Role of the HDI in HDD 2 - IDEMA | International Disk ... · Role of the Head Disk ... slider-disk clearance should be treated as a critical parameter by which to design a mechanically

Role of the Head Disk Interface in HDD Role of the Head Disk Interface in HDD ReliabilityReliability

George W. TyndallReliability Technology

Samsung Information Systems AmericaSan Jose, CA

IDEMA Reliability Symposium 2

IntroductionIntroductionMotivation: Designing a reliable HDD requires a rigorous understanding of the

most prevalent failure modes/mechanisms, and a means to predict their impact on end-user failure rates.

HDD reliability information can be obtained from a variety of sources End user – Reliability information from customers can be the hardest to obtain, is often times the most

ambiguous, and can be difficult to feedback into product design.Customer qualification – Reliability Demonstration Test (RDT) on well controlled sample distribution to

determine intrinsic reliability of product (AFR% / MTBF) . Sample size can be limited.Internal qualification – Highly Accelerated Stress Testing (HAST) during product development to

investigate design margins. Drives may not be sufficiently mature, but a substantial number are available. Subsystem testing – component-level testing (H/M, platform, firmware, channel) to evaluate product

feasibility and uncover design limitationsFundamental studies – Research and development into the scientific underpinnings of current

technology, and evaluation of new technologies / alternative designs to meet future product specifications

Failure rates from drive-level testing (end-user, customer qualification, and internal qualification) are used to identify the predominant failure mechanisms

Failure of the head-disk interface is found to be the dominant factor impacting HDD reliability

A course-grained model for HDD reliability is developed to address the head-disk interface and the influence of a few vital parameters.

Results of drive level testing are presented to vailidate the modelModel Refinement – results from component level and fundamental studies


Failure Modes SummaryFailure Modes SummaryHighly Accelerated Stress testing results

Conducted during product development (>10,000 HDD’s)Dominant failure modes (64%) are HDI – related

RDT testing results (~100 HDD’s)

Majority of failures (75%) are HDI – related

Field Returns – Generic failure pareto (1000’s HDD)

Large fraction of “real” HDD failures are HDI-related

64%

18%

18%

HDI-relatedCND

other

Failure Modes

HDI-relatedCND

Other

767K1.14%2F - Disk defects1F - Weak write1F - Head Degradation

848K1.03%1F - CND2F - Disk Scratch1F - Head Degradation

80GB/platter (3.5”)

1.11 M0.79%1F - PCBA2F - Disk Scratch1F - Head Degradation

MTBF(hrs)

AFR(%)Failure modesProduct

6%

28%

49%

11%6%PCBA

CND

TA / scratch

WW

HDI-related

Head Degradation

Sample HDD Failure Mode Pareto

Scratch

TA

Head degradation

Grown defect

Motor

PCBMishandling

CND

High-fly writeWrite abort

HDI-related

35.4% CND

Mishandling

All results point to the head-disk interface as the dominant contributor to HDD reliability

Can we develop a deterministic model to predict these failures?


Dominant HDI-related failure mechanismsand approximate failure rates

Failure Analysis Failure Analysis –– HDI FailuresHDI Failures

Scratch (50%)

Weak write / modulation (15%)

Head degradation (30%)

6%

28%

49%

11%6%PCBA

CND

Head Degradation

TA / scratch

WW

-2 0 2 4 6 8 10

x 10-5

-0.1

-0.08

-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08Read Signal

Time

Vol

tage

Particle-induced scratch (5%)

Head-disk contact (45%)

90 – 95% of scratching results from direct contact between flying head and diskMajority of head degradation failures result from head-disk contacts

100% of all modulation failures result from flying too low - “near”contact

Majority of HDI-related failures result from the head and disk coming into contact, or near contact


Clearance is key to HDI ReliabilityClearance is key to HDI ReliabilityClearance, space between the slider and disk, is the primary factor for HDI reliability.

