Implementing Physics of Failure into the Design Process

9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com

Implementing Physics of Failure into the Design ProcessSeptember 15, 2016


Nathan Blattau, Ph.D.

Senior Vice President of DfR Solutions, has been involved in the packaging and reliability of electronic equipment for more than ten years. His specialties include best practices in design for reliability, robustness of Pb-free, failure analysis, accelerated test plan development, finite element analysis, solder joint reliability, fracture, and fatigue mechanics of materials.

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Role of Modeling in Design

o Nobody can afford to repeatedly test and redesign to create reliable, cost

effective products

o Working with models allows an interdisciplinary design team to create a more

reliable design smarter, faster & cheaper!

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Simulation & Modeling

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o Performing thermal, mechanical & electrical simulations & extracting the results into a time-to-failure prediction


Physics of Failure (PoF)

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o PoF Definition: Use of science to capture an understanding of failure

mechanisms & evaluate useful life under actual operating conditions

o Using PoF, design, perform, and interpret the results of accelerated life

tests

o Starting at design stage

o Continuing through lifecycle of the product

o Start with standard industry specifications

o Modify or exceed them

o Tailor test strategies specifically for product design & materials, use

environment, and reliability needs


Physics of Failure Definitions

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o Failure of a physical device or structure attributed to

o Gradual or rapid degradation of the material(s) in the device

o In response to the stress or combination of stresses the device is exposed

to, such as:

o Thermal, Electrical, Chemical, Moisture, Vibration, Shock, Mechanical Loads . . .

o Failures May Occur:

o Prematurely

o Gradually

o Erratically


Using Physics of Failure During the Design Stage

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o Design the product for robustness and to meet the

environmental requirements

o Vibration

o Mechanical Shock

o Thermal Cycling


Designing for Mechanical Loads

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o Unlike other materials, solder is a poor engineering

material

o Extreme dimensional variations

o Presence of voids is normal and expected

o Is constantly being subjected to inelastic deformations

under thermal cycling and shock

o You would never use steel, titanium or aluminum under these

types of conditions, unless you want it to fail

o During vibration we need to prevent inelastic deformations

(plasticity)

o This makes the field of electronics reliability unique


Vibration Fatigue

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o Due to the high number of cycles we need to avoid

inelastic deformations at all cost

o Inelastic deformations (plasticity and creep) are low

cycle fatigue (< 100,000 cycles)

o During vibration cycles accumulate quickly

o Example, 100 Hz vibration – 100 cycles per second

o Time to accumulate 100,000 cycles, 16.67 minutes


Designing for Vibration

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o Octave rule: the PCB natural frequencies should be at

least 2X the chassis natural frequencies to prevent

coupling

o Recommended reading Steinberg’s Vibration Analysis

of Electronic Equipment

o If the chassis resonant frequency is close to the PCB then

there can be significant amplification of the PCB

deformations


Natural Frequency

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o Do it by hand

o Limited shapes

o Simple support

conditions

o Use FEA to handle

complex shapes and

boundary conditions

Vibration Analysis of Electronic Equipment

David S. Steinberg


Using Sherlock or FEA for Vibration During Design Phase

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o There are many factors that can be adjusted to modify

the natural frequency response of the printed circuit

board

o Component placement

o Boundary conditions (mount points)

o PCB properties (thickness, laminate)


Boundary ConditionsComponent Mass – HeatsinksPrinted Circuit Board Properties

Typically, the higher natural frequency the more robust the design

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Boundary Conditions

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o Chassis typically have

lower natural frequencies

than circuit boards

o Usually looking for circuit

boards having natural

frequencies greater than

150 Hz

o Natural frequency should

not coincide with peaks

in the expect vibration

input

23 Hz is too low for most

applications, need to

make changes


Boundary Conditions

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o Current, PCI-E type

o Add an additional

mount at high

deflection area

Add more support


Modifying Boundaries in Sherlock

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9000 Virginia Manor Rd Ste 290, Beltsville MD 20705 | 301-474-0607 | www.dfrsolutions.com17

o Almost a 4X

increase with

one additional

mount

o Changing

heatsink from

Copper to

Aluminum


o Heatsink from

Copper to

Aluminum

o 82 to 93Hz


Additional Mount Point and Increasing PCB Thickness

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1.544 to 1.65 mm

NF increases to 108.5 Hz


Conduct Physics of Failure Assessment

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o Once an acceptable NF is achieved

o During vibration the assembly is assumed to deform

elastically so that strain on the PCB is proportional to the

strain in the solder and leads

o This allows PCB strain to be used to make fatigue

predictions


Vibration Fatigue

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o HCF failures typically occur in the lead or solder joint

Component Motion Board bending


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o Lifetime under vibration

is divided into two regimes

o Low cycle fatigue (LCF)

o High cycle fatigue (HCF)

o LCF is driven by inelastic strain

(Coffin-Manson)

o HCF is driven by elastic strain

(Basquin) b

f

f

e NE

2

c

ffp N2

-0.5 < c < -0.7; 1.4 < -1/c > 2

-0.05 < b < -0.12; 8 > -1/b > 20


Harmonic

Steinberg D.S. Vibration analysis for electronic

equipment.

John Wiley & Sons, 2000.

Random

MIL-STD-810G Figure 514.6C-1

US Highway truck vibration exposure

1 hour is equivalent to 1000 miles

Typical Vibration Levels

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Enter profile into Sherlock


Physics of Failure Results – Random Vibration

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Random Vibration Results

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o U1 is failing because

it is in areas of high

bending (red)

o Further design

changes are

necessary to get this

board to survive the

expected field

environment

o Additional mount

points, stiffeners, etc..


