What Is Chip DFT Design?

Design for Testability (DFT) is a key technology in chip design, which stands for Design for Testing. It refers to the insertion of relevant test logic in the design stage during the chip manufacturing process due to unavoidable manufacturing defects, such as metal wire short circuits, circuit breaks, or abnormal doping concentrations, which may lead to circuit logic failures and chip system failures, so relevant test logic is inserted in the design stage to test during or after the manufacturing process, screen out defective chips, and avoid entering the market or customers. With the dramatic increase in the complexity of integrated circuits and the large number of logic gates, how to ensure that each chip works properly in the manufacturing process has become an important topic, and DFT plays a key role in this context.

The need for testing

Chip testing is to judge whether the chip has faults by applying a known excitation signal to the input of the chip and observing the response of the output. Testing is mainly divided into manufacturing testing and functional testing: manufacturing testing is conducted before the chip leaves the factory to screen scrap wafers due to process defects, including wafer testing and packaging testing; Functional testing ensures the correctness of the chip in real-world applications, verifying that the chip design is 100% correct by verifying the use case.

However, with the advent of nanotechnology, the chip manufacturing process is becoming more and more complex, and the transistor density increases, resulting in an increased probability of short circuit or circuit breakage, and the possibility of chip failure greatly increases. The cost of testing can reach more than 50% of the manufacturing cost.

Various physical defects can occur during the manufacturing process, such as bridging or breaking interconnects, gate oxygen short circuits in CMOS transistors, mask lithography errors, and silicon wafer defects, which can lead to electrical failures and ultimately chip failure. In critical applications, such as medical devices, automotive electronics, or aerospace, chip failure can have serious consequences, so testing is performed under extreme conditions.

Test costs follow a tenfold principle, increasing from chip level to board level to system level, so early detection of defects can significantly reduce losses. DFT optimizes the chip manufacturing process by adding test capabilities to the design phase to make testing feasible and cost-effective, enabling quality control and manufacturing capability monitoring.

Basic principles and concepts of DFT

At its core, DFT enhances chip controllability and observability. Controllability refers to the ability to apply test excitation to the internal logic node that needs to be tested through an external input signal, so that it is assigned to any desired value; Observability refers to the ability to monitor the response values of internal nodes through external output ports, making it easy to observe and compare. These two features enable the testing process to fully cover the internal logic of the chip without worrying about the actual function of the chip, reducing test complexity and improving the versatility of design methods.

Defects can lead to failures in chip manufacturing, which are electrical manifestations of defects, and common fault models include fixed faults (such as pin ports shorting to power or ground), tripping faults and path delay faults (such as slow rise and fall of gate ports), and quiescent current type faults (causing high quiescent current leakage). If a fault can propagate backwards and be observed, causing the chip to behave not as expected, it is called failure. Not all failures cause failures, only those that affect functionality cause problems.

Main DFT technologies and methods

Scan testing is a common method for DFT by replacing normal registers with scan registers and concatenating them into a scan chain. In test mode, perform the move-in operation to move the test data into the internal register through the scan chain, and use the low-frequency clock to ensure accuracy. Then the capture operation is carried out, the data is captured at the functional clock frequency, and the low-speed clock (10~50MHz) is used for fixed faults, and the system function clock frequency (10MHz~GHz) is used for tripping or delayed faults. Finally, the captured data is moved out for analysis by the move-out operation.

Built-in self-test (BIST) targets memory units, such as SRAM and DRAM, by inserting specific test logic, generating test vectors internally and comparing the results, detecting physical defects such as short circuits and circuit breaks, without the need for external test equipment.

Boundary scanning is used to verify chip pin connectivity, enabling IO testing and board-level testing by inserting and chaining scanning test units for each input and output of the IO port.

Automatic Test Vector Generation (ATPG) automatically generates test vectors through software and is applied to production testing to judge the quality of chips by comparing the actual output with the expected output.

Together, these technologies solve the problem of timing circuit testing and convert hard-to-test timing circuits into easy-to-test combination circuits. The testing process involves applying a test vector to the circuit under test and then comparing the output response with the expected response.

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Applications of DFT:

DFT technology is widely used in various types of chips, such as processors, memory, and specialized chips. For example, Zhongke Benyuan's real-time control series DSP chips integrate scanning chains, BIST, and boundary scanning with a complete DFT architecture, ensuring high reliability and stability in high-risk fields such as industrial control, automotive electronics, and aerospace. DFT design improves testing efficiency, reduces production testing costs and time, and enhances chip resistance to interference in harsh environments such as high temperature, high humidity, and strong electromagnetic interference. It also supports the full lifecycle management of the chip, ensuring consistent performance from design and manufacturing to field applications.

Testability design plays a crucial role in chip manufacturing and validation. Through reasonable design, DFT greatly improves the efficiency and accuracy of testing, ensuring that each chip can work stably and reliably. With the continuous development of chip technology, testing technology will also advance, and DFT will escort the steady development of the chip industry.