Transcript IPC-2221 3.5.3.5 Physical Test Concerns

IPC-2221
3.5.3.5 Physical Test Concerns
Printed board assembly functional test
equipment is usually very expensive
and requires highly skilled personnel
to operate. If printed board assembly
testability is poor, the printed board
assembly test operation can be very
expensive. There are some simple
physical considerations that can
decrease the debug time and
therefore the overall test costs.
IPC-2221
3.5.3.5 Physical Test Concerns
The orientation of polarized parts
should be consistent so that the
operator does not get confused with
parts being oriented 180° out of phase
with other parts on the printed board
assembly. Non-polarized parts still
need to have the pin #1 identified so
that the test operator knows which
end to probe when guided probe
software says to probe a specific pin.
IPC-2221
3.5.3.5 Physical Test Concerns
Test connectors are much preferred over
test points which require the use of test clips
or test hook-up wires. However, test points
such as riser leads are preferred over
clipping on to the lead of a part. If riser leads
are used for temporary testing, such as
determining a select-by-test resistor, it is
suggested that the risers remain after the
installation of the selected component. This
allows verification of the selected item
without re-fixturing the assembly.
IPC-2221
3.5.3.5 Physical Test Concerns
Signals that are not accessible for probing (such
as can happen with leadless parts) can greatly
increase fault isolation problems. If scan registers
are not used, it is recommended that every signal
have a land or other test point somewhere on the
printed board assembly where the signal can be
probed. It is also recommended that lands used
for test points be located on grid and placed so
that all the probing can be done from the
secondary side of the printed board assembly.
IPC-2221
3.5.3.5 Physical Test Concerns
If it is not feasible to provide capability
for probing every signal, then (1) only
the strategic signals should have
special probing locations and (2) the
test vectors need to be increased or
other test techniques need to be
utilized to assign fault isolation to one
component or a small set of
components.
IPC-2221
3.5.3.5 Physical Test Concerns
Many faults are often due to shorts between the
leads of adjacent parts, shorts between a part
lead and an external layer conductor on the
printed board or shorts between two printed
board conductors on the external layers of the
printed board. The physical design must
consider these normal manufacturing defects
and not impair the isolation of the faults due to
lack of access or inconvenient access to
signals. As with design for in-circuit testability,
probe pad test points should be on grid to allow
automated probing to be used in the future.
IPC-2221
3.5.3.5 Physical Test Concerns
Partitioning of the design into functions, perhaps
digital separated from analog, is sometimes
required for electrical performance. Testing
concerns also are helped with physical separation
of dissimilar functions. Separation of not just the
circuitry but also the test connectors or at least
grouping the pins on the connectors can help
improve testability. Designs that mix digital design
with high performance analog design may require
testing on two or more sets of test equipment.
Separating the signals will not only help the test
fixturing but will help the operator in debugging
the printed board assembly.
IPC-2221
3.5.3.5 Physical Test Concerns
As with in-circuit test fixturing, functional
test fixturing can have a significant cost
impact. Normally a standard board size or
only a few board sizes are used for all
designs on a program. Similarly one, or at
most a few, test fixtures are typically used
for a program. Generating test fixtures
can be costly and debugging noise
problems in the fixtures or tuning the
fixtures to the tester can be expensive.
IPC-2221
3.5.3.5 Physical Test Concerns
If the test fixturing is not adequately
engineered, it may not be possible to
accurately measure the board under test.
Typically much effort is expended in
generating a few test fixtures and it is
expected that the fixtures will be used for
all the printed board assembly designs.
Therefore the test fixturing restrictions
must be considered in the printed board
assembly design.
IPC-2221
3.5.3.5 Physical Test Concerns
The fixturing restraints can be
significant. Such as (1) requiring
ground and voltage supplies on
specific connector pins, (2) limiting
which pins can be used for high speed
signals, (3) limiting which pins can be
used for low noise applications, (4)
defining power switching limitations,
(5) defining voltage and current
limitations on each pin, etc.