Mixed-Signal Test and DFT

May 17, 2001 Mixed-Signal Test and DFT: va@agere.com

Vishwani D. AgrawalAgere Systems, Murray Hill, NJ 47974

va@agere.comhttp://cm.bell-labs.com/cm/cs/who/va

May 17, 2001

VLSI Realization ProcessVLSI Realization Process

Determine requirements

Write specifications

Design synthesis and Verification

FabricationManufacturing test

Chips to customer

Customer’s need

Test development

Costs of TestingCosts of Testing Design for testability (DFT)

Chip area overhead and yield reduction Performance overhead

Software processes of test Test generation and fault simulation Test programming and debugging

Manufacturing test Automatic test equipment (ATE) capital cost Test center operational cost

Cost of Manufacturing Testing in 2000AD

0.5-1.0GHz, analog instruments,1,024 digital pins: ATE purchase price = $1.2M + 1,024 x $3,000 = $4.272M

Running cost (five-year linear depreciation) = Depreciation + Maintenance + Operation

= $0.854M + $0.085M + $0.5M = $1.439M/year

Test cost (24 hour ATE operation) = $1.439M/(365 x 24 x 3,600) = 4.5 cents/second

Testing PrincipleTesting Principle

Characterization TestCharacterization Test

Worst-case test Choose test that passes/fails chips Select statistically significant sample of

chips Repeat test for every combination of 2+

environmental variables Plot results in Schmoo plot Diagnose and correct design errors

Continue throughout production life of chips to improve design and process to increase yield

Manufacturing TestManufacturing Test

Determines whether manufactured chip meets specs

Must cover high % of modeled faults Must minimize test time (to control cost) No fault diagnosis Tests every device on chip Test at speed of application or speed

guaranteed by supplier

Burn-in or Stress TestBurn-in or Stress Test

Process: Subject chips to high temperature &

over-voltage supply, while running production tests

Catches: Infant mortality cases – these are

damaged chips that will fail in the first 2 days of operation – causes bad devices to actually fail before chips are shipped to customers

Freak failures – devices having same failure mechanisms as reliable devices

Test Specifications & PlanTest Specifications & Plan Test Specifications:

Functional Characteristics Type of Device Under Test (DUT) Physical Constraints – Package, pin

numbers, etc. Environmental Characteristics – supply,

temperature, humidity, etc. Reliability – acceptance quality level

(defects/million), failure rate, etc. Test plan generated from specifications

Type of test equipment to use Types of tests Fault coverage requirement

Automatic Test Equipment Components

Automatic Test Equipment Components Consists of:

Powerful computer Powerful 32-bit Digital Signal

Processor (DSP) for analog testing Test Program (written in high-level

language) running on the computer Probe Head (actually touches the bare

or packaged chip to perform fault detection experiments)

Probe Card or Membrane Probe (contains electronics to measure signals on chip pin or pad)

ADVANTEST Model T6682 ATE

LTX FUSION HF ATELTX FUSION HF ATE

Test Process SummarizedTest Process Summarized

Parametric tests – determine whether pin electronics system meets digital logic voltage, current, and delay time specs

Functional tests – determine whether internal logic/analog sub-systems behave correctly

ATE Cost Problems Pin inductance (expensive probing) Multi-GHz frequencies High pin count (1024)

ATE Cost Reduction Multi-Site Testing DFT methods like Built-In Self-Test

Mixed-Signal Testing Problem

Differences from Digital Testing

Size not a problem – at most 100 components Much harder analog device modeling

No widely-accepted analog fault model Infinite signal range Tolerances depend on process and

measurement error Tester (ATE) introduces measurement error Digital / analog substrate coupling noise Absolute component tolerances +/- 20%,

relative +/- 0.1% Multiple analog fault model mandatory

No unique signal flow direction

Present-Day Analog Testing Methods

Specification-based (functional) tests Main method for analog – tractable and

does not need an analog fault model Intractable for digital -- # tests is huge

Structural ATPG – used for digital, just beginning to be used for analog (exists)

Separate test for functionality and timing not possible in analog circuit Possible in digital circuit

DSP Tester ConceptDSP Tester Concept

Waveform SynthesisWaveform Synthesis

Needs sin x / x (sinc) correction – Finite sample width

Waveform SamplingWaveform Sampling

Sampling rate > 100 ks/s

A/D and D/A Test Parameters

A/D -- Uncertain map from input domain voltages into digital value (not so in D/A) Two converters are NOT inverses

Transmission parameters affect multi-tone tests Gain, signal-to-distortion ratio,

intermodulation distortion, noise power ratio, differential phase shift, envelop delay distortion

Intrinsic parameters – Converter specifications Full scale range (FSR), gain, # bits, static

linearity (differential and integral), maximum clock rate, code format, settling time (D/A), glitch area (D/A)