Small clearance results in an increased risk of head-disk contact

Zero or negative clearance results in eventual failure due to scratch or head degradationFailure rates increase exponentially with decreasing clearance

Air flow

Lubricant

Slider

Head-diskclearance

slider

+ + +

Magnetic layer

Under layers + substrate

COC layer

(b)

Population FH Distribution

DISK

DISK GLIDE AVALANCHE

Mean Clearance

No head-disk contact

Clearance (Z)

DISK

DISK GLIDE AVALANCHE

∆ Ζ

Head-disk interference-4 -3 -2 -1 0 1 2 3 4

0.0

0.2

0.4

0.6

0.8

1.0

16% Failure Rate

50% Failure Rate

σCum

ulat

ive

Failu

re R

ate

Z-value

Reduced HDI failure rate requires minimizing head-disk contacts by maximizing clearance


HDI reliability modelHDI reliability modelObjective: Provide reliability design criteria by modeling clearance, and predicting failure rates, under any given set of operating conditions.

AssumptionsFinite clearance 0% failure rateZero clearance 100% failure rateNot a dynamic model, i.e. no explicit time dependence is incorporated into model

Parameters identified as affecting drive clearance (z) include:Incoming clearance distribution (zo)TemperatureAltitude Humidity Duty cycle: Clearance loss during seek (not covered)

Sensitivity of clearance on each parameter measured independently


4000 m altitude

2

3

4

5

6

7

25 35 45 55 65 75 85

Drive temperature (C)

Cle

aran

ce (n

m)

Temperature sensitivity = -0.035nm/ºC

Drive temperature (ºC)

Cle

aran

ce (n

m)

Engineering Sample Data

HeadHead--Disk Clearance vs. Altitude and TemperatureDisk Clearance vs. Altitude and Temperature

60708090100

0

2

4

6

8

Pressure (kPa)

Cle

aran

ce (n

m)

Touch-down

Altitude sensitivity = 0.13nm/kPa

Disk

Head

Cle

aran

ce d

rop

(nm

)

Clearance also decreases with increasing temperature

Consistent with temperature deratingdiscussed above.

Clearance decreases w/ increasing altitudeLess lift is generated by the ABS as the pressure drops

InitialClearance


Theoretical model developed to describe the effect of humidity on clearance Compression under ABS can result in local supersaturation of water vaporSupersaturated water vapor is incapable of supporting ABSEffective ABS lift drops – fly height decreases

Clearance is Sensitive to HumidityClearance is Sensitive to Humidity

The head-disk clearance decreases with drive humidity

0.1453

Po (atm)T (ºC)

0.1453

Po (atm)T (ºC)

MeasurementSimulation

-1

0

1

2

3

4

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14Partial pressure of water (atm)

Cle

aran

ce d

rop

(nm

)

For this HDI design, clearance decreases with water vapor pressure as – 0.2 nm/kPa(comparable to altitude effect)

For example, clearance at 60 C drops by 3. 5 nmwhen the humidity is increased from 0 90%RH

(Strom et al., IEEE Trans. Magn. (2007), 43, 3301)

-5

-4

-3

-2

-1

0

0 20 40 60 80 100

Ambient Humidity RH (%)

FH C

hang

e (n

m)

-80

-60

-40

-20

0

20

Pitc

h/R

oll C

hang

es (u

rad)

Gap FH (nm)Pitch (urad)Roll (urad)

ABS Compression SupersaturationFly height drop


Factors impacting failure rates:Initial clearanceTemperature sensitivityAltitude sensitivityHumidity sensitivity

Data input into Monte Carlo simulations

)()( 0 RHfPbTaznmz −∆−∆−=

Reliability / failure rate modelReliability / failure rate model

Model structure

0

50

100

150

200

250

300

-1.01 -0.03 0.96 1.95 2.94

nm

Freq

uenc

y

0

50

100

150

200

250

0.84 1.71 2.59 3.46 4.33

nm

Freq

uenc

y

Head-disk contact

T=65ºC, RH=30% (Altitude=sea level) T=65ºC, RH=80%

05 0.1 0.15 0.2Altitude sensitivity (nm/kPa)

