controlling pad damage - cleaning technologies€¦ · basic damage control strategies – probe...
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International Test Solutions
Impact and Control of Pad Damage at Wafer Sort
International Test SolutionsJerry Broz, Ph.D.
VP of WW ApplicationsReno, NV USA
Probing Process Workshop
October 13 and 14, 2014Ieper, Belgium
J. BrozOct-2014
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International Test Solutions
Overview• International Test Solutions Profile
• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations
• Basic Damage Control Strategies– Probe card parameters– Operational parameters
• Summary / Discussion
2J. BrozOct-2014
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International Test Solutions
Overview• International Test Solutions Profile
• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations
• Basic Damage Control Strategies– Probe card parameters– Operational parameters
• Summary / Discussion
3J. BrozOct-2014
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International Test Solutions
International Test Solutions Corporate Profile
• Global supplier of high quality yield and utilization improvement products for wafer sort and package test since 1997
– Cost-effective cleaning solutions and industry leading technical services.– Strong IP position for front-end, wafer-sort, and back-end test.
• Managed by highly experienced semiconductor industry personnel with a combined experience of +100 years.
– ITS Branch Offices in Taiwan, Japan, Korea, China, and Singapore.– World-wide sales support network of authorized agents.
• Manufacturing Center for advanced polymer materials research and development– Controlled Compliance Manufacturing methods. – Materials characterization, development, and testing laboratories.
• Award winning Test Analysis Center for electrical test and process characterization– Analytical laboratory focused on probe technology and contactor performance testing.– Cleaning recipe assessment and optimization
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International Test Solutions
ITS – Increasing Yields and Reducing Costs
Maximized wafer yield– Controlled and stable CRES– Reduced site-to-site failure
Increased throughput– Minimize off-line cleaning– Extend probe card lifetime
Improved tool uptime– Reduce operator intervention– Reduce spare inventories
Increased litho tool output– Clear “hot spots” w/o downtime– Perform regular PM cycles
Higher etch tool output– Clean ESC with closed chamber– Accelerate wet clean recovery
Lower operating costs– Extend time between wet cleans– Lower process kit part usage
Improved first-pass yields– Improved contact– Reduced rescreen
Greater throughput– Minimize off-line cleaning– Maintaining high UPH
Reduced Cost of Test– ACC for low downtime– Tri-temperature capable
Probe Card Clean Test Socket Clean Chuck Cleaning Wafer
5J. BrozOct-2014
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International Test Solutions
Overview• International Test Solutions Profile
• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations
• Basic Damage Control Strategies– Probe card parameters– Operational parameters
• Summary / Discussion
6J. BrozOct-2014
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International Test Solutions
Introduction• Probe card technologies are more advanced; however,
the contact basics of wafer sort really have not changed.
• ALL probe technologies have a contact area substantially harder than the pads, solder balls, or pillars.
• “Contact and slide” CRITICAL for electrical contact, but results in localized plastic deformation.
• Volume of material displaced during the slide is a function of probe mechanics, metallic interactions, frictional effects, and surface properties.
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International Test Solutions
Controlled Contact is One of the TOP GOALS
• To control wafer test is to control the mechanical contact and the electrical contact between the probes and the DUT.
• Process Monitors• Probe Yield• Binout Metric• Contact Resistance• Probe Mark• Re-Probe / Re-Test• I/O Damage
Apply the least mechanical contact that ensures a reliable electrical contact.
• Control Variables• Probe Force• Overtravel• Probe Placement (XYZ)• Current / Duration• Temperature• Cleaning Execution
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International Test Solutions
Copper Bonding Wire Implementation• Cost Benefit?
– Au price at $400: little adoption– Au price at $700: some movement– Au price at $1000: significant motivation
• Advantages– High thermal conductivity– Low electrical resistance– Higher breaking stress– Smaller wire diameters for higher density– Excellent intermetallic formation
• Challenges– Bonding temperature can increase
oxidation– High pad Impact Force – Bonding force and ultrasonic energy
control is critical
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International Test Solutions
Local Plastic Deformation (a.k.a., Probe Marks)
• Stress distribution during the scrubbing has been shown by cause underlying damage and cracking.