Ideal Transfer Functions

A/D Converter D/A Converter

Offset ErrorOffset Error

Gain ErrorGain Error

D/A Transfer Function Non-Linearity Error

Differential Linearity Error

Differential linearity function – How each code step differs from ideal or average step (by code number), as fraction of LSB

Subtract average count for each code tally, express that in units of LSBs

Repeat test waveform 100 to 150 times, use slow triangle wave to increase resolution

Linear Histogram and DLE of 8-bit ADC

DSP-Based TestingDSP-Based Testing DSP-based tester has:

Waveform Generator Waveform Digitizer High frequency clock with dividers for

synchronization A/D and D/A Test Parameters

Transmission Intrinsic

A/D and D/A Faults: offset, gain, non-linearity errors Measured by DLE, ILE, DNL, and INL

A/D Test Histograms – static linear and sinusoidal D/A Test –- Differential Test Fixture

DSP-Based Test Concepts

Quantization Error – Introduced into measured signal by discrete sampling

Quantum Voltage – Corresponds to flip of LSB of converter

Single-Tone Test -- Test of DUT using only one sinusoidal tone

Tone – Pure sinusoid of f, A, and phase Transmission (Performance) Parameter --

indicates how channel with embedded analog circuit affects multi-tone test signal

UTP – Unit test period: joint sampling period for analog stimulus and response

Spectral Test of A/D Converter

Spectral Components in DSP-Based Testing

A/D Converter Spectrum

Audio source at 1076 Hz sampled at 44.1 kHz

Coherent Multi-Tone Testing

Analog Test BusAnalog Test Bus PROs:

Usable with digital JTAG boundary scan Adds analog testability – both controllability and

observability Eliminates large area needed for analog test points

CONs: May have a 5 % measurement error C-switch sampling devices couple all probe points

capacitively, even with test bus off – requires more elaborate (larger) switches

Stringent limit on how far data can move through the bus before it must be digitized to retain accuracy

Analog Test Bus DiagramAnalog Test Bus Diagram

Analog Boundary ModuleAnalog Boundary Module

Chaining of 1149.4 ICsChaining of 1149.4 ICs

Partitioning for TestPartitioning for Test

Partition according to test methodology: Logic blocks Memory blocks Analog blocks

Provide test access: Boundary scan Analog test bus

Provide test-wrappers (also called collars) for cores.

Test-Wrapper for a Core

Test-wrapper (or collar) is the logic added around a core to provide test access to the embedded core.

Test-wrapper provides: For each core input terminal

A normal mode – Core terminal driven by host chip An external test mode – Wrapper element observes core

input terminal for interconnect test An internal test mode – Wrapper element controls state of

core input terminal for testing the logic inside core For each core output terminal

A normal mode – Host chip driven by core terminal An external test mode – Host chip is driven by wrapper

element for interconnect test An internal test mode – Wrapper element observes core

outputs for core test

A Test-WrapperA Test-Wrapper

Wrappertest

controller

Scan chain

to/from TAP

from/toExternalTest pins

Wrapperelements

Overhead EstimateOverhead Estimate

Rent’s rule: For a logic block the number of gates Gand the number of terminals t are related by

t = K G

where 1 < K < 5, and ~ 0.5.

Assume that block area A is proportional to G, i.e.,t is proportional to A 0.5. Since test logic is addedto each terminal t,

Test logic added to terminals

Overhead = ------------------------------------------------- ~ A –0.5

DFT Architecture for SOC

User defined test access mechanism (TAM)

Module

Testsource

Testsink

Module

Test access port (TAP)

Functionalinputs

FunctionaloutputsFunc.

inputs

Func.outputs

SOC inputs SOC outputsTD

Instruction register control

Serial instruction data

DFT ComponentsDFT Components

Test source: Provides test vectors via on-chip LFSR, counter, ROM, or off-chip ATE.

Test sink: Provides output verification using on-chip signature analyzer, or off-chip ATE.

Test access mechanism (TAM): User-defined test data communication structure; carries test signals from source to module, and module to sink; tests module interconnects via test-wrappers; TAM may contain bus, boundary-scan and analog test bus components.

Test controller: Boundary-scan test access port (TAP); receives control signals from outside; serially loads test instructions in test-wrappers.

SummarySummary Functional test: verify system hardware, software,

function and performance; pass/fail test with limited diagnosis; high (~100%) software coverage metrics; low (~70%) structural fault coverage.

Diagnostic test: High structural coverage; high diagnostic resolution; procedures use fault dictionary or diagnostic tree.

SOC design for testability: Partition SOC into blocks of logic, memory and analog

circuitry, often on architectural boundaries. Provide external or built-in tests for blocks. Provide test access via boundary scan and/or analog

test bus. Develop interconnect tests and system functional tests. Develop diagnostic procedures.

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