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Accelerated Stress Testing and Reliability Conference

Thin MLCC (Multi-Layer Ceramic Capacitor) Reliability Evaluation Using an Accelerated Ramp

Voltage Test

John Scarpulla

The Aerospace Corporation

[email protected]

September 28- 30 2016, Pensacola Beach, Florida January-4-17 1


Thin* Layer MLCCs

www.ieee-astr.org September 28- 30 2016, Pensacola Beach, Florida

15 μF

22 μF

0.1 μF

Standard MLCC DUT

(end-bell terminations)

*~2-4 µm thick vs >20 µm in heritage devices

Ubiquitous

Construction


Sample MLCC Cross-Sections

www.ieee-astr.org September 28- 30 2016, Pensacola Beach, Florida 3

• Concerns:

• A: Irregular metal layers

• B: Thin polycrystalline ceramic

• C: Voids & defects

• ?: Oxygen vacancies?

• ?: Other potential failure mechanisms?

A

B

C


Thinner and Thinner MLCCs


• Dielectric thickness are steadily decreasing

– 1970: 25 – 50µm

– 1980: 18 – 25 µm

– 1990: 10 – 20 µm

– 2000 8 – 15 µm

– 2010 4 – 8 µm

– 2016 2 – 5 µm

– Future submicron BME = base metal electrode

PME = precious metal electrode

IDC = inter-digitated capacitor

BMEs

PMEs

• Our technique useful for:

MLCCs

end bell capacitors

BMEs

PMEs

IDCs


Need for Improved Test Methods

• Limited failures in traditional long constant stress tests yield ambiguous reliability results

– 1000’s-hour lifetests often give essentially zero failures

• Our method :

– is faster & provides 100% failures

– can predict “teenage mortality”

• (an elevated failure rate in 1st 10,000hrs)

– can determine screening efficacy



The effective thickness concept:

Each defect is assigned an “effective thickness” irrespective of its physical nature or observability


kT

EBEt An

F exp


Extracting xeff

• Recall the P&V* model

• Electric field

• Solve for xeff

– which states that the effective thickness can be extracted from the failure time (at constant Voltage)

– But …Voltage is not constant in actual usage

– …P&V model parameters are unknown


kT

EBEt An

F exp

*T. Procopowicz and R. Vaskas, 1968

effxVE

nkT

EB

tVx

An

nF

eff

exp1

1 EA

n

B

coefficients:


Ramp Voltage Technique

• Generalize the P&V model to non-constant stress

• Assume a ramp voltage with T constant

• Ramped test produce 100% failures

– (We reliability engineers live or die with failures!)


F Ft t

dttkT

E

x

tV

Bdttf A

n

eff0 0

1exp1

)( Cumulative damage function

RttV

nkT

ERnBVx Annn

Feff exp111 Extract xeff from ramped

breakdown voltages


Ramped Breakdown Test Results

• Test setup • Ramped breakdowns


This array is called BV and is referred to as the "long" BV vector

50 100 150 200 250 3000

0.2

0.4

0.6

0.8

1

Failure Voltage (Volts)

Fra

ctio

n F

aile

d

120C

103

77

… +

DUT board in temperature chamber

Agilent 34980ASwitch/Measure

system

computerBallast resistor and switch board

Kepco BHK 300-0.6MGramp voltage generator

32 MLCCs per test “cell”

32 9 cells = 288 failures


Extract P&V coefficients from Ramp data

• Key assumption:

– VF95’s are at nominal thickness x0

– Allows the P&V coefficients to be extracted


kTn

Ex

n

nRnB

nV A

F1

ln1

ln1lnln1

1ln 095

bxmxmy 2211transformation and multiple linear regression

95th percentiles of VF from ramped breakdown test


P&V regression results

• VF95 9-cell multiple regression

regression

coefficients

P&Vmodel

coefficients

units

m1 0.115 n 7.723 dimensionless

m2 0.132 EA 1.154 eV

b 0.679 B 1.7510-3 hr(V/m)n


0 1 2 3 4 54.6

4.8

5

5.2

5.4

5.6

x1 = ln(R)

y =

ln

(V9

5)

T = 77C

103

120

Now the effective thicknesses can be extracted, but first….