2 4 6 8

Prob

abili

ty

Incoming clearance (nm) RH sensitivity

5.3 nm

0 0.02 0.04 0.06 0.08Temperature sensitivity (nm/oC)

0.0035nm/C 0.13 nm/kPa clea

ranc

e

No failure 6.2% failure rate

Model Output

RH

The failure predictions were then compared directly with results obtained from standard HDD reliability testing conducted under the specified temperature / altitude / humidity conditions


)()( 0 RHfPbTaznmClearance −∆−∆−=

4 6 8 10 12 14 16

0

20

40

60

80

100

T = 0 C T = 60C

Failu

re R

ate

(%)

Altitude (ft)

Model Verification Model Verification –– Failure rate Failure rate vsvs Altitude and TemperatureAltitude and Temperature

)()( 0 RHfPbTaznmClearance −∆−∆−=21.2%

63.7%

94.3% 99.5% 100.0%

100.0%100.0%100.0%

66.7%

16.7%0%

20%

40%

60%

80%

100%

120%

8 10 12 14 16Altitude (kft)

Failu

re ra

te (%

)

Simulation

Experiment

3.0%24.4%

68.9%

95.0%99.5%

100.0%

95.2%

66.7%

28.6%

0.0%0%

20%

40%

60%

80%

100%

120%

8 10 12 14 16

Altitude (kft)

Failu

re ra

te (%

)

Simulation

Experiment

Engineering Sample Data

Conditions: Variable altitude test conducted at two temperatures

Quantitative agreement found between model and experiment

Failure rate increases with increasing tempFailure rate increases with increased altitude

Experimental vs Model results

T = 25CT = 60C

∆ T


Conditions: Fixed Temperature (T = 70oC) / variable humidity ( 5 – 95%)

Model failure rate predictions vs Experimental Data

Excellent agreement observedIndicates validity of approach for clearance / humidity dependence

120 140 160 180 200 220 240Water Vapor Pressure (torr)

Cum

ulat

ive

Failu

re R

ate

0.0

0.2

0.4

0.6

0.8

1.0

Humidity impact on driveHumidity impact on drive reliabilityreliability

Predicted

)()( 0 RHfPbTaznmClearance −∆−∆−=

Relative Humidity Predicted Failure Rate Experiment

data 60 0.0% 0.0%65 6.3% 3.9%70 12.2% 12.0%75 25.5% 30.6%80 66.4% 65.8%90 99.0% 100.0%


Summary of clearanceSummary of clearance--based reliability modelbased reliability model

A clearance-based reliability model was developed and successfully tested.

The following parameters affecting clearance have been measured and incorporated in the model:

Initial clearance distribution Operating temperatureAltitudeInternal drive humidity (Developed theoretical model of humidity affect on clearance)

On the basis of the excellent correlation with experiment we conclude that. slider-disk clearance should be treated as a critical parameter by which to design a mechanically reliable magnetic hard disk drive.

Cautionary note: The impact of humidity and temperature on drive failure rates in the field cannot be attributed exclusively to the clearance effects discussed above.


“Fly-height On Demand” (FOD) conceptHead-disk spacing is controlled by resistive heating of the pole tip region of the head

Head-disk Spacing Control

Thermal Expansion

Element spacingNominal FH

FOD Off FOD On

4.5nm11nm

Disk

Thermal actuation is only performed during read and write operations

Since head – disk occur only during FOD actuation, the “effective” duty cycle, and AFR are reduced

Clearance measured by magnetic signal change via the Wallace spacing loss equation

FOD allows for a fourfold decrease in the std. deviation of the clearance.