Mechanical Shock

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o Very similar to vibration

o The higher the board stiffness

(Natural Frequency is directly related) the

more robust with regards to mechanical

shock

o Lower component mass, Increase board

thickness

o Due to today’s low profile

surface mount components,

shock failures are primarily

driven by board flexure

o BGAs don’t care about in-plane

shock, unless it causes the board

to bend

o Shock tends to be an overstress

event (though, not for car doors)

o Failure distribution is ‘random’


In Plane Shock

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Design - Board Thickness Effects

Board thickness 1.575 mm

0.97 mm displacement



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Using Sherlock During Design for Thermal Cycling Fatigue

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Examples

o Package selection

o Printed circuit board properties

o Solder pad design

o Plated through hole


Predictive Models: Physics of Failure (PoF)

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o Modified Engelmaier for Pb-free Solder (SAC305)o Semi-empirical analytical approach

o Energy based fatigue

o Determine the strain range (Dg)

o C is a correction factor that is a function of dwell time and

temperature, LD is diagonal distance, a is coefficient of

thermal expansion (CTE), DT is temperature cycle, h is

solder joint height

Th

LC

s

D DDD ag


Predictive Models: Physics of Failure (PoF)(cont.)

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o Determine the shear force applied to the solder joint

o F is shear force, L is length, E is elastic modulus, A is the area, h

is thickness, G is shear modulus, and a is edge length of bond

pad

o Subscripts: 1 is component, 2 is board, s is solder joint, c is bond

pad, and b is board

o Takes into consideration foundation stiffness and both

shear and axial loads

D

aGGA

h

GA

h

AE

L

AE

LFLT

bcc

c

ss

s

9

2

221112

aa


Predictive Models – Physics of Failure (PoF)(cont.)

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o Determine the strain energy dissipated by the

solder joint

o Calculate cycles-to-failure (N50), using energy

based fatigue models

10019.0

D WN f

sA

FW DD g5.0


Thermal Cycling Design – Component Choices

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o Plastic components typically perform better than ceramic

components

o Smaller components usually perform better than larger

components

o Larger solder joints (pad size, thickness) perform better


2512 resistor or two 1206 size resistors?

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2512 1206


Bond Pad Influence2512 Resistor

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o Increase bond

pad length from

1.28 mm to 2.00

mm

o 862 cycles

o 1296 cycles

o 54% increase in

life


PCB Influence2512 Resistor

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o Decrease PCB CTE

from 16 ppm to

15 ppm

o 1296 cycles

o 1632 cycles

o 26% increase in

life

o 14 ppm - 2117

cycles, 63%

increase over

baseline


Plated Though Holes

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o Design influences

o PCB thickness

o Plating thickness

o Hole diameter


IPC TR-579

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Round Robin Reliability Evaluation of Small

Diameter (<20 mil) Plated Through Holes in

PWBs

Activity initiated by IPC and published in 1988

Objectives

Confirm sufficient reliability

Benchmark different test procedures

Evaluate influence of PTH design and

plating (develop a model)


o Determine applied stress applied (σ)

o Determine strain range (∆ε)

o Apply calibration constantso Strain distribution factor, Kd(2.5 –5.0)

o PTH & Cu quality factor KQ(0 –10)

o Iteratively calculate cycles-to-failure (Nf50)

Plated Through Hole Via Barrel Cracking Fatigue Life Based On IPC TR-579

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PoF Durability/Reliability Risk AssessmentsPCB Plated Through Hole Via Fatigue Analysis

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When a PCB experiences thermal cycling the

expansion/ contraction in the z-direction is much

higher than that in the x-y plane. The glass fibers

constrain the board in the x-y plane but not

through the thickness. As a result, a great deal of

stress can be built up in the copper via barrels

resulting in eventual cracking near the center of

the barrel


IMEC Plated Through Hole Fatigue Model

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o Alternative model

o Better accuracy when predicting

fatigue of large plated through holes

o Less reliance on correction coefficients

o TR-579 can be overly conservative in

certain cases

Kd and Kq are IPC-TR-579 correction factors


Plating Thickness

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20 microns – 1621 cycles 30 microns – 2166 cycles


Printed Circuit Board Thickness

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1.6 mm – 2166 cycles 2.0 mm – 1623 cycles


Printed Circuit Board Expansion (z-axis)

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60 ppm – 1293 cycles 45 ppm – 2984 cycles


Accuracy of PoF-Based Models

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o Once a physics of failure model has been developed and validated, it typically displays accuracy similar to a validated finite element model


Traditional Reliability Growth in Product Development Empirical “TRIAL & ERROR” Method to Demonstrate Statistical Confidence

Today, This Reactive Approach Is Not Enough!

o Testing doesn’t truly simulate actual usage.

o Can not afford the time or money to test to high reliability.

o Problems found too late for effective corrective action, quick fixes often used.

o Testing more parts & more/longer tests “seen as only way” to increase reliability.

DESIGN - BUILD - TEST - FIX (D-B-T-F)

6) REPEAT 3-5 Until Nothing Else Breaks Or

You Run Out Of Time/Money.

Yes

No4)

Faults Detected

?

5) Fix Whatever Breaks.

2) Build 3) Test1) Design

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Traditional Reliability Growth in Product Development Empirical “TRIAL & ERROR” Method to Demonstrate Statistical Confidence

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Implementing Physics of Failure gets you through the loop the

first time

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DESIGN - BUILD - TEST - FIX (D-B-T-F)

Yes

No4)

Faults Detected

?

5) Failure Analysis

2) Build 3) Test1a) Design

1b) PoF

6) Fix and PoF

Implementing Physics of Failure into the Design Process

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