• Repeated probing has been shown to displace material and create under pad damage.
Probe Mark Anatomy
Pile-Up Created DuringForward Motion
Pile-Up Created DuringBackward Motion
ForwardMotion
BackwardMotion
Depth CreatedDuring Scrub
10J. BrozOct-2014
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International Test Solutions
“Traditional” Definition of Pad Damage• Large and multiple probe marks affect ball bond adhesion
and cause long term reliability issues.
Damaged Area Due to Probe
• Pad Area = pad-X dimension × pad-Y dimension
Pad Damage (%) = 𝑷𝑷𝑷𝑷𝑷𝑷 𝑨𝑨𝑨𝑨𝑨𝑨𝑷𝑷𝑨𝑨𝑷𝑷
K. Karklin, J. Broz, and B. Crump, SW Test 2008
AaA sTD
nnd ∑
=−=
11
1Where:
Ad - disturbed area
TD - touchdowns
a - scaling coefficient
As - scrub mark 2D size
• Modeling the Disturbed Area due to Probe:
11J. BrozOct-2014
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International Test Solutions
Damage Area Affects Bondability• Probe damage area positively correlated to bondability issues.
– Reduced ball shear strength and wire pull strength– Increased NSOP (no stick on pad) and LBB (lifted ball bond)
Assembly Parameter vs. Probe Mark Area
% L
BB R
ejec
ts%
NSO
P Re
ject
s
% AREA Pad Damage
Ball
Shea
r (gr
ams)
Wire
Pul
l (gr
ams)
Sources …Tran, et al., ECTC -2000Tran, et al., SWTW-2000Langlois, et al, SWTW-2001Hotchkiss and Broz, ECTC-2001Hothckiss and Broz., IRPS-2001Among others …
Critical ValueDamage = 25%Ball Shear
Wire Pull
% LBB% NSOP
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International Test Solutions
Probe Mark Area Impact on Bond Integrity
0
278
8333
19444
0 5,000 10,000 15,000 20,000 25,000
30%
40%
50%
60%
Lifted Balls
Damage Area vs. Au Wire Bond FailureInter-metallic
CoveragePad
Damage
80-90%
70-80%
70-80%
50-60%
Gillaed, et al., SW Test 2007
Source: TIPI C027 Study
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International Test Solutions
Damage Area vs. Cu Wire Bond Failure
• Ball shear and pull strength showed no significant differences vs. pad damage.• Ball shear and ball pull strength values were more than twice the required strength. • Non-stick on pad (NSOP) bonding failure was not observed with 15,840 wire bonds at
each level of pad damage.
Beleran, et al., ECTC 2013
14J. BrozOct-2014
Specification RequirementSpecification Requirement
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International Test Solutions
Damage Depth Affects Bondability!• A probe mark can have a small damage
area, but exceed critical depth.– % Damage < 9 % (which is within limits)– Depth = 10kÅ (which is excessively deep)
• Multiple touchdowns displace the pad material and expose the barrier metal
6.0kÅ aluminum + 5.5kÅ thermal oxide = 11kÅ
Probe Depth = 10kÅ
Miller, et al., SWTW-2007
Exposed Barrier Metal
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International Test Solutions
No. Touch Downs Impact on Bond Integrity
0
5556
8333
13889
0 5,000 10,000 15,000 20,000 25,000
1
4
8
12
Lifted Balls
Depth Damage vs. Au Bond Failure
Inter-metallic Coverage
No. ofTouchdown
Source: TIPI C027 Study
60-70%
60-70%
65-75%
80-90%
Gillaed, et al., SW Test 2007
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International Test Solutions
Damage Area vs. Au Wire Intermetallic (%)• Insufficient aluminum-gold intermetallics form at the deepest portion of
the probe mark.• Bonding to pads with > 25% probe damage produces a higher incidence
of lifted balls during production.