Seeds model


• Seeds* model -- yield worsens with defect density and area - basic semiconductor yield model

– (it approximates a Poisson distribution of defects)

• Solve for cumulative defect density D :

– Expressed as a function of the xeff

– Typical units are defects/cm2

DAYPf

1

111

f

feff

P

P

AxD

1

1)(

DATA TRANSFORMATION

VF xeff Pf D

*R. Seeds 1967


D vs. xeff curve MLCC properties

quantity value units

Capacitance C 0.1 µF

Case size - 0402 -

Rated voltage Vrat 10 V

Rated temperature

Trat 125 °C

Active electrode length

L 0.77 mm

Active electrode width

W 0.27 mm

Number of layers N 73 -

Total active area A 0.152 cm2

Nominal dielectric thickness

x0 3.70 µm


1 2 3 4 50.01

0.1

1

10

100

1 103

1 104

effective thickness, xeff (um)

def

ect den

sity

, D

(cm

-2)

90% C.I.


An MLCC Birth to Retirement in 4 phases

phase & duration

V (V)

T (C)

effective thickness

xeff (m)

defect density

Du

(cm-2)

Cum. failure

prob. (%)

Cond. failure

prob. (%)

1

DWV test

1 sec

25 25 0.0586 1.31210-2 0.2 0.2

2

Part screen 168 hrs 10 125 0.566 1.71610-2 0.26018 6.1310-4

3

Board burn-in 320 hrs

8 85 0.567 1.71710-2 0.26026 7.9910-7

4 Mission 15 yrs

8 85 1.14 2.32210-2 0.352 9.1710-2



xeff for the four MLCC Life Phases


1

1

111 exp

nkT

E

B

tVx A

n

eff

n

AnAneff

kT

E

B

tV

kT

E

B

tVx

1

2

22

1

112 expexp

n

AnAnAnAneff

kT

E

B

tV

kT

E

B

tV

kT

E

B

tV

kT

E

B

tVx

1

4

44

3

33

2

22

1

114 expexpexpexp

n

AnAnAneff

kT

E

B

tV

kT

E

B

tV

kT

E

B

tVx

1

3

33

2

22

1

113 expexpexp

DWV is part of manufacturing

screening

mission


Unconditional D vs. xeff curve

• Usage times at rated conditions (125C, 10V) map to ever increasing xeff

• Curve is unconditioned for the DWV screen test (0.2%)

• From this curve any desired reliability predictions can be made


1 sec

1 min

1 hr

1 day

1 wk

1 mo

1 yr

DWV screen

5 10 15 20 yrs

0 1 2 3 4 5 0.01

0.1

1

10

100

1 10 3

´

1 10 4

´

effective thickness, xeff (um)

def

ect

den

sity

, D

(cm

-2)


0.1

1

10

100

0 5 10 15

ave

rage

fai

lure

rat

e (

FITs

)

mission time (years)

Example Reliability Prediction

• Failure rate


• Effective thicknesses mapped

V= 8 V

T = 85C 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15

effe

ctiv

e t

hic

kne

ss x

eff(

m)


“Standard” burn-in

7 FITs

@15 yrs


Example Reliability Prediction (Fluctuating Temperature)

• Failure rate



0.1

1

10

100

0 5 10 15

ave

rage

fai

lure

rat

e (

FITs

)


V= 8 V

T fluctuates yearly

between 85C & 50C 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15

effe

ctiv

e t

hic

kne

ss x

eff(

m)


“Standard” burn-in

Temperature


1

10

100

1000

0 5 10 15

ave

rage

fai

lure

rat

e (

FITs

)


Example Reliability Prediction with reduced burn-in

• Failure rate



V= 8 V

T = 85C 0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 5 10 15

effe

ctiv

e t

hic

kne

ss x

eff(

m)


With burn-in reduced 10X in duration and 25C in temperature

xeff smaller in first year –

Higher failure rate

“teenage” mortality


Compare to the Traditional Constant Failure Rate


1E-091E-08

0.00000010.000001

0.000010.0001

0.0010.01

0.11

10100

100010000

0 1 2 3 4 5

def

ect

den

sity

,D(c

m-2

)

effective thickness, xeff (m)

traditional method – inconceivably low defect densities

ramp method

• Assume an exponential distribution with = 7 FITs

• Use same P&V coefficients (not available traditionally)

• RESULT:

– Shows that conventional method is overly optimistic

– Cannot predict “teenage mortality” despite actual P&V coefficients


Conclusions

• A new method of MLCC testing has been proposed

– Effective thickness concept

– Ramp voltage test technique is fast (2 wks for this example)

– Seeds defect model (from the semiconductor industry)

– Allows more realistic computations of failure rate • under realistic variable usage conditions

– Permits essentially 100% failures (We live or die on failures)

– Extracts P&V model coefficients

– Evaluates screening stratagems



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