dfmea, thermal derating caf, via fatigue dfr... · dfmea basics - failure mode and effect analysis...
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DFMEA, Thermal Derating CAF, VIA Fatigue
Dr. Nathan Blattau
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o DFMEA – Design Failure Mode Effects Analysis
o Potential failure modes of the components
o The effect that a failure of the component will have on the circuit
o Thermal Derating
o Component temperature ratings for storage and operation
o Comparison of those ratings to the applied thermal environments
o CAF – Conductive Anodic Filament Formation
o The potential for conductive filaments forming internal to the circuit
board
o Typically due to drill damage and hole spacing
o VIA Fatigue
o Thermal cycling fatigue of the plated through holes of the PCB
o IMEC and IPC-TR-579 models
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DFMEA
o Analyzes potential reliability problems early in the development cycle of
a product, when it is easier to fix problems and therefore enhance
overall reliability.
o FMEA is used to
o Identify potential failure modes
o Determine their effect on the operation of the product
o Identify actions to mitigate the failures
o Design Failure Modes and Effects Analysis (DFMEA) focuses on
components and subsystems
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What DFMEA is….
o A systematic group of
activities designed to:
o Recognize and evaluate
potential failures of systems,
products, or processes
o Identify the effects and
outcomes of the failures
o Identify actions that could
eliminate or mitigate the
failures
o Provide a historical written
record of the work performed
Why perform a DFMEA?
o Purpose of an DFMEA Study is to analyze:
o What might go wrong?
o How bad might the effect be?
o How might it be prevented, mitigated or detected at the earliest possible moment?
o With lowest cost, impact, safety risk….
o Develop a DFMEA process for use in future designs
DFMEA Basics - Failure Mode And Effect Analysis
o DFMEA is a widely used and powerful analysis & design review technique.
o Extremely comprehensive element by element review of:
o What can go wrong
o What will happen
o How the situation can be improved
o For improving a design
o For each design component or element:
o List how failures can occur (Failure Mode, what can go wrong, how the
failure manifests itself)
o List what could happen (Failure Effect, consequences of the mode).
o List how processes or the system itself can detect & prevent the problem
o Generate Recommendations for Improvements.
Some Key DFMEA Terms
o Failure: The loss of expected or intended function under stated conditions
o Failure mode: The way in which a failure is observed; generally describes the way the failure occurs.
o Failure effect: The immediate consequences of a failure on operation, function or functionality
o Failure cause: Defects in design, system, process, quality, or part application, the underlying cause of the failure or things which initiate a process which leads to failure.
o Severity: The consequences of a failure mode. Severity considers the worst case outcome of a failure as determined by the degree of injury, property damage, or harm that could ultimately occur.
DFMEA Basics - Failure Mode And Effect Analysis
o Calculate a Risk Priority Number (RPN) for Each Line Item using
3 Criteria,
o Severity of the Failure Effect “S” (Scale of 1 (Low) - 10 (High)).
o Frequency of Failure Occurrence “O” (Scale of 1(Infrequent) -
10 (Frequent)).
o Detectability/Preventability/Warning “D” (Scale or 1(Very
Detectable) - 10 (Not Detectable)).
o RPN = S x O x D, range (1 (good) to 1000 High Risk).
o An unacceptable range is defined.
o Example: RPN’s > 150 are unacceptable and require a
corrective action redesign.
o Often a Pareto Ranking of the RPN is performed and used
to prioritize corrective action efforts.
DFMEA – The Old Way
DFMEA - Sherlock
o Ties the Netlist and the
Sherlock Parts list
automatically
o Generates template based
worksheets that can be
imported and exported into
Excel using a custom markup
language
o User configurable parameters
to control failure modes,
causes, effects
o Can group components into
subcircuits
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The Netlist, Subcircuits
o Sherlock will
automatically extract it
from the ODB++
o For Gerbers a IPC
netlist will need to be
uploaded
o Subcircuit information
o PRP fields in the
ODB++, need to
specify this from
your designers
o Manual entry
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DFMEA Sherlock Input Configuration
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DFMEA Sherlock Default Failure Modes
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Populate the DFMEA Using the Partlist
o Multiple levels
o Safety critical
applications
o Component and I/O
level analysis
o Specify how far away
to check for adjacent
pins
o Ignore pins without
connections, or adjacent
pins on the same net
o Component failure only15
DFMEA Inputs
o Input
subcircuit
details
o User should
enter function
and update
the default
information
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0.5 mm pin to pin short distance
Formatting DFMEA Outputs
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Generate DFMEA Output
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Edit the Template
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Output with New Template
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Plated Through Hole Fatigue
o IPC-TR-579
o Standard model
o IMEC PTH model – license
enabled
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IPC-TR-579 [Engelmaier Model for PTH fatigue]
o Beam model formulation, main concern is the effective
area assumption of the PCB
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IMEC [Model for PTH fatigue]
o Axisymmetric model formulation, better accounts for the area effect of
the PCB around the PTH
o The effective PCB area Equation
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IMEC Model
o Better captures
the drop in strain
as the hole size
increases
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PTH Fatigue Parameters
o Drill size, extracted from the drill file
o Z axis coefficient of thermal expansion (CTE) of the
PCB computed in the stackup
o Temperature ranges from the
Life Cycle
o Analysis Parameters
o PTH Quality
o Copper properties
o Hole size filter
o PTH wall thickness
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Parameters
o There are two qualitative factors
o IST/HAT Qualification
o This doesn’t change the probability of failure prediction
o Limits the maximum score one can achieve based on
qualification testing conducted
o PTH Quality Factor
o For the IMEC model this only changes the Weibull slope
used to generate the prediction curve
o For the IPC model it changes the stress concentration factor
and the Weibull slope parameter
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Conductive Anodic Filament Formation (CAF)
o Conductive anodic filament
formation (CAF), is a failure
observed within the glass-
reinforced epoxy printed
wiring board (PWB)
laminates
o It is an electrochemical
process involving the ionic
transport of metal under
the influence of an electric
field
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CAF Failures
o Failures are typically the result of damage during
drilling of the printed circuit board
o Drilling causes fracture of the laminate in which
plating chemicals get entrapped
o Failures can occur very rapidly in the presence of
humidity and electric bias
o Sherlock does a spacing analysis coupled with fiber
direction to generate a score based on distance and
overlap
o Qualitative analysis
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Sherlock CAF Analysis
o Sherlock uses the drill file for hole locations and sizes
o Analysis user inputs
o Holes size filter, removes holes from
the analysis
o Damage zone tells Sherlock how
far out to look for intersections
o Qualification changes the score based on the
qualification tests done
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Sherlock CAF Results
o Scoring is based on a
15 mil wall to wall
spacing
o Maximum score limited
by qualification testing
conducted (or not)
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Thermal Derating
o Data check to compare the part temperature ratings to
the thermal cycles defined in the life cycle
o Need to verify that parts have operating and storage
parameters in the parts list
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Thermal Parameters
o Set the Life Cycle State
in the Thermal Event
Editor
o Storage is assumed to
be non-operating
conditions
o Therm tab in the Part
Properties
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Temperatures
o Sherlock will look at
temperature inputs from the
Life Cycle or thermal inputs
(images, csv files assigned to
a temperature cycle) to
compare against
o Margin above the minimum
operating/storage
temperature and margin
below the maximum
operating/storage
temperature used for scoring
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Margin on the cold side is not as
critical as the hot side