Basic Failure
Analysis (FA) Flows
Every
experienced failure analyst knows that every FA is unique. Nobody can
truly say that he or she has developed a standard failure analysis flow for every FA
request that will come his or her way. FA's have a tendency of directing
themselves, with each subsequent step depending on the outcome of the
previous step.
The flow of
failure analysis is influenced by a multitude of factors: the device itself, the
application in which it failed, the stresses that the device has undergone
prior to failure, the point of failure, the failure rate, the failure
mode, the failure attributes, and of course, the failure mechanism.
Nonetheless, FA is FA, so it is indeed possible to define to a certain
degree a 'standard' FA flow for every failure mechanism.
This article
aims to give the reader a basic idea of how the FA flow for a given
failure mechanism may be standardized. 'Standardization' in this context
does not mean defining a step-by-step FA procedure to follow, but rather
what to look for when analyzing failures depending on what the observed or
suspected failure mechanism is.
Basic
Die-level FA Flow
1)
Failure
Information Review.
Understand thoroughly the customer's description of the failure.
Determine: a) the specific electrical failure mode that the customer is
experiencing; b) the point of failure or where the failure was encountered
(field or manufacturing line and at which step?); c) what conditions the
samples have already gone through or been subjected to; and d) the failure
rate observed by the customer.
2)
Failure
Verification.
Verify the customer's failure mode by electrical testing. Check the
datalog results for consistency with what the customer is reporting.
3)
External Visual
Inspection.
Perform a thorough external visual inspection on the sample. Note
all markings on the package and look for external anomalies, i.e.,
missing/bent leads, package discolorations, package
cracks/chip-outs/scratches, contamination, lead oxidation/corrosion,
illegible marks, non-standard fonts, etc.
4)
Bench Testing.
Verify the electrical test results by bench testing to ensure that all ATE
failures are not due to contact issues only.
The ideal case
is for the customer's reported failure mode, ATE results, and bench test
results to be consistent with each other.
5)
Curve Tracing.
Perform curve tracing to identify which pins exhibit current/voltage
(I/V) anomalies. The objective of curve tracing is to look for open
or shorted pins and pins with abnormal I/V characteristics (excessive
leakage, abnormal breakdown voltages, etc.). FA may then be focused
on circuits involving these anomalous pins. Dynamic curve tracing, wherein
the unit is powered up while undergoing curve tracing, may be performed if
static curve tracing does not reveal any anomalies.
6)
X-ray
Inspection.
Perform x-ray inspection to
look for internal package anomalies such as broken wires, missing wires,
incorrect or missing die, excessive die attach voids, etc, without having
to open the package. Xray inspection results must be consistent with
curve trace results, e.g., if x-ray inspection revealed a broken wire at a
pin, then curve tracing should reveal that pin to be open.
7)
CSAM.
Perform CSAM
on plastic packages to determine if the samples have any internal
delaminations that may lead to other failure attributes such as corrosion,
broken wires, and lifted bonds.
8)
Decapsulation.
Once
all the non-destructive steps such as those above have been completed, the
samples may be subjected to decapsulation to expose the die and other
internal features of the device for further FA.
9)
Internal Visual
Inspection.
Perform
internal visual inspection after decap. This is usually done using a
low-power microscope and a high-power microscope, proceeding from low
magnification to higher ones. Look for wire/bond anomalies, die cracks,
wire and die corrosion, die scratches, EOS/ESD sites, fab defects, and the
like. SEM inspection may be needed in some instances.
10)
Hot Spot
Detection.
If
curve trace results indicate some major discrepancies between the I/V
characteristics (especially with regard to power dissipation) of the
samples and known good units, then the samples may have localized heating
on the die. For example, an abnormally large current flowing between
an input pin and GND may mean a short circuit from this input pin to GND.
Shorts such as this will emit heat that can be located by hot spot
detection techniques.
11)
Light Emission
Microscopy.
If
the device does not exhibit abnormalities in power dissipation that may
indicate hot spots, light emission microscopy may be performed to look for
defects that emit light.