Current FOD systems actively compensate for changes in drive temp, altitude, and humidity

Future designs promise to supply real time feedback on spacing fluctuations (analogous to position error signal)

4.5nm5.2 nm

FH

02468

10121416

0 20 40 60 80 100

120

140

160

180

200

220

240

FOD [DAC]FH

[nm

]

14IDEMA Reliability Symposium

Does FH control solve all our problems?Finer Details – Glide Avalanche

Physico-chemical properties of disk lubricant Long-term effects of temperatureOther adverse impacts of high humidity

The Future Challenge

The effect of intermolecular forces when macroscopic objects areseparated by nanoscopic distances – van der Waals interaction forces

FH Distribution

DISK

Distribution of DISK GLIDE AVALANCHE

Mean Clearance

4nm

5nm


Molecularly Molecularly --Thin PFPE Lubricant FilmsThin PFPE Lubricant FilmsEnergetics of molecularly thin PFPE films are dependent on the amount of material present

Surface-induced confinement results in properties that deviate substantially from bulk material. Anomalous materials properties – presence of disjoining pressure unique to molecularly-thin films Tthickness dependent film properties

• Lubricant displacement by flying head• diffusion coefficients (effective viscosity), • Adhesive forces• Friction forces

Oscillatory polar surface energy of PFPE films indicates layering phenomenon

Thermodynamic instabilities developReadily apparent in terraced spreading

Free

Ene

rgy

Film Thickness

1st Monolayer

2nd layer (unstable)

3rd layer(Stable)

(Stable)

Film

Thi

ckne

ss (A

)

0 10 20 30 4015

16

17

18

19

20

21

22

∆γd

Dis

pers

ive

Sur

face

Ene

rgy

(mJ/

m2 )

Lubricant Film Thickness (A)

Dispersive Surface Energy


Lubricant Stability Lubricant Stability –– AutophobicAutophobic dewettingdewettingOscillatory polar surface energy, χp results in a disjoining pressure ( Π = - dχ/dh) that changes sign as a function of film thickness Thermodynamic stability requires that Π >0When Π < 0, film is unstable and will spontaneously dewet.

Spontaneous dewetting

Π < 0 : Dewetted lubricant droplets

Disjoining Pressure Zdol (MW =4000)

dewetting

26.0 A

30.0 A

OSA ImagesΠ > 0No dewetting

Waltman, Khurshudov, TyndallTrib. Lett. (2002), 12, 163.


Impact of Impact of dewettingdewetting

9.6 A

9.9 A

10.3 A

Zdol 1350

Dewetting of Zdol (MW = 1350)Homogenous films observed when film

thickness, h, < 10 Lubricant dewetting occurs at h > 10

Clearance measurementsAcoustic emission results

0 5 10 15 20 25-5

-4

-3

-2

-1

0

1

Cle

aran

ce C

hang

e (n

m)

Lubricant Thickness (A)

Clearance severely reduced when lubricant droplets formed

Lubricant Lubricant dewettingdewetting produces produces ““soft particlessoft particles”” that can interact with the that can interact with the flying sliderflying slider

∆Z > 4nm


Lubricant Stability Lubricant Stability -- DewettingDewettingPFPE lubricant layering on surfaces is ubiquitous

Other surfaces

0

60

40

20

1.0 0.8 -0.20.6 0.4 0.2 0

Zdol 2800 on Cu

Thic

knes

s (A

)

Distance [mm]

Impact of PFPE Molecular Weight

Dew

ettin

gTh

ickn

ess (

A)Effect of carbon composition

0 1000 2000 3000 4000 500005

101520253035404550

OSATitrationDisjoining Press.

Mon

olay

er T

hick

ness

(A)

Zdol Molecular Weight (Mn)

gTerraced Spreading on Cu Fractal dewetting on Si


Moisture Moisture ––related failures modesrelated failures modesHigh moisture levels inside drive interior can adversely impact reliabilityFailure mechanism #1: Corrosion (well-known phenomenon)

Failure mechanism #2: Clearance loss due to supersaturation under flying head (included in failure model discussed above)Failure mechanism #3: Condensation of long-lived water microdroplets on both heads and disks

Droplets are abnormally long-livedDroplets reduce clearance / increase head-disk interactionDecreased reliability

45 min After Exposure

160 min After Exposure

6 hr After Exposure

21 hr After Exposure

45 hr After Exposure

4 days After Exposure




Schematics of Lubricant BondingSchematics of Lubricant Bonding

Carbon over coat

Free-Lube

Bonded-Lube

Z-DOL

-OH functionality = Lower affinity for disk Di-hydroxyl = Higher affinity for disk

Z-Tetraol

PFPE lubricants are available with a variety of backbone structures, molecular weights and functional end-groups.

