DFT or “Design For Testability” is a technique, which facilitates a design to become testable after production. It is the extra logic that we put in the normal design, during the design process, which helps its post-production testing.
DFT is independent of design verification. Verification is required to verify the functionality of the chip while DFT plays the role in ensuring that each and every node in the design for structural and other faults. This includes designing test features for the chip and generating the test inputs and outputs required. These are used for testing the chip for errors, once the chip is fabricated.
The importance of DFT for chip design and manufacturing companies is that it helps to avert the huge cost and time delays involved in the re-spin of the ASICs because the errors were found only after the chip is fabricated. It is a preventive tool to weed out the defective dies as well as individual packaged units.
The DFT Flow
- During chip design, DFT architecture is created based on an understanding of different blocks on the SoC. This includes adding some extra blocks on the chip to increase its testability.
- The functionality of these blocks is then verified.
- A link of logic is created which can test each and every node of the chip.
- These schemes are verified using simulations.
- SCAN/JTAG insertions are performed and ATPG (Automatic Test Pattern Generation) patterns are generated using the appropriate tools.
- ATPG patterns are simulated and actual outputs during simulations are compared with expected outputs.
- Wafer test – The dies are tested on the wafer itself before they are cut and sorted.
- Package Chip test – Individual dies that have passed the wafer test are then packaged and re-tested with ATPG patterns.
Controllability and Observability
Controllability and observability are the two most important tenets of DFT.
- Controllability – The DFT architecture and input should be such that each and every node in the chip can be toggled by the APTG pattern inputs.
- Observability– The changes in output corresponding to the toggling of a node should be observable at the packaged part pins.
Scope of DFT in VLSI
Any company which is into silicon chip design requires DFT engineers. The proportion of DFT engineers in the entire team depends on the type of chip being manufactured, whether it is re-doing an existing chip, adding new functionalities to a chip, or creating a new chip altogether. But DFT is a vital process in any VLSI design – DFT engineers and expertise is needed for both product companies like Intel, Broadcom, Qualcomm, etc. and service companies that support these product companies.
Though the number of DFT engineers in a company will be less than Verification or Physical design engineers, there is a mismatch between the number of DFT engineers needed in the industry and the number of trained engineers. This mismatch is due to the fact that the output of skilled DFT engineers from good institutes or colleges is much less compared to other streams. Hence trained DFT students to have a good opportunity for placement.
Qualifications needed to become a DFT Engineer
Most companies are looking for BE, B-Tech, or M-tech students specialized in Electronics. This is because the basic requirement for a DFT engineer profile is a good understanding of Digital Design and CMOS devices. Other than freshers, engineers who have experience in other VLSI Domains have an added advantage in shifting to DFT as they already have an understanding of post-silicon testing which is valued by companies. But the majority of hiring is happening for freshers, in the DFT domain.
Initial Package and Profile of a DFT Engineer
Service companies have a standard package of around 3 l.p.a for freshers. This package is consistent for almost all fields of VLSI like DFT, Verification, and PD, etc. Due to certain urgent requirements or quality of the candidate, this may vary. Those with a testing background might get a higher package.
In product companies, the starting package for the same profile will be around 8-10 l.p.a but the number of vacancies is less comparatively.
Future of a DFT engineer
A DFT engineer with good performance can increase their salary by over three times, over a 5 year period, be it in a service company or a product company. Unless one stagnates and does not improve in one’s output and skills, the VLSI industry provides consistent and streamlined growth opportunities.
In product companies, after a few years, engineers decide on pursuing career growth either as a technical specialist or move into management. This decision has to be taken keeping one’s strengths, weaknesses, and interests in mind. Both these verticals have the same remuneration but different roles, within the organization.
Potential for offshore assignments is there once a person has got sufficient experience and has moved up the career ladder.
While companies follow a standard 5 day week with perks similar to other professional industries, the VLSI industry requires employees to be willing to work long hours and consecutive days depending on the urgency of the work. During Tape-out, which is when the chip is finally sent for manufacturing, long weeks of 10+ hours of work each day are not uncommon. Unexpected bugs or errors may require the engineers to work at a stretch without prior notice.
Leaves and holidays are standard in nature. While product companies have their own leave calendars, service companies work according to the schedules of their client. VLSI companies normally have an extended holiday season for 10-15 days at the end of the year.