Note that an emission site does not mean that it is the failure site.
12)
Microprobing.
Microprobing
becomes necessary if no hot spots nor abnormal photoemissions were seen
from the samples. Microprobing
may entail extensive circuit analysis wherein the failure site is
pinpointed by analyzing the die circuit stage by stage or section by
section. The thought process used when troubleshooting a full-size circuit
also applies to die circuit troubleshooting.
13)
Die
Deprocessing.
Perform
die deprocessing to look for subsurface damage or defects if the above FA
steps were not successful in locating the failure site.
Basic
Ball Lifting FA Flow
1)
Failure Information Review.
Check the customer's description of the failure for telltale signs of
ball lifting, i.e., a) functional or catastrophic failures that may
indicate an open bond; b) pins that become intermittently open when
pressure is applied to the package or if the device is subjected to
elevated or extremely low temperature; or c) high-resistance or
permanently open pins.
2)
Device/Lot History Review.
Check the FA history of the device to determine if it has exhibited
ball lifting returns previously. Check the assembly and test history
of the lot to determine if the lot has exhibited any yield or process
issues potentially related to ball lifting. Sad to say, most ball lifting
issues have assignable causes and are non-random in nature, so containment
or bounding of the problem must be meticulously pursued.
3)
Failure Verification.
Verify the customer's failure mode by electrical testing. If ball lifting
is suspected but the unit is passing e-test, test the unit several times
because the unit may have intermittently good bonds that allow it to pass.
E-test must also be performed at elevated temperature if possible.
4)
External Visual Inspection.
Perform a thorough external visual inspection on the sample. Note
all package anomalies that may indicate the unit having been subjected to
thermo-mechanical stresses.
5)
Bench Testing.
Verify the electrical test results by bench testing at the temperature
where the failure was seen. If e-test at high temperature did not
verify the failure reported by the customer, perform the bench test at
elevated temperature as well.
6)
Curve Tracing.
Perform curve
tracing at ambient, elevated (125C-150C) and low temperature (-10C to
-40C). This is the turning point of any ball lifting FA, because a lifted
ball bond should be seen as an open pin at elevated, if not at ambient,
temperature. Some lifted balls manifest at low temperature, although
not as frequently. Note that the sample is unlikely to be a ball
lifting failure if none of its pins is open, whether permanently or
intermittently.
7)
X-ray Inspection.
Perform x-ray
inspection as part of the FA routine. Don't expect to find any lifted
balls in the xray image if no open pins were seen during curve tracing.
On the other hand, if you see a lifted ball during xray inspection, then
consider this as a gross case of ball lifting and ask yourself how this
could have passed electrical testing.
8)
CSAM.
Perform CSAM
on plastic packages to determine if the samples have any internal
delaminations that may lead to ball lifting. Delaminations play an
important part in aggravating, if not directly causing, lifted ball bonds.
Movement of the plastic compound parallel to or away from the die surface
as a result of delamination can shear ball bonds off their bond pads.
9)
Decapsulation/Internal Visual Inspection.
Perform
internal visual inspection after decap. SEM inspection is most
useful in verifying lifted ball bonds, since some lifted balls may not be
visible optically due to the poor depth of field of optical microscopes.
Once a lifted ball is found, perform further visual inspection on the
affected bond pad, looking for signs of contaminants, deep probe
marks/exposed oxide, cratering, metal lifting, corrosion, and other
attributes that may lead to ball lifting.
10)
Microprobing
(optional).
Some ball
bonds will not appear to be 'lifted' visually, even under SEM inspection.
In such cases, it is necessary to confirm that the ball bond has no
electrical contact with the bond pad by microprobing. Of course,
this works best if you've already identified which pin is anomalous during
curve tracing.
11)
Aspect Ratio
Quantification.