Lubricants with functionalized end-groups will spontaneously bond to the disk.

Extent of lubricant bonding is dictated by the lube, the temperature and RH.

10000 20000 30000 40000 50000 600000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

80% Humidity

40% Humidity

Zdol 4000

Bon

ded

Frac

tion

time (min)0 20000 40000 60000 8000

0.2

0.4

0.6

0.8

1.0Z-Tetraol

Bon

ded

Frac

tion

Time (min)

Z-Tetraol 80% RH

0 20000 40000 60000 800000.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Bond

ed F

ract

ion

Time (min)


Lubricant bonding Lubricant bonding Exposure to elevated temperature induces a

(reversible) bonding of the lubricant to the disk.

Bonding adversely impacts lubricant mobility and contact forces 1 10 100 1000 10000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

k(t) = kBt-0.5

∆ Bo

nded

Fra

ctio

n

Time (min)

T = 50C

20% Bonded Lube 50% Bonded Lube 90% Bonded Lube

Adhesion hysteresis increases with bonded fractionForce required to separate the head and disk surfaces

increases as the lube bonding ratio increases.Severity of contact (time in contact) increases as bonded

fraction increasesHDI failure rate will scale with the adhesion hysteresis

0 20 40 60 80 1000.0

0.2

0.4

0.6

0.8

1.0

Total Zdol Thickness = 10.5 +/- 0.5 A

Rel

ativ

e C

SS D

urab

ility

% Bonded Zdol

Adhesion hysteresis


Phase transitions in PFPE lubricantsPhase transitions in PFPE lubricants

0 20 40 60 80 100 120

78

80

82

84

86

88

90

"liquid-like""glass-like"

Phas

e sh

ift, d

egre

es

Temperature, °C

1 10 100 1000 10000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

k(t) = kBt-0.5

(T = 90C)k(t) = kBt

-0.5

(T = 50C)∆ Bo

nded

Fra

ctio

n

Time (min)

PPFE bonding kinetics are non-classicalTime dependent rate coefficients1D diffusion – limited reactionFunctional form of rate coefficients can change

as a function of temperatureResults indicative of a phase transition

Shear modulated AFM measurements confirm a transition from solid-like to liquid-like behavior

Transition temperature is dependent on the lubricant molecular weight, PFPE structure and nature of carbon surface

Impact of this change in lubricant mobility on HDI reliability has not been addressed


Future (current) Challenge Future (current) Challenge -- Intermolecular ForcesIntermolecular Forces

0 2 4 6 8 1 0 1 2

0

5

1 0

1 5

2 0

V D W A t t ra c t iv e F o rc e s

(A = 3 .6 x 1 0 -1 9 J )

8 A Z d o l 4 0 0 0 /C N x

∆ Fr

eque

ncy

d ( n m )

Quartz Resonator (f = 10 - 30 MHz)v = 0.5 - 1.5 m/sA = 20 nm

Si3N4R = 2.3 mm

PFPE + carbon

Z Resolution = 0.1 nm

d

Force Transducer (FN)

It is well known that attractive, intermolecular forces are generated when macroscopic bodies are separated by nanoscopic distances (d < 10 – 20nm). For van der Waals forces, the attractive force is dependent on three factors:

g = Geometric factor defining the ‘effective’ interaction areaA = defines interaction strength between materials on the two surfaces

We operate the head-disk interface in a regime where these forces are not insignificant, and will only get more prominent in future products

2dgAF ∝

Experimental


Impact of VDW forces at the headImpact of VDW forces at the head--disk interfacedisk interface

