Design for testing: Difference between revisions

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'''Design for Test''' (aka "Design for Testability" or "DFT") is a name for [[Integrated circuit design | design]] techniques that add certain testability features to a [[integrated circuit | microelectronic]] hardware product design. The premise of the added features is that they make it easier to develop and apply manufacturing tests for the designed hardware. The purpose of manufacturing tests is to validate that the product hardware contains no defects that could adversely affect the product’s correct functioning.

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Tests are applied at several steps in the [[Semiconductor fabrication|hardware manufacturing]] flow and, for certain products, may also be used for hardware maintenance in the customer’s environment. The tests generally are driven by [[Automated testing|test programs]] that execute in [[Automatic test equipment|Automatic Test Equipment]] (ATE) or, in the case of system maintenance, inside the assembled system itself. In addition to finding and indicating the presence of defects (i.e., the test fails), tests may be able to log [[diagnostic]] information about the nature of the encountered test fails. The diagnostic information can be used to locate the source of the failure.

'''Design for Test''' (aka "Design for Testability" or "DFT") is a name for design techniques that add certain testability features to a microelectronic hardware product design. The premise of the added features is that they make it easier to develop and apply manufacturing tests for the designed hardware. The purpose of manufacturing tests is to validate that the product hardware contains no defects that could adversely affect the product’s correct functioning.

Tests are applied at several steps in the hardware manufacturing flow and, for certain products, may also be used for hardware maintenance in the customer’s environment. The tests generally are driven by test programs that execute in Automatic Test Equipment (ATE) or, in the case of system maintenance, inside the assembled system itself. In addition to finding and indicating the presence of defects (i.e., the test fails), tests may be able to log diagnostic information about the nature of the encountered test fails. The diagnostic information can be used to locate the source of the failure.

DFT plays an important role in the development of test programs and as an interface for test application and diagnostics. [[Automatic test pattern generation]], or ATPG, is much easier if appropriate DFT rules and suggestions have been implemented.

== History ==

DFT techniques have been used at least since the early days of electric/electronic data processing equipment. Early examples from the 1940s/50s are the switches and instruments that allowed an engineer to “scan” (i.e., selectively) probe the voltage/current at some internal nodes in an analog computer [analog scan]. DFT often is associated with design modifications that provide improved access to internal circuit elements such that the local internal state can be controlled ([[controllability]]) and/or observed ([[observability]]) more easily. The design modifications can be strictly physical in nature (e.g., adding a physical probe point to a net) and/or add active circuit elements to facilitate controllability/observability (e.g., inserting a [[multiplexer]] into a net). While controllability and observability improvements for internal circuit elements definitely are important for test, they are not the only type of DFT. Other guidelines, for example, deal with the [[Electromechanics|electromechanical]] characteristics of the interface between the product under test and the test equipment. Examples are guidelines for the size, shape, and spacing of probe points, or the suggestion to add a [[Tri-state buffer|high-impedance state]] to drivers attached to probed nets such that the risk of damage from back-driving is mitigated.

Over the years the industry has developed and used a large variety of more or less detailed and more or less formal guidelines for desired and/or mandatory DFT circuit modifications. The common understanding of DFT in the context of [[Electronic design automation|Electronic Design Automation]] (EDA) for modern microelectronics is shaped to a large extent by the capabilities of commercial DFT software tools as well as by the expertise and experience of a professional community of DFT engineers researching, developing, and using such tools. Much of the related body of DFT knowledge focuses on digital circuits while DFT for analog/mixed-signal circuits takes somewhat of a backseat.

== Objectives of DFT for Microelectronics Products ==

DFT affects and depends on the methods used for test development, test application, and diagnostics.

Most tool-supported DFT practiced in the industry today, at least for digital circuits, is predicated on a ''Structural test'' paradigm. Structural test makes no direct attempt to determine if the overall functionality of the circuit is correct. Instead, it tries to make sure that the circuit has been assembled correctly from some low-level building blocks as specified in a structural [[netlist]]. For example, are all specified [[logic gatesgate]]s present, operating correctly, and connected correctly? The stipulation is that if the netlist is correct, and structural testing has confirmed the correct assembly of the circuit elements, then the circuit should be functioning correctly.

