2. OBJECTIVES
What is DFT?
Why we need DFT?
DFT Methods
Testing Economics
Goal of DFT
ATPG
BIST
Faults Models
Stuck at Faults Model
Path Sensitization
3. WHAT IS DFT?
Design for testability (DFT) refers to those
design techniques that make test generation and
test application cost-effective.
DFT consists of IC design techniques that add
testability features to a hardware product
design.
The purpose of manufacturing tests is to
validate that the product hardware contains no
manufacturing defects that could adversely
4. WHY DESIGN FOR TESTABILITY?
Testability is a design characteristic that
influences various costs associated with
testing.
It allows for:
Device status to be determined
Isolation of faults
Reduce test time and cost
5. CONTROLLABILITY
Ability to establish a specific signal value at
each node by setting circuit’s inputs
Circuits typically difficult to control: decoders,
circuits with feedback, oscillators, clock
generators …
7. PREDICTABILITY
Ability to obtain known output values in
response to given input stimuli
Factors affecting predictability
Initial state of circuit
Hazards
8. DIFFICULT TEST CASES
Sequential logic is more difficult to test than
combinational logic
Control logic is more difficult to test than data-
path logic
Random logic is more difficult to test than
structured bus-oriented designs
Asynchronous design is more difficult to test
than synchronous design
9. TESTING ECONOMICS
Chips must be tested before they are assembled onto
PCBs, which, in turn, must be tested before they are
assembled into systems.
The rule of ten:
If a chip fault is not detected by chip testing, then
finding the fault costs 10 times as much at the PCB
level as at the chip level.
Similarly, if a board fault is not found by PCB testing,
then finding the fault costs 10 times as much at the
10. GOAL OF DESIGN FOR TESTABILITY
(DFT)
Improve
Controllability
Observability
Predictability
11. DFT METHODS
DFT methods for digital
circuits:
Ad-hoc methods
Structured methods:
Scan
Partial Scan
Built-in self-test (BIST)
12. AD-HOC DFT METHODS
Good design practices learnt through experience are used as
guidelines:
Avoid asynchronous (unclocked) feedback
Make flip-flops initializable
Avoid redundant gates
Avoid large fan-in gates
Provide test control for difficult-to-control signals
Avoid gated clocks
Design reviews conducted by experts or design auditing
tools
Disadvantages of ad-hoc DFT methods:
Experts and tools not always available
Test generation is often manual with no guarantee of high fault
coverage
13. SCAN DESIGN
Circuit is designed using pre-specified design rules.
Test structure (hardware) is added to the verified design:
Add a test control (TC) primary input.
Replace flip-flops by scan flip-flops (SFF) and connect to form
one or more shift registers in the test mode.
Make input/output of each scan shift register
controllable/observable from PI/PO.
Use only clocked D-type of flip-flops for all state variables
At least one PI pin must be available for test; more pins, if
available, can be used
14. BUILT-IN SELF-TEST
Advances in microelectronics technology have
introduced a new paradigm in IC design: System-on-
Chip (SoC)
Many systems are nowadays designed by embedding
predesigned and preverified complex functional blocks
(cores) into one single die
Such a design style allows designers to reuse previous
designs and will lead to shorter time-to-market and
reduced cost
System-on-Chip
Embedded
DRAM
Int er f ace
Cont r ol
opmplex
cor e
UDL
gacy
cor e
DSP
cor e
Self - t est
cont r ol
49. 1
UDL
SoC structure breakdown:
10% UDL
75% memory
50% in-house cores
60-70% soft cores
15. BIST TECHNIQUES
BIST techniques are classified:
on-line BIST - includes concurrent and non-concurrent techniques
off-line BIST - includes functional and structural approaches
On-line BIST - testing occurs during normal functional operation
Concurrent on-line BIST - testing occurs simultaneously with normal operation mode, usually
coding techniques or duplication and comparison are used
Non-concurrent on-line BIST - testing is carried out while a system is in an idle state, often by
executing diagnostic software or firmware routines
Off-line BIST - system is not in its normal working mode, Usually
on-chip test generators and output response analysers or micro diagnostic routines
Functional off-line BIST is based on a functional description of the Component Under Test
(CUT) and uses functional high-level fault models
Structural off-line BIST is based on the structure of the CUT and uses structural fault models
(e.g. SAF)
16. GENERAL ARCHITECTURE OF BIST
BIST
Control Unit
Circuitry Under Test
CUT
Test Pattern Generation (TPG)
Test Response Analysis (TRA)
BIST components:
Test pattern generator (TPG)
Test response analyzer (TRA)
TPG & TRA are usually
implemented as linear
feedback shift registers
(LFSR)
Two widespread schemes:
test-per-scan
test-per-clock
17. BIST BENEFITS
Reduced testing and maintenance cost
Lower test generation cost
Reduced storage / maintenance of test patterns
Simpler and less expensive ATE
Can test many units in parallel
Shorter test application times
Can test at functional system speed
18. DRAWBACKS OF BIST
Additional pins and silicon area needed
Decreased reliability due to increased silicon area
Performance impact due to additional circuitry
Additional design time and cost
19. BOUNDARY SCAN
An outline of a typical test procedure using a boundary scan is
as follows:
– A boundary-scan test instruction is shifted into the IR
through the TDI.
