Test:
Electrical Test
Electrical testing
is the identification and segregation of electrical failures from a
population of devices. An electrical failure is any unit that does
not meet the electrical specifications defined for the device. In
simplified terms, electrical testing consists of providing a series
of electrical excitation to the
device
under test
(DUT) and
measuring the response of the DUT. For every set of electrical
stimuli, the measured response is compared to the expected response,
which is usually defined in terms of a lower and an upper limit.
Any DUT that exhibits a response outside of the expected range of
response is considered a failure.
In
production mode, electrical testing is usually performed using a
test system or platform, consisting of a tester (see Fig. 1) and a
handler (see Fig. 2). Such a test system is also referred to as an
automatic (or automated) test equipment,
or
ATE.
The tester performs the electrical testing itself, while the handler
takes care of transferring the unit to the test site and positioning
it for proper testing, as well as reloading it back into another
tube after the testing process is completed.
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Figure 1. Example of an IC Tester (Left) and
Three (3) Examples of Test Handlers (Right) |
The
testing process executed by the tester is controlled by the
test
program
or test software. The test program is usually written in a high
level language such as C++ or Pascal. It consists of a series of
several
test
blocks,
each of which tests the DUT for a certain parameter. Every test
block sets up the DUT fixtures for proper testing of the DUT for
the corresponding parameter. It also tells the tester what
electrical excitation needs to be applied to the DUT, as well as
the correct timing of applying them.
There are
usually two versions of the test program. One is a
production
version and the other is a
quality
assurance
version. The production version has stricter limits compared to the
QA version, while the QA version more or less tests the DUT to the
datasheet specification limits. The differences in production and QA
limits, or the
guardbands,
should be large enough to take into account errors attributed to
over-all testing variability and noise, but not large enough to
result in over-rejection. If the guardband is chosen properly, any
unit passing the production test is almost sure to pass the
datasheet limits, regardless of which test equipment on the floor is
used.
The test
program usually consists of two types of test blocks, namely,
parametric and functional.
Functional
testing checks if the device is able to perform its basic
operation.
Parametric
testing checks if the device exhibits the correct voltage, current,
or power characteristics, regardless of whether the unit is
functional or not. Parametric testing usually consists of forcing a
constant voltage at a node and measuring the current response
(force-voltage-measure-current, or FVMC) at that node, or forcing a
constant current at a node and measuring the voltage response
(force-current-measure-voltage, or FCMV).
Electrical testing is normally done at ambient temperature, but
testing at
other
temperatures is also being done depending on the screening
requirements. For instance, latch-up problems have better chances
of being detected at an elevated temperature while hot carrier
failures are easier detected at low temperatures. Aside from 25C,
other standard test temperatures include -40C, 0C, 70C, 85C, 100C,
and 125C.
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