Use your SEM to estimate the aspect ratio of your ball bond. Ball
bond aspect ratio is defined as the ratio of the ball diameter to the ball
height, so flatter bonds will exhibit higher aspect ratios. Well-formed
ball bonds would exhibit aspect ratios between 3 to 5. Balls are
considered underbonded (AR<2.5) or overbonded (AR>5.5) if way outside this
range. Poorly formed bonds mean a processing problem at wirebond that can
lead to ball lifting.
12)
IMC
Quantification.
Use your optical microscope to quantify the intermetallic coverage (IMC)
of the ball bond. This is done by getting the percentage of the
intermetallic formation on the ball bond surface. An IMC of at less than
50% (i.e., less than 50% of the bonded surface has intermetallics)
indicate insufficient intermetallic formation. Try to correlate the amount
and geometry of the IMC with whatever visual attributes are observed on
the bond pad. Remember that poor IMC formation is most often due to
bond pad anomalies that impede bonding.
13)
EDX Analysis.
Perform
EDX analysis on the bond pads and ball bond surface to look for
contaminants that may have impeded intermetallic formation. Note
that silicon over the bond pad (unetched glass or Si saw dust) is a very
common cause of ball lifting, so don't immediately presume that the
silicon peak came from the wafer/substrate. Silicon is on top of the bond
pad if its peak increases relative to that of aluminum when the SEM EHT is
lowered.
14)
Wire Pull Test/Ball Shear Test.
If only one or
two bonds have lifted, it may be useful to check the strengths of the
other bonds of the sample(s). This will indicate whether the bonding
problem is localized to a particular area of the die or it affects all
the bonds. This is highly destructive, and must only be done as
one of the last steps (if not the last one) of the analysis.
15)
Conclusion.
As may be
discerned from above, the basic flow of a ball lifting FA consists of the
following: a) looking for intermittent or open pins prior decap; b)
visually and electrically confirming the ball lifting after decap; c)
assessment of the IMC; d) identification of the physical and chemical
abnormalities on the bond pad and the ball itself that correlate with the
IMC observed; and e) subsequent investigations/simulations/evaluations to
identify the root cause of these anomalies.
Basic Die
Cracking FA Flow
1)
Failure Information/Device and Lot History Review.
Understand the customer's description of the failure, i.e., the
failure mode, where it was encountered, what conditions the sample was
subjected to, etc. Check the FA history of the device to determine if it
has exhibited die cracking returns before. Check the assembly and
test history of the lot to determine if the lot has exhibited any yield or
process issues potentially related to die cracking.
2)
Failure
Verification.
Verify the customer's failure mode by electrical testing.
3)
External Visual
Inspection.
Perform a thorough external visual inspection on the sample. Note
all package anomalies that may indicate the unit having been subjected to
thermo-mechanical stresses, i.e., package cracks/chip-outs, tool marks,
bent leads, discolored/burned package, etc.
4)
Bench Testing.
Verify the electrical test results by bench testing.
5)
Curve Tracing.
Perform curve tracing at ambient, elevated (125C-150C) and low
temperature (-10C to -40C). Look for open or shorted pins which may
indicate gross die cracking. Note, however, that some die crack
failures may only exhibit subtle I/V curve anomalies.
7)
X-ray
Inspection.
Perform x-ray inspection on the sample. Check for die attach
problems such as excessive voids, die overhang, insufficient die attach
coverage, and insufficient fillet. Check also for molding compound voids
and cracks. Gross die cracks may also be found using sophisticated x-ray
equipment.
8)
CSAM.
Perform CSAM on plastic packages to determine if the samples have any
internal delaminations that are indicative of the unit having been
subjected to extremely high temperatures. Units with severe die attach
abnormalities will exhibit die cracking upon exposure to temperature
extremes.
9)
Decapsulation/Internal Visual Inspection.
Perform internal visual inspection after decap to confirm the die crack.
The crack pattern on the die surface as well as the die edge must be fully
understood through extensive optical and SEM inspection.
10)
Full
Decapsulation.