3X TDs 6X TDs
Regions of little or no intermetallic formation and voidsmatch the locations of the probe marks
1X TD
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International Test Solutions
Damage Area vs. Cu Wire Intermetallic (%)
• No significant differences of intermetallic formation vs. pad damage.
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Beleran, et al., ECTC 2013
J. BrozOct-2014
Specification Requirement
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International Test Solutions
OT = 30um
OT = 60um
Probe MarkHeel
Metallization Affects Probe “Pile-up”
• OT = 30um• Length = ~10 to ~14um• Depth = ~3.5 to ~3.8kÅ
• OT = 60um• Length = ~18 to ~20um• Depth = ~5.0 to 5.4kÅ
• OT = 30um• Length = ~12 to ~14um• Depth = ~8.0 to ~8.3kÅ
• OT = 60um• Length = ~20 to ~22um• Depth = ~12.0 to 12.3kÅ
• OT = 30um• Length = ~16 to ~18um• Depth = ~14.6 to 16.2kÅ
• OT = 60um• Length = ~22 to ~24um• Depth = ~20.0 to 22.3kÅ
6kÅ Wafer 15kÅ Wafer 30kÅ Wafer
OT = 30um
OT = 60um
Probe MarkHeel
OT = 30um
OT = 60um
Probe MarkHeel
Bischoff, et al., SWTW-2012
19J. BrozOct-2014
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International Test Solutions
“Pile-up” Height Effects• Pile-up has been correlated to bondability issues.
– Reduced ball shear strength and wire pull strength– Increased NSOP (no stick on pad) and LBB (lifted ball bond)
Assembly Parameter vs. Aluminum Pile-Up
% L
BB
Rej
ects
% N
SO
P R
ejec
ts
Height of Pile - Up
Bal
l She
ar (g
ram
s)W
ire P
ull (
gram
s)
Ball ShearWire Pull
% LBB% NSOP
Critical Value“unknown”
20J. BrozOct-2014
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International Test Solutions
Low Sensitivity of Cu Wire to “Pile-up”
• With Cu wire bonding the soft Al pad material readily deforms.
• The pile-up and depth of the probe mark are flattened under high force.
• Under high loads, the Al pad material gets squeezed out from under the ball bond.
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Beleran, et al., ECTC 2013Breach, Gold Bulletin, 2010
J. BrozOct-2014
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International Test Solutions
Depth Effects Create Hidden Damage• Probe induced cracking of underlying structures and the impact on
package long term reliability is an ongoing test industry issue.
• Damage to Low-k and CUP / BOAC during probe and assembly– Low fracture toughness = high probability of cracking– CUP damage = leakage or shorts electrical nodes.
• Cu wire bonding requires higher forces and more ultrasonic energy exacerbating the problems of pad damage and underlying cracks.– A weak pad structure cannot withstand robust copper wire bonding process – In fact, some unprobed pads will crater under high force and energy
requirements of a Cu wire bond.
22J. BrozOct-2014
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International Test Solutions
Hidden Damage due to Probe• Under-layer micro-scratches and cracking attributed to probe.
Slight Medium Serious
OD = 65μm TD = 6 times
Tip Dia.= 8μmBCF = 4gw/mil
Hwang, et al., SW Test 2006
ForwardScrub Backward
Scrub
• TaN Crack > Underlying Deformation > Pad Void• z-Force is well determined from probe needle properties.• Transverse loading conditions were not characterized.
