 Test
strategies improve when boundary scan teams up with
functional test
by Dave Bonnett
ScanWorks Technical Marketing Manager
Today's high-density, complex electronic
products are becoming increasingly difficult to test.
To maintain high test coverage, new test methods are
being developed while the more established methods are
being applied in new ways. For example, boundary scan
has been implemented extensively in PCB assembly verification
for some time. Now though, new ways of creating and
running boundary-scan tests during functional testing
are being tried every day.
While automated optical inspection
(AOI) probably will discover missing, misaligned, incorrectly
oriented or incorrect devices, boundary scan detects
most shorts and opens before the printed circuit board
ever reaches functional test. So why would a manufacturer
deploy boundary scan again during functional test? The
same question is asked when boundary scan is implemented
at the same time as ICT. And the answer is the identical
for both questions: time and money is saved during manufacturing.
Every new product must have a test
strategy designed specifically for that product. AOI
and automated x-ray inspection (AXI) can effectively
spot defects before power is applied to a board and,
of course, neither test method requires costly fixturing.
In-circuit test (ICT) makes sense for many products,
but for many others a more cost-effective test strategy
might rely on AOI, boundary scan and functional test
rather than invest in ICT testers, fixtures and test
development. Choosing a test strategy is not an easy
decision. There are many variables to consider, and
often the best answer is not the most obvious one.
Once boundary scan and functional test
have been identified as playing a major role in a test
strategy, the best way of devising an efficient overall
test strategy often is to step back and examine the
most cost-effective test methods. Both boundary scan
and functional test require power and electrical connections
to the board. If both can be done on the same test platform,
the expense of setting up two different test platforms
can be eliminated, in addition to the time required
to set up and run tests on the two platforms.
When boundary scan and functional tests
are combined, each retains its traditional role. Boundary
scan supports structural tests to verify a board was
built correctly and functional test verifies functional
performance. Boundary-scan testing can be deployed in
a number of ways, all of which are effective. For example,
it can be implemented on a PC-based bench-top debug
station, deployed with a flying probe test system, integrated
into an Agilent 3070 ICT system or combined with a functional
test platform. In any case, the re-use of test vectors
developed earlier in the product's life cycle makes
boundary scan's implementation during functional testing
very cost effective.
Just as ICT requires a fixture, functional
testing requires a design-specific test environment.
The complexity of a design's test setup varies with
the board as well as with the functional test methods.
Because boundary scan requires only a four-wire connection
and power to the board, the complexity of the test setup
is driven by functional test requirements.
Not only is the hardware setup for
boundary-scan test compatible with that of functional
test, but the software packages commonly used for functional
test also can be used to control boundary-scan test
application. Several National Instruments products,
including LabVIEW, Lab Windows CVI, and Test Stand,
provide easy control of commercial test equipment. With
ASSET's ScanWorksAPI and Process Automation Scripting,
ScanWorks can be treated as any other piece of test
equipment. Commands can be invoked to drive specific
patterns at certain times or responses can be expected
at predefined times. ScanWorks also operates from a
boundary-scan controller card that is in the PXI or
Compact PCI (CPCI) form factor. This means that ScanWorks'
boundary scan tools can be included in the same PXI
or CPCI equipment rack as other commercial functional
test equipment.
With boundary-scan testing on the same
test platform as functional tests, and controlled through
the same test operator interface, the combining of boundary
scan and functional tests can be simplified and appear
to the test operator as a single test operation instead
of two separate and distinct operations. Generally speaking,
this combined process would begin with boundary-scan
tests to detect and isolate structural faults. This
is followed by boundary scan-based in-system programming
to load the latest version of firmware into the design's
PLDs and to store program data in flash memory. Boards
can even be configured for a specific application and
subsequently tested functionally for that application.
Boundary-scan structural test can be
combined easily with functional test, and, in addition
it also can assist during functional testing. Functional
tests consist of test patterns which stimulate the inputs
to a board and monitor the outputs at the correct time
to validate that the inputs have been processed correctly.
The inputs can be provided by commercial test equipment
or by known-good portions of the system where the board
is implemented. Likewise, a board's outputs can be measured
by commercial test equipment or by the next stage in
the system. In certain cases, some inputs may be held
at a static level while others are being toggled. In
other instances, output signals may remain static while
inputs are toggled. In all of these cases, boundary-scan
is often the most convenient way to control or monitor
static signals.
Boundary-scan also can be very helpful
when functional tests are exercising a system consisting
of several boards. To test a multi-board, the boards
must be connected either by cables or over a backplane.
Ensuring the system is connected properly before performing
functional tests saves many hours of chasing failures
induced by improper connections. Boundary-scan is ideally
suited to verifying the connections between boards (if
boundary-scan is available at the board edges). As an
extra bonus, boundary-scan interconnect tests that were
developed earlier in the product's life cycle, such
as during development to debug prototype designs, can
be re-used during functional test and run from the same
test executive, making boundary scan an integral part
of the test plan.
Even though Boundary scan will never
replace functional testing, just as it will not replace
ICT, it can be combined on the same test platform with
functional test, just as it is combined with ICT. The
results are significantly reduces test costs. In addition,
boundary scan also can make functional test simpler
and more effective by providing easy access to static
signals and validating system connections are properly
made before functional testing begins.
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