Note that this is very different from ''[[Acceptance test|Functional testing]]'', which attempts to validate that the circuit under test functions according to its functional specification. This is closely related to [[functional verification]] problem of determining if the circuit specified by the netlist meets the functional specifications, assuming it is built correctly.

One benefit of the Structural paradigm is that test generation can focus on testing a limited number of relatively simple circuit elements rather than having to deal with an exponentially exploding multiplicity of functional states[[State (computer science)|state]]s and state transitions. While the task of testing a single logic gate at a time sounds simple, there is an obstacle to overcome. For today’s highly complex designs, most gates are deeply embedded whereas the test equipment is only connected to the primary [[Input/output]]s (I/Os) and/or some physical test points. The embedded gates, hence, must be manipulated through intervening layers of logic. If the intervening logic contains state elements, then the issue of an exponentially exploding [[state space]] and state transition sequencing creates an [[Computational complexity theory|unsolvable problem]] for test generation. To simplify test generation, DFT addresses the accessibility problem by removing the need for complicated state transition sequences when trying to control and/or observe what’s happening at some internal circuit element.

Depending on the DFT choices made during circuit design/implementation, the generation of Structural tests for complex logic circuits can be more or less [[Automatic test pattern generation|automated]]. One key objective of DFT methodologies, hence, is to allow designers to make trade-offs between the amount and type of DFT and the cost/benefit (time, effort, quality) of the test generation task.

== Looking forward ==

One challenge for the industry is keeping up with the [[Moore's law |rapid advances in chip technology]] (I/O count/size/placement/spacing, I/O speed, internal circuit count/speed/power, thermal control, etc.) without being forced to continually upgrade the test equipment. Modern DFT techniques, hence, have to offer options that allow next generation chips and assemblies to be tested on existing test equipment and/or reduce the requirements/cost for new test equipment. At the same time, DFT has to make sure that test times stay within certain bounds dictated by the cost target for the products under test.

== Diagnostics ==

Especially for advanced semiconductor technologies, it is expected some of the chips on each manufactured [[Wafer (electronics)|wafer]] contain defects that render them non-functional. The primary objective of testing is to find and separate those non-functional chips from the fully functional ones, meaning that one or more responses captured by the tester from a non-functional chip under test differ from the expected response. The percentage of chips that fail test, hence, should be closely related to the expected functional yield for that chip type. In reality, however, it is not uncommon that all chips of a new chip type arriving at the test floor for the first time fail (so called zero-yield situation). In that case, the chips have to go through a debug process that tries to identify the reason for the zero-yield situation. In other cases, the test fall-out (percentage of test fails) may be higher than expected/acceptable or fluctuate suddenly. Again, the chips have to be subjected to an analysis process to identify the reason for the excessive test fall-out.

In both cases, vital information about the nature of the underlying problem may be hidden in the way the chips fail during test. To facilitate better analysis, additional fail information beyond a simple pass/fail is collected into a fail log. The fail log typically contains information about when (e.g., tester cycle), where (e.g., at what tester channel), and how (e.g., logic value) the test failed. Diagnostics attempt to derive from the fail log at which logical/physical ___location inside the chip the problem most likely started.

In some cases (e.g., PCBs[[Printed circuit board]]s, [[Multi-Chip Module]]s (MCMs), embedded or stand-alone [[Memory (computers)|memories]]) it may be possible to repair a failing circuit under test. For that purpose diagnostics must quickly find the failing unit and create a work-order for repairing/replacing the failing unit.

DFT approaches can be more or less diagnostics-friendly. The related objectives of DFT are to facilitate/simplify fail data collection and diagnostics to an extent that can enable intelligent failure analysis (FA) sample selection, as well as improve the cost, accuracy, speed, and throughput of diagnostics and FA.

== Scan design ==

The most common method for delivering test data from chip inputs to internal "''circuits under test"'' (CUTs, for short), and observing their outputs, is called scan-design. In scan-design, registers ([[Flip-flop (electronics)|flip-flopsflop]]s or latches[[latch]]es) in the design are connected in one or more "[[scan chains"chain]]s, which are used to gain access to internal nodes of the chip. Test patterns are shifted in via the scan chain(s), functional [[clock signalssignal]]s are pulsed to test the circuit during the "capture cycle(s)", and the results are then shifted out to chip output pins and compared against the expected "good machine" results.

== Debug using DFT features ==