Slider

Carbon OvercoatPFPE

ΠFN d

5 10 15 20 25 30 35-20

-15

-10

-5

0

5

10

15

20

unperturbed

Zdol 4000/ CNxImpact of macroscopic body at distance d

d = 8nm

d = 6nm

d = 4nm

d = 7nm

d = 5 nm

Πef

f (at

m)

Lubricant Thickness ( )

The attractive VDW forces that develop at the HDI reduces the magnitude of the disjoining pressure resulting in a vertical expansion of the lubricant film

Slider

Carbon OvercoatPFPE

Lube Jum p

Slider

Carbon OvercoatPFPE

ApproachAt sufficiently close distances, the lube will “jump” from the disk to the head.

This phenomenon will limit the minimum separation distance, and thereby contribute to the glide avalanche.


Effect of Molecular Weight on Glide AvalancheEffect of Molecular Weight on Glide AvalancheFr

ictio

n, g

Linear Velocity, m/sec

13.6 7.310.5

12 A Zdol 5391

12 A Zdol 3216

10 A Zdol 1350

h

Lower MW Zdol Higher MW Zdol

slider

disk

Clearance Measurements vs Lubricant Molecular WeightA) Reduced rotational velocity B) Reduced pressure

Both techniques show clearance decreases with increasing PFPE molecular weight

(Khurshudov and Waltman, Trib. Lett. (2001), 11, 143)

∆Z ~ 2nm


Effect of film thickness on Slider disk interactions

Disjoining pressure characterizing film adhesion decreases rapidly with increasing film thickness

Thicker films should jump to contact sooner than thinner filmsExperimental results confirm this expectation

These results also indicate that Intermolecular forces are currently limiting glide avalanche.

Nor

mal

For

ce (a

rb.u

nits

)

0

10

20

Zdol 4000/CNx

7A

13A

35A

0 2 4 6 8 10 12

Physical Separation (nm)

contact

28A


Intermolecular Forces: AdhesionIntermolecular Forces: AdhesionPrevious results suggest that decreased lubricant film thickness would be

beneficial from a clearance perspectiveThe Adhesive Force generated between the disk and slider (stickiness of interface), increases with decreasing film thickness, however

Increased adhesive (and frictional) force at low film thickness will increase the severity of any head-disk contact

0 5 10 15 20 25 30 350.0

0.2

0.4

0.6

0.8

1.0

1.2F ad

h / g

(arb

. uni

ts)

∆γd + ∆γp (mJ-m-2)

Thick lube (20A)

Thin lube (7.5A)

)(4 hRFadh γπ ∆=


PrePre--contact Frictional Forcescontact Frictional ForcesPre – contact Frictional forces are generated at low clearances in films of

thicknesses used at the HDI

CNx/Zdol 4000

file:CNxqcm1

35A

11A

0 1 2 3 4 5 6 7 8 9 10

15A

0.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Fric

tion

Forc

e (a

rb. u

nits

)

Separation Distance (nm)

28A

Physical contactFriction detected

d = 1.5nm

Friction starts at nominally 1.5nm prior to establishment of physical contact

Friction signal shows displays a maximum value prior to contact

Magnitude of friction maximum scales inversely with film thickness

The origin of these phenomenon are not well understood

This could also contribute to defining our minimum separation distance.

Friction maximum


SummarySummaryHDD failure rates can be dominated by the HDIHDI failure rates depend strongly on clearance

Mean clearanceStd. deviation of clearanceSensitivity to environmental parameters

Interface materials (lube + carbon) dictate HDI performance and can impact clearance

Lube thickness, type and molecular weightCarbon type and thicknessTemperature and humidity

Physics underlying many of the interface materials properties is still not understood completely

Intermolecular forces (both adhesive and friction) will fundamentally limit the lowest head-disk clearance that can be obtained (2 – 3nm)

Role of the HDI in HDD 2 - IDEMA | International Disk ... · Role of the Head Disk ... slider-disk clearance should be treated as a critical parameter by which to design a mechanically

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