Objective:
To present the concept of failure analysis for ASIC work and
for use as an example methodology.
Failure Analysis (F/A) takes place after you have discovered an
ASIC-related failure. The failure may actually originate at any of the
following places:
- the ASIC
- other active components in the circuit around the ASIC
- passive components in the circuit around the ASIC
- circuit interconnect
- operation of the circuit with the ASIC outside of specified
operating conditions of voltage, current, temperature, etc.
ASIC failure analysis determines the cause of the failure. Although
the visible failure effect may be in the ASIC, the cause may be
external to the ASIC. When verified within the ASIC, F/A continues
on the device. If the device passes all specifications and the failure
cannot be found in the ASIC, then failure analysis must focus on
other parts of the circuit surrounding the ASIC. Formally stated:
failure analysis examines electronic parts to determine the cause of
performance variations outside previously established limits, for the
purpose of identifying failure mechanisms and failure activating
causes.
Because of the high cost, F/A usually cannot be performed on every
failed part. In response, QPL and QML have devised a system to account for
expected failures due to random process variations. Thus engineers
need perform F/A only when the failure is most likely due to a
systemic (i.e. recurring and correctable) problem. Failures assumed
due to random errors are expressed as a percentage of parts from a
given lot, called the lot tolerance percent defective
(LTPD), and from a given screen, called the percent defective allowable (PDA). This approach
saves significant money by foregoing F/A on statistically expected
part failures that have no remedy, due to their random nature.
However, some failures that forego analysis could result from subtle
but correctable problems in the fabrication process if the LTPD and
PDA are not statistically accurate. Therefore, determining when and
when not to perform F/A calls for sound engineering judgment.
F/A consists of logical and systematic examination of the failed part
in order to identify and analyze the failure mode and mechanism and
to recommend an appropriate corrective action to prevent recurrence
of the failure. This procedure breaks into five major tasks:
- Verify the failure.
- Determine failure mode (i.e., the symptoms or the way a part has
failed).
- Diagnose the failure mechanism (i.e., the physical cause of the
failure).
- Design stress conditions to perform a simulation (i.e., a
reproduction of the failure), if needed.
- Suggest corrective actions to prevent or minimize the possibility
of similar or like failures.
Many examinations and tests are used to systematically deduce the
cause of a failure. They can be broken into four types:
- failure confirmation
- nondestructive (parts are not physically altered)
- semidestructive (parts that are physically altered, but should still
function)
- destructive (parts are physically altered and will no longer
function)
Examinations and tests used for each of these categories are outlined
below, along with the contents of the F/A report.
- Part Failure History: The
F/A group carefully reviews all information associated with the
failed part and how it failed prior to the examination and testing of
the part.
- External Visual
Examination: The identification markings and any package
discrepancies on the suspect part are recorded and photographed as
needed.
- Electrical
Characterization: This includes curve tracer measurements,
functional tests, parametric tests, or any environmental tests as
required to verify the reported failure.
- Radiographic
Examination: Two X-ray views of the package are taken for
possible internal anomalies and for assisting in opening of the
package.
- Hermeticity Tests: Gross and
fine leak tests are conducted to check the package seal integrity.
- Particle Detection Test:
The particle impact noise detection (PIND) test is performed when
particle(s) is suspected.
- Package Gas Analysis: This
test qualitatively and quantitatively analyzes the gaseous
components in the package cavity and especially the water-vapor
content.
- Package Opening and Internal
Examination: Delidding of the package and internal optical and
scanning electron microscopic (SEM) examinations are performed to
identify any physical symptoms of the failure or any manufacturing
defects.
- Emission Microscope
Test: The emission microscope for multilayer inspection (EMMI)
locates failure sites having current leakages down to 0.5 microamps
(MA).
- Electromechanical
Probing: This technique isolates the failure area when none of
the other techniques (e.g., the emission microscope, the liquid crystal
or the SEM voltage contrast) have been successful in locating the
failure site.
- Material Analysis: The SEM
energy dispersive spectroscopy (EDS) performs qualitative and
quantitative elemental composition analysis of contaminants or any
suspect materials. EDS provides a hard copy listing of all analytical
findings and/or photographs with elements identified by individual
colors.
- Removal of Layers: Chemical
etching or dry plasma etching are used to remove layers (e.g.,
glassivation, metallization, oxide) in order to uncover the failure site
obscured by these materials.
- Cross-Sectioning: This technique
exposes the interior of a suspect solid material for further
examination and tests.
- Mechanical Tests: Destructive or
nondestructive wire bond pull tests, or die shear tests, are performed
as needed for failure (or defect) identification.
The failure analysis report, together with the analysis results and
conclusions, will cover all significant measurements and data
including optical and SEM photographs necessary to prove the
determination of cause of failure. Recommendation for corrective
action is the last significant part of the failure analysis report.
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