OD = 65μm TD = 4 times
Tip Dia.= 8μmBCF = 4gw/mil
OD = 65μm TD = 2 times
Tip Dia.= 8μmBCF = 4gw/mil
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International Test Solutions
Assessing Transverse Loading Conditions
Pad TD X-Axis Video
Pad TD Y-Axis Video
y-Force vs. OT
z-Force vs. OT
z-Force and y-Transverse Force vs. OT are clearly differentiated
Khavandi, et al., SW Test 2014
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International Test Solutions
Overview• International Test Solutions Profile
• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations
• Basic Damage Control Strategies– Probe card parameters– Operational parameters
• Summary / Discussion
25J. BrozOct-2014
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International Test Solutions
Damage Control Strategies• Probe card parameters …
– Probe technology– Tip shape / Geometry– Scrub “mechanism” – Low force
• Operational parameters …– Optimized probe to pad interactions– Z-stage motion control– Number of Touchdowns
• Probe pad properties … – Robust BEOL stacks designed to resist cracking– Alternate pad materials, for example, Ni-Pd or Ni-Pd-Au– Thicker pad metal layers to protect the underlying stack
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International Test Solutions
Pad Damage Depth vs. Technology
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Cantilever
Vertical
MEMS
Pad Surface
2000
1000
500SCRUB HEIGHT
- 400
SCRUB DEPTH
- 500- 600 - 800- 1200
nm
Horn, SW Test 2008
Metal Thickness
J. BrozOct-2014
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International Test Solutions
Probe Tip Geometry Optimization• Reduce probe tip diameter• Reduce spring force and overdrive
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Benefits:• Smaller probe mark• Minimize probe mark size• Reduce probe mark depth
Concerns:• CRES stability • Probe process control• Probe card maintenance• Reduced card life
J. BrozOct-2014
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International Test Solutions
Probe Tip Texture / Surface Roughness(Example: 15kA Al-thickness)
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Horizontal Force Horizontal Force
Textured Probe TipSmooth Probe Tip
Vertical ForceVertical Force
Moment
• Textured probe tips “dig into” the aluminum layer; while smooth probe tip “skate” across.• As the textured surface resists the forward scrub of the tip, a moment is generated and the
heel of the probe penetrates the surface of the pad.
Khavandi, et al., SW Test 2014
J. BrozOct-2014
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International Test Solutions
Advanced Scrub Control
• Macroscopically, punch through level was found to be a direct function of tip pressure (FORCE / AREA)
– Tip area– Spring constant– Planarity – Over travel
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Tip Size
Low High
Standard
Large
KWang, et al., SWTW-2007
J. BrozOct-2014
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International Test Solutions
Probe Count and Total Probe Force• Probe force needs to stay in a manageable region• Probe force per probe has to decrease• Certain probe force is needed for stable CRES
Probe Force [g]
C re
s [O
hm]
Characterization of Different Probe Types
Type 1Type 2Type 3
M. Huebner, SWTW 2009
Unstable CRES Stable CRES
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International Test Solutions
What Operational Steps Can I Take ?
• Can reasonable steps be taken with existing technologies (e.g., an existing probe card and a prober) to reduce pad damage in a cost-effective manner?
• Is it possible to identify an optimized combination of prober operational settings to reduce the overall area and volumetric probe damage?
