DFMEA, Thermal Derating CAF, VIA Fatigue Dr. Nathan Blattau

DFMEA – Design Failure Mode Effects Analysis Potential failure modes of the components The effect that a failure of the component will have on the circuit Thermal Derating Component temperature ratings for storage and operation Comparison of those ratings to the applied thermal environments CAF – Conductive Anodic Filament Formation The potential for conductive filaments forming internal to the circuit board Typically due to drill damage and hole spacing VIA Fatigue Thermal cycling fatigue of the plated through holes of the PCB IMEC and IPC-TR-579 models

DFMEA Analyzes potential reliability problems early in the development cycle of a product, when it is easier to fix problems and therefore enhance overall reliability.  FMEA is used to Identify potential failure modes Determine their effect on the operation of the product Identify actions to mitigate the failures  Design Failure Modes and Effects Analysis (DFMEA) focuses on components and subsystems

What DFMEA is…. A systematic group of activities designed to: Recognize and evaluate potential failures of systems, products, or processes Identify the effects and outcomes of the failures Identify actions that could eliminate or mitigate the failures Provide a historical written record of the work performed

Why perform a DFMEA? Purpose of an DFMEA Study is to analyze: What might go wrong? How bad might the effect be? How might it be prevented, mitigated or detected at the earliest possible moment? With lowest cost, impact, safety risk…. Develop a DFMEA process for use in future designs

DFMEA Basics - Failure Mode And Effect Analysis DFMEA is a widely used and powerful analysis & design review technique. Extremely comprehensive element by element review of: What can go wrong What will happen How the situation can be improved For improving a design For each design component or element: List how failures can occur (Failure Mode, what can go wrong, how the failure manifests itself) List what could happen (Failure Effect, consequences of the mode). List how processes or the system itself can detect & prevent the problem Generate Recommendations for Improvements.

Some Key DFMEA Terms Failure: The loss of expected or intended function under stated conditions Failure mode: The way in which a failure is observed; generally describes the way the failure occurs. Failure effect: The immediate consequences of a failure on operation, function or functionality 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. 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 Calculate a Risk Priority Number (RPN) for Each Line Item using 3 Criteria, Severity of the Failure Effect “S” (Scale of 1 (Low) - 10 (High)). Frequency of Failure Occurrence “O” (Scale of 1(Infrequent) -10 (Frequent)). Detectability/Preventability/Warning “D” (Scale or 1(Very Detectable) - 10 (Not Detectable)). RPN = S x O x D, range (1 (good) to 1000 High Risk). An unacceptable range is defined. Example: RPN’s > 150 are unacceptable and require a corrective action redesign. Often a Pareto Ranking of the RPN is performed and used to prioritize corrective action efforts.

DFMEA – The Old Way

DFMEA - Sherlock Ties the Netlist and the Sherlock Parts list automatically Generates template based worksheets that can be imported and exported into Excel using a custom markup language User configurable parameters to control failure modes, causes, effects Can group components into subcircuits

The Netlist, Subcircuits Sherlock will automatically extract it from the ODB++ For Gerbers a IPC netlist will need to be uploaded Subcircuit information PRP fields in the ODB++, need to specify this from your designers Manual entry

DFMEA Sherlock Input Configuration

DFMEA Sherlock Default Failure Modes

Populate the DFMEA Using the Partlist Multiple levels Safety critical applications Component and I/O level analysis Specify how far away to check for adjacent pins Ignore pins without connections, or adjacent pins on the same net Component failure only

DFMEA Inputs Input subcircuit details User should enter function and update the default information 0.5 mm pin to pin short distance

Formatting DFMEA Outputs

Generate DFMEA Output

Edit the Template

Output with New Template

Plated Through Hole Fatigue IPC-TR-579 Standard model IMEC PTH model – license enabled

IPC-TR-579 [Engelmaier Model for PTH fatigue] Beam model formulation, main concern is the effective area assumption of the PCB

IMEC [Model for PTH fatigue] Axisymmetric model formulation, better accounts for the area effect of the PCB around the PTH The effective PCB area Equation

IMEC Model Better captures the drop in strain as the hole size increases

PTH Fatigue Parameters Drill size, extracted from the drill file Z axis coefficient of thermal expansion (CTE) of the PCB computed in the stackup Temperature ranges from the Life Cycle Analysis Parameters PTH Quality Copper properties Hole size filter PTH wall thickness

Parameters There are two qualitative factors IST/HAT Qualification This doesn’t change the probability of failure prediction Limits the maximum score one can achieve based on qualification testing conducted PTH Quality Factor For the IMEC model this only changes the Weibull slope used to generate the prediction curve For the IPC model it changes the stress concentration factor and the Weibull slope parameter

Conductive Anodic Filament Formation (CAF) Conductive anodic filament formation (CAF), is a failure observed within the glass-reinforced epoxy printed wiring board (PWB) laminates It is an electrochemical process involving the ionic transport of metal under the influence of an electric field

CAF Failures Failures are typically the result of damage during drilling of the printed circuit board Drilling causes fracture of the laminate in which plating chemicals get entrapped Failures can occur very rapidly in the presence of humidity and electric bias Sherlock does a spacing analysis coupled with fiber direction to generate a score based on distance and overlap Qualitative analysis

Sherlock CAF Analysis Sherlock uses the drill file for hole locations and sizes Analysis user inputs Holes size filter, removes holes from the analysis Damage zone tells Sherlock how far out to look for intersections Qualification changes the score based on the qualification tests done

Sherlock CAF Results Scoring is based on a 15 mil wall to wall spacing Maximum score limited by qualification testing conducted (or not)

Thermal Derating Data check to compare the part temperature ratings to the thermal cycles defined in the life cycle Need to verify that parts have operating and storage parameters in the parts list

Thermal Parameters Set the Life Cycle State in the Thermal Event Editor Storage is assumed to be non-operating conditions Therm tab in the Part Properties

Temperatures Sherlock will look at temperature inputs from the Life Cycle or thermal inputs (images, csv files assigned to a temperature cycle) to compare against Margin above the minimum operating/storage temperature and margin below the maximum operating/storage temperature used for scoring Margin on the cold side is not as critical as the hot side