– The instruction is decoded by the decoder associated with
the IR to generate the required control signals so as to
properly configure the test logic.
– A test pattern is shifted into the selected data register
through the TDI and then applied to the logic to be tested.
– The test response is captured into some data register.
– The captured response is shifted out through the TDO
for observation and, at the same time, a new test pattern can
be scanned in through the TDI.
20. AUTOMATIC TEST PATTERN
GENERATION
Automatic test equipment (ATE) is computer-controlled equipment used in
the production testing of ICs (both at the wafer level and in packaged devices)
and PCBs.
Test patterns are applied to the CUT and the output responses are compared
to stored responses for the fault free circuit.
Generating effective test patterns efficiently for a digital circuit is thus the
goal of any Automatic-Test-Pattern- Generation (ATPG) system.
The effectiveness of ATPG is measured by the number of modeled defects, or
fault models, detectable and by the number of generated patterns.
22. FAULT MODELS
Stuck-at fault model
Transistor faults
Bridging faults
Delay faults
• Delay faults can be classified as:
1) Gate delay fault
2) Transition fault
3) Hold Time fault
4) Slow/Small delay fault
23. 0
1
1
1
0
1/0
1/ 0
stuck-at-0
Fault-free ResponseTest Vector
Faulty Response
Assumptions: Only one line is faulty.
Faulty line permanently set to 0 or 1.
Fault can be at an input or output of
a gate.
STUCK-AT FAULT MODEL
In this model, one of the
signal lines in a circuit is
assumed to be stuck at
a fixed logic value,
regardless of what
inputs are supplied to
the circuit.
24. MULTIPLE STUCK-AT FAULTS
Several stuck-at faults occur at the same time
Important in high density circuits
For a circuit with k lines
There are 2k single stuck-at faults
There are 3k-1 multiple stuck-at faults
ATPG algorithms for multiple s-a-faults are
much more complex and not as well developed
25. WHY SINGLE STUCK-AT FAULTS?
Complexity is greatly reduced.
Many different physical defects may be modeled by the same
logical single stuck-at fault.
Single stuck-at fault is technology independent.
Can be applied to TTL, ECL, CMOS, etc.
Single stuck-at fault is design-style independent.
Gate Arrays, Standard Cell, Custom VLSI
Even when single stuck-at fault does not accurately model
some physical defects, the tests derived for these faults may
still be effective for these defects.
Single stuck-at tests cover a large percentage of multiple
stuck-at faults.
26. TRANSISTOR FAULTS
This model is used to describe faults for CMOS logic gates. At transistor level,
a transistor maybe stuck-short or stuck-open.
In stuck-short, a transistor behaves as it is always conducts (or stuck-on), and
stuck-open is when a transistor never conducts current (or stuck-off). Stuck-
short will produce a short between VDD and VSS.
BRIDGING FAULTS
A short circuit between two signal lines is called bridging faults. Bridging to
VDD or VSS is equivalent to stuck at fault model.
If one driver dominates the other driver in a bridging situation, the dominant
driver forces the logic to the other one, in such case a dominant bridging fault
is used.
27. COMBINATIONAL ATPG
The combinational ATPG method allows testing the individual
nodes (or flip-flops) of the logic circuit without being concerned
with the operation of the overall circuit.
During test, a so-called scan-mode is enabled forcing all flip-
flops (FFs) to be connected in a simplified fashion, effectively
bypassing their interconnections as intended during normal
operation.
This allows using a relatively simple vector matrix to quickly test
all the comprising FFs, as well as to trace failures to specific FFs.
28. SEQUENTIAL ATPG
Sequential-circuit ATPG searches for a sequence of test vectors to
detect a particular fault through the space of all possible test vector
sequences.
Even a simple stuck-at fault requires a sequence of vectors for
detection in a sequential circuit.
Due to the presence of memory elements, the controllability and
observability of the internal signals in a sequential circuit are in
general much more difficult than those in a combinational logic
circuit.
30. Try path f – h – k – L. This path is
blocked at j, since there is no way to
justify the 1 on i
1
0
D
D1
1
1
D
D
D
PATH SENSITIZATION
31. Try simultaneous paths f – h – k – L and
g – i – j – k – L. These paths blocked at k
because D-frontier (chain of D
or D) disappears
1
DD
D
D
D
1
1
1
PATH SENSITIZATION
32. Final try: path g – i – j – k – L – test found!
0
D
D D
1 D
D
1
0
1
PATH SENSITIZATION