32J. BrozOct-2014
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International Test Solutions
Factors (Prober Operational Settings)• Number of Touchdowns
Single vs. Double
• Overtravel MagnitudeLow (50um) vs. Middle (63um) vs. High (75um)
• Undertravel MagnitudeLow (0um) vs. Middle (10um) vs. High (20um)
• Pin-Update Execution– Abbreviated pin alignment to compensate for thermal movement – On vs. Off
• Wafer Chuck SpeedLow (6000 um/sec) vs. High (18000 um/sec)
• Chuck Revise Execution– Re-zero of the wafer chuck to compensate for thermal movement– On vs. Off
33J. BrozOct-2014
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International Test Solutions
Significant Factor Estimates13.3180
1.6657
-1.1663
-0.9465
-0.7554
-0.6623
2.0472
-1.9320
1.0323
-0.0079
-12.4100
0 2 4 6 8 10 12 14 16
ut[20]
ut[10]
cr[Off]
speed[Low]*cr[Off]
pu[Off]
speed[Low]
speed[Low]*ot[50]
ot[63]
speed[Low]*ot[63]
ot[50]
td[Double]
Probe Mark Volume
Scaled Estimates
1.6605
2.0336
2.7873
5.0350
5.4209
39.9630
-38.75266
-1.5891
-4.8920
-7.1279
-9.0581
0 5 10 15 20 25 30 35 40 45
ut[10]
ut[20]
cr[Off]
pu[Off]
speed[Low]*cr[Off]
speed[Low]*ot[63]
speed[Low]*ot[50]
ot[63]
speed[Low]
ot[50]
td[Double]
Probe Mark Area
Scaled Estimates
• Single vs. Double Touchdown• Minimum vs. Maximum Overtravel
Primary Responses Secondary Responses• No clear chuck speed dependency for volume was surprising.• Speed was the third largest factor probe mark area response.
34J. BrozOct-2014
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International Test Solutions
1.3116
1.5082
1.8716
1.9195
1.9346
2.5774
20.6415
-4.4643
-11.6636
-1.4899
-0.4748
0 5 10 15 20 25
speed[Low]*ot[50]
speed[Low]*ot[63]
ot[63]
ut[10]
cr[Off]
pu[Off]
speed[Low]
speed[Low]*cr[Off]
ut[20]
ot[50]
td[Double]
Pile Up Area
Scaled Estimates
13.3180
1.6657
-1.1663
-0.9465
-0.7554
-0.6623
2.0472
-1.9320
1.0323
-0.0079
-12.4100
0 2 4 6 8 10 12 14 16
ut[20]
ut[10]
cr[Off]
speed[Low]*cr[Off]
pu[Off]
speed[Low]
speed[Low]*ot[50]
ot[63]
speed[Low]*ot[63]
ot[50]
td[Double]
Pile-up Volume
Scaled Estimates
Significant Factor Estimates
• Single vs. Double Touchdown• Minimum vs. Maximum Overtravel
Primary Responses Secondary Responses• Speed was a low level contributor to pile-up volume.• No clear huck speed dependency was surprising.• Undertravel was a more contributing factor than chuck speed.
35J. BrozOct-2014
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International Test Solutions
50100150200250300
Mea
n(Sc
rub
Area
)64
.055
95±1
9.72
275
Doub
le
Sing
le
Singletd
Low
High
Lowspeed
Off
On
Offcr
50 63 75
50ot
10 20 off
10ut
Off
On
Onpu
0
20
40
60
Mea
n(D
ivot
Vol
ume)
8.73
2232
±8.4
3241
3
Dou
ble
Sing
le
Singletd
Low
Hig
h
Highspeed
Off
On
Oncr
50 63 75
50ot
10 20 off
20ut
Off
On
Onpu
Off
Best Case Combinations• Modeled response data can be used to investigate the effects of changing one
parameter and keeping the other constant.
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Miller, Broz, Robinson, SWTW-2007
Number of TDs Overtravel
J. BrozOct-2014
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International Test Solutions
Overview• International Test Solutions Profile
• Pad Damage at Wafer Sort– Defining the Problem– Packaging and Assembly Considerations
• Basic Damage Control Strategies– Probe card parameters– Operational parameters
• Summary / Discussion
37J. BrozOct-2014
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International Test Solutions
Two-Pronged Strategy for Reduced Damage
• Well-controlled probe card metrics can mitigate some of the damage effects.– Tip size / shape– Scrub length– Probe force– Contact material
• Reasonable steps can be taken with “existing” hardware to pad damage in a cost-effectively.– Reduced overtravel– Reduced touchdown counts
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Recall a GOAL for wafer test …Apply the least contact that ensures reliable electrical connection.
J. BrozOct-2014
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International Test Solutions
Summary / Discussion• I/O pad damage has been aggravated by smaller pads,
sharper needles, and new process node technologies.
