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What Is Not Testable Is Not Fixable

Gabe Moretti, Senior Editor

In the past I have mused that the three letter acronyms used in EDA like DFT, DFM, DFY and so on are superfluous since the only one that counts is DFP (Design For Profit).  This of course may be obvious since every IC reason for existence is to generate income for the seller.  But it is also true that the above observation is superficial since the IC must be testable, not only manufacturable and must also reach a yield figure that makes it cost effective.  Breaking down profitability into major design characteristics is an efficient approach, since a specific tool is certainly easier to work with than a generic one.

Bassilios Petrakis, Product Marketing Director at Cadence told me that: “DFT includes a number of requirements including manufacturing test, acceptance test, and power-on test.  In special cases it may be necessary to test the system while it is in operation to isolate faults or enable redundancies within mission critical systems.  For mission critical applications, usage of logic and memory build-in-self-test (BIST) is a commonly used approach to perform in-system test. Most recently, a new IEEE standard, P1687, was introduced to standardize the integration and instrument access protocol for IPs. Another IEEE proposed standard, P1838, has been initiated to define DFT IP for testing 3D-IC Through-Silicon-Via (TSV) based die stacks.”

Kiran Vittal, Senior Director of Product Marketing at Atrenta Inc. pointed out that: “The test coverage goals for advanced deep submicron designs are in the order of 99% for stuck-at faults and over 80% for transition faults and at-speed defects. These high quality goals can only be achieved by analyzing for testability and making design changes at RTL. The estimation of test coverage at RTL and the ability to find and fix issues that impact testability at RTL reduces design iterations and improves overall productivity to meet design schedules and time to market requirements.”

An 80% figure may seem an under achievement, but it points out the difficulty of proving full testability in light of other more demanding requirements, like area and power to name just two.

Planning Ahead

I also talked with Bassilios  about the need for a DFT approach in design from the point of view of the architecture of an IC.  The first thing he pointed out was that there are innumerable considerations that affect the choice of an optimal Design for Test (DFT) architecture for a design.   Designers and DFT engineers have to grapple with some considerations early in the design process.

Bassilios noted that “The number of chip pins dedicated to test is often a determining factor in selecting the right test architecture. The optimal pin count is determined by the target chip package, test time, number of pins supported by the automated test equipment (ATE), whether wafer multi-site test will be targeted, and, ultimately, the end-market segment application.”

He continued by noting that: “For instance, as mixed signal designs are mostly dominated by Analog IOs, digital test pins are at a premium and hence require an ultra low pin count test solution. These types of designs might not offer an IEEE1149.1 JTAG interface for board level test or a standardized test access mechanism. In contrast, large digital SoC designs have fewer restrictions and more flexibility in test pin allocation.  Once the pin budget has been established, determining the best test compression architecture is crucial for keeping test costs down by reducing test time and test data volume. Lower test times can be achieved by utilizing higher test compression ratios – typically 30-150X – while test data volume can be reduced by deploying sequential-based scan compression architectures. Test compression architectures are also available for low pin count designs by using the scan deserializer/serializer interface into the compression logic. Inserting test points that target random resistant faults in a design can often help reduce test pattern count (data volume).”

The early exploration of DFT architectures to meet design requirements – like area, timing, power, and testability – is facilitated by modern logic synthesis tools. Most DFT IP like JTAG boundary scan, memory BIST collars, logic BIST and compression macros are readily integrated into the design netlist and validated during the logic synthesis process per user’s recipe. Such an approach can provide tremendous improvements to designer productivity. DFT design rule checks are run early and often to intercept and correct undesirable logic that can affect testability.

Test power is another factor that needs to be considered by DFT engineers early on. Excessive scan switching activity can inadvertently lead to test pattern failures on an ATE. Testing one or more core or sub-block in a design in isolation together with power-aware Automatic Test Pattern Generation (ATPG) techniques can help mitigate power-related issues. Inserting core-wrapping (or isolation logic) using IEEE1500 is a good way to enable a core-based test, hierarchical test, and general analog mixed signal interactions.

For designs adopting advanced multi-voltage island techniques, DFT insertion has to be power domain-aware and construct scan chains appropriately levering industry standard power specifications like Common Power Format (CPF) and IEEE1801. A seamless integration between logic synthesis and downstream ATPG tools helps prime the test pattern validation and delivery.

ATPG

Kiran delved in greater details into the subject of testability by giving particular attention to issues with Automatic Test Pattern Generation (ATPG).  “For both stuck-at and transition faults, the presence of hard to detect faults has a substantial impact on overall ATPG performance in terms of pattern count, runtime, and test coverage, which in turn has a direct impact on the cost of manufacturing test. The ability to measure the density of hard to detect faults in a design early at the RTL stage, is extremely valuable. It gives RTL designers the opportunity to make design changes to address the issue while enabling them to quickly measure the impact of the changes.”

The performance of the ATPG engine is often measured by the following criteria:

- How close it comes to finding tests for all testable faults, i.e. how close the ATPG fault coverage comes to the upper bound.  This aspect of ATPG performance is referred to as its efficiency. If the ATPG engine finds tests for all testable faults, its efficiency is 100%.

- How long it has to run to generate the tests.  Full ATPG runs need to be completed within a certain allocated time, so the quest for finding a test is sometimes abandoned for some hard to test faults after the ATPG algorithm exceeds a pre-determined time limit.

- The larger the number of hard to test faults, the lower the ATPG efficiency.  The total number of tests (patterns) needed to test all testable faults. Note that a single test pattern can detect many testable faults.

To give a better idea of how test issues can be addressed Kiran provided me with an example.

Figure 1 (Courtesy of Atrenta)

Consider Figure 1, which has wide logic cones of flip flops and black boxes (memories or analog circuits) feeding a downstream flip flop. ATPG finds it extremely difficult to generate ‘exhaustive’ patterns and the test generation time is either long or the fault coverage is compromised. These kinds of designs can be analyzed early at RTL to find areas in the design that have poor controllability and observability, so that the designer can make design changes to improve the efficiency (test data and time) of downstream ATPG tools to generate optimum patterns to not only improve the quality of test, but also be economical to lower the cost of manufacturing test.

Figure 2 (Courtesy of Atrenta)

Figure 2 shows the early RTL analysis using Atrenta’s SpyGlass DFT tool suite. This figure highlights the problem through the schematic representation of the design and shows a thermal map on the low control/observe areas, which the designer can fix easily by recoding the RTL.

The analysis of the impact of hard to test faults at RTL can save significant design time in fixing low fault coverage and improving ATPG effectiveness for runtime and pattern count early in the design cycle, resulting in over 50x more efficiency in the design flow to meet the required test quality goals.

Conclusion

Bassilios concluded that “further improvements to testability can be achieved by performing a “what if” analysis with test point insertion and committing the test points once the desired coverage goals are met. Both top-down and bottom-up hierarchical test synthesis approaches can be supported. Early physical placement floorplan information can be imported into the synthesis cockpit to perform physically aware synthesis as well as scan ordering and congestion-free compression logic placement.”

One thing is certain: engineers will not rest.   DFT continues to evolve to address the increased complexity of SoC and 3D-IC design, its realization, and the emergence of new fault models required for sub-20nm process nodes.  With every advance, whether in the form of a new algorithm or new IP modules, the EDA tools will need to be updated and, probably, the approach to the IC architecture will need to be changed.  As the rate of cost increase of new processes continues to grow, designers will have to be more creative in developing better testing techniques to improve the utilization of already established processes.

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