Many die cracking issues involve die cracks that originate from the
backside of the die. If SEM inspection of the die surface and die
edge indicates that the cracks most likely originated from the die
backside, then full decapsulation must be done. Full decapsulation
consists of immersing the entire unit in acid to disintegrate the entire
package, leaving behind the die only. The die backside crack pattern
may then be inspected freely once full decap is completed.
11)
Fractography.
Fractography is the systematic and scientific process of determining the
origination and propagation of the cracking mechanism by studying the
attributes of the fracture surface of the die. Fractography is a
complicated process and can only be done reliably through years of study
and experience. Once mastered, fractography would be an indispensable tool
for analyzing die crack issues.
Note
that Steps 9, 10, and 11 all have one objective: to understand the crack
origin and propagation pattern to determine what stresses were applied to
the die.
12)
Conclusion.
As
may be discerned from above, the basic flow of a die cracking FA consists
of the following: a) taking note of all electrical and visual/mechanical
attributes of the sample before decap; b) confirmation of the die crack
after decap; c) determination of the point of origin and propagation
pattern of the die crack; d) determination of the points of application
and direction of the
stresses most likely
experienced by the die based on the crack origin and propagation; and e)
subsequent investigations, simulations, or evaluations to identify the
root cause of the stresses.
Basic
Package Cracking FA Flow
1)
Failure Information/Device and Lot History Review.
Understand the customer's description of the package crack failure.
Check the FA history of the device to determine if it has exhibited
package cracking occurrences before, whether in the field or in the
manufacturing line. Check the assembly and test history of the lot
to determine if the lot has exhibited any yield or process issues
potentially related to package cracking.
2)
Failure
Verification.
Perform external visual inspection on the sample to confirm the package
cracks reported by the customer. Note the similarities and
differences between the customer's description of the package crack and
the actual package crack.
3)
External Visual
Inspection.
Perform a more thorough external visual inspection on the sample to
completely characterize the package crack. Check how many distinct
crack lines there are, where they originate and where they end, and how
they propagated from these end points. Note also all other package
anomalies that may indicate the unit having been subjected to
thermo-mechanical stresses, i.e., package chip-outs, tool marks,
bent/non-coplanar leads, discolored/burned package, etc.
4)
Look for
Origin/Propagation Patterns.
Check how many distinct crack lines there are, where they originate and
where they end, and how they propagated from these end points. If there
are several units affected, check for specific patterns with regard to how
the cracks are localized. Are they on one side of the package only? Do
they affect certain pins only? Do they always occur at certain features of
the package only, e.g., at the top-bottom package interface, at the tie
bar, at the leads, etc.?
5)
CSAM.
Perform CSAM on the samples to check for any internal delaminations that
are indicative of the unit having been subjected to extremely high
temperatures. Check also for localized delaminations that correlate with
the locations of the package cracks.
6)
Stress Analysis.
Analyze the package crack characteristics and internal delaminations to
formulate your best hypothesis (or hypotheses) on how the unit was
stressed. A good guideline to follow for this is that fractures always
occur under tensile stresses. List down as many possible scenarios
or conditions that can result in these cracks. Pay particular attention to
the possibility that these have been caused in the manufacturing line.
Be sure to enlist the help of the Back-end Assembly experts in generating
the list of hypotheses.
7)
Simulations.
Perform simulations on good units to verify each of your hypothetical
root causes. For example, if you think that debris under the package
during DTF caused the problem, then perform DTF on units with debris
underneath them. You know you've pinned down the actual cause if you've
duplicated the exact package crack pattern.
FA
Techniques: Failure
Verification;
Optical
Inspection;
Xray
Radiography;
Curve Tracing;
Decapsulation;
Sectioning;
Microthermography; LEM;
Microprobing;
Die
Deprocessing;
Focused
Ion Beam;
SEM/TEM;
Acoustic
Microscopy;
Other
FA Techniques
See Also:
Failure
Analysis; Ball
Lifting FA Flow; Die Crack
FA Flow;
Package
Crack FA Flow;
Package Failures; Die
Failures;
Reliability Engineering;
Reliability Modeling
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