• Changes and improvements to probe card specification have been developed to mitigate some of the problems.
• Significant new probe methods, new probe card technologies, and design and layout tricks are now being implemented.
• Reasonable steps can be taken with “existing” hardware to reduce pad damage in a cost-effective manner.
39J. BrozOct-2014
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International Test Solutions
“I'm smart enough to know that I'm dumb as the next guy.”- Richard Feynman
Nobel Winner in Physics, 1965
Questions ???
40J. BrozOct-2014
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International Test Solutions
IEEE SW Test Workshop – www.swtest.orgJune 7 - 10, 2015, in San Diego, CA, USA• Want to Learn More !
– All aspects of wafer level testing– Three Day Technical Program – EXPO that showcases key suppliers
• EXPO does not compete with technical program
• Topics include but are not limited to:
– New probe card and contractor technologies– Challenges of 300-mm wafer probing– Monitor and reduction of chip I/O pad damage– Productivity improvements for production– Probe data collection, analysis, and management– Cleaning and cost of ownership – e-Test, Parametric, and Test Structure Testing
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25 Years of Probe Technology
J. BrozOct-2014
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International Test Solutions
About the AuthorJerry Broz, Ph.D., has been the Applications Engineering Team Leader and VP of Applications at International Test Solutions since 2003. Dr. Broz is responsible for the ITS branch office teams located in Taiwan, Korea, Japan, China, and Singapore that are focused on optimal on-line cleaning solutions for wafer sort and package test. Previously, Dr. Broz was a Member of Technical Staff with the Worldwide Probe Development Team at Texas Instruments, Inc. He has authored numerous publications and presentations in the areas of wafer level test, package test, and IC packaging. Dr. Broz holds a number of US and International patents as well as several pending patent applications related to wafer sort, package test, and front-end processes. Dr. Broz earned a Ph.D. in Mechanical Engineering from the University of Colorado at Boulder and has over 20 years of experience in various high volume manufacturing and applied research environments.
Dr. Broz is the General Chair for IEEE SW Test Workshop and a Sr. Member of the IEEE as well as an IEEE Golden Core member. The SW Test web site http://www.swtest.org is an on-line repository for many probe technology presentations.
Jerry Broz, Ph.D.VP World Wide ApplicationsInternational Test SolutionsReno, NV 89502
4242J. BrozOct-2014
Impact and Control of Pad �Damage at Wafer SortOverviewOverviewInternational Test Solutions �Corporate ProfileSlide Number 5OverviewIntroductionControlled Contact is One of the TOP GOALSCopper Bonding Wire ImplementationLocal Plastic Deformation �(a.k.a., Probe Marks)“Traditional” Definition of Pad DamageDamage Area Affects BondabilityDamage Area vs. Au Wire Bond FailureDamage Area vs. Cu Wire Bond FailureDamage Depth Affects Bondability!Depth Damage vs. Au Bond FailureDamage Area vs. Au Wire Intermetallic (%)Damage Area vs. Cu Wire Intermetallic (%)Metallization Affects Probe “Pile-up”“Pile-up” Height EffectsLow Sensitivity of Cu Wire to “Pile-up”Depth Effects Create Hidden DamageHidden Damage due to ProbeAssessing Transverse Loading ConditionsOverviewDamage Control StrategiesPad Damage Depth vs. TechnologyProbe Tip Geometry OptimizationProbe Tip Texture / Surface Roughness�(Example: 15kA Al-thickness)Advanced Scrub ControlProbe Count and Total Probe ForceWhat Operational Steps Can I Take ?Factors (Prober Operational Settings)Significant Factor EstimatesSignificant Factor EstimatesBest Case CombinationsOverviewTwo-Pronged Strategy for Reduced DamageSummary / DiscussionSlide Number 40IEEE SW Test Workshop – www.swtest.org�June 7 - 10, 2015, in San Diego, CA, USAAbout the Author