on a device, if it is found faulty, trash it, don’t ship to customers, if it is found to be good, ship it to customers. Since it is a production fault, there is assumed to be no cure. So it is just a detection, not even a localization of the fault. That is our intended purpose of DFT. For the end customer, the DFT logic present on the device is a redundant logic.To further justify the need of DFT logic, consider an example where a company needs to provide 1 Million chips to its customer. If there isn’t any DFT logic in the chip, and it takes for example, 10 seconds (Its very kind and liberal to take 10 seconds as an example, in fact it can be much larger than that) to test a physical device, then it will take approx. three and a half months just to test the devices before shipping.So the DFT is all about reducing three and a half months to may be three and a half days. Of course practically many testers will be employed to test the chips in parallel to help reduce thetest time.