Tin Whisker Phenomenon, Mitigation And Testing Overview V2

Tin Whiskers
Properties
P ti
Risk Factors
Mitigation
Industry Perspective
JEDEC Test Requirements

- Patrick Neyman
y

Outline
•Ti
TinWhiskers Fundamentals
Whi k F d t l
•Physical & Electrical Properties
•Growth and Accelerated Growth Factors
•Risk Factors
•Mitigation
t gat o
- Conformal Coating Update
•The Role of Intermetallics
•Industry Perspective
- Consumer, Telecom, Automotive
- Supplier and Customer
•JEDEC Testing Requirements
•Test Flow: Ni-Underplated devices with 4/less leads
•Reference List
Information is Patrick Neyman Confidential & Proprietary page 2 /
Do Not Reproduce or Distribute - Patrick Neyman

Tin Whiskers Basics

•TinWhiskers are single-crystal Sn filaments that extrude
from the Sn surface via grain boundary diffusion

•Grain boundary diffusion occurs due to compressive
stresses at tin grain boundaries

•Tin whiskering is a stress relief mechanism

•TinWhisker mitigation techniques
are stress relief mechanisms


Tin Whisker Examples

Whisker with striations Whisker with rings
g

Smooth Whisker with a consistent cross section.


Physical and Electrical Properties

•Diameter: ~1 – 5 microns

•Length: up to ~1 mm
1

•Fusing Current: ~5 mA

•LinearResistance:
0.5 Ohm/micron
(for
(f 2 micron di
i diameter)
t )

•~700 G required to
break-off
break off from surface note: G
Gold plating required for electrical contact
f
Oxide difficult to breach with whisker
(7 G force required to breach oxide)


Typical Time to Whiskering
• iNEMI h d t
has determined rough t i l ti
i d h typical time t whiskering f various h idit
to hi k i for i humidity
& temperature conditions
• Matte Tin over Copper, 150 °C anneal – NO UNDERPLATE
• No Corrosion on measured regions
Temperature Humidity Tin Thickness Time to Whisker

85 °C
C 85% RH 3 micron ~ 2000 hours
10 micron ~ 2000 hours
60 °C 87% RH 3 micron ~ 2000 – 4500 hours
10 micron ~ 4500 – 7000 hours
60 °C 60% RH 3 micron ~ 7000 – 10,000 hours
10 micron ~ 7000 – 10,000 hours
30 °C 90% RH 3 micron ~ 11,000 hours
45 °C
C 60% RH 10 micron ~ 10 000 hours
10,000


Tin Whisker Risk Factors
•Mechanical Processing
•Compressive stress at bending and singulation points

•IntermetallicFormation
•Cu6Sn5 volume is greater than 6Cu+5Sn
•Compressive stress forms at the Cu6Sn5 – Sn interface

•Surface Oxidation
•SnO burrows into the Sn layer during oxidation
•SnO volume is greater than Sn + O

•CTE Mismatch (Coefficient of Thermal Expansion)
•Compressive stress at solder fillet / device interface.
interface

•Plating Conditions
•Organic Contamination (
g (internal compressive stresses)
p )


Tin Whisker Mitigation Techniques
•Plating Conditions
•!!! No other techniques work if this is not right !!!
•Eliminate / reduce organics,...
•M tt Tin ( f no Ni underplate: min 7 micron, 10 nominal. With Ni: thinner is better)
Matte Ti (if )
•Larger grains result in larger free volume (void volume)
•Free volume allows short-range stress relief
•U d
Underplate with Ni k l (min 0.5 micron, 1.27 recommended)
l t ith Nickel
•Ni protects Sn from Cu
•Ni3Sn4 intermetallic forms tensile stress regions
•Relieves compressive stress from oxides or damage on thin (<<10 μm) Sn
•Anneal @ 150 C for 1 hr within 24 hrs of plating
•Stress relaxation
•Fuse in hot oil above Tmelt(Sn) within 24 hrs of plating (time varies)
•Sn flows to fill in voids and cool to non-stressed state
•Alloy with Lead
•Telecom and Automotive current (JUN07) method of choice


Conformal Coating NOT a mitigator
• No coatings yet found to mitigate whisker growth
• Conformal coatings CAN protect against intrusion by
whiskers from neighboring components


Tin Whiskers & Intermetallics
•IntermetallicFormation
•Cu6Sn5 takes up more volume
than 6Cu+5Sn
•Compressive stress forms at the
Cu6Sn5 – Sn interface

Sn Deposit

Cu6Sn5

Cu Substrate


Tin Whiskers & Intermetallics
•Occasionally, a whisker extrudes
•The extrusion die is the
path of least resistance


Tin Whisker Formation

Questions ???

Outline
•Ti
TinWhiskers F d
Whi k Fundamentals
t l
•Risk Factors
•Mitigation
t gat o
•Reference List

Telecom / Automotive (AEC Jun07)
• Sn Pb components preferred by most
Sn-Pb
– Pb-free being forced by suppliers and customers

• Long-term reliability concern
– Uncertain if Whisker Growth is Limited / Unlimited

• 3% Sn Pb requirement being pushed by some
Sn-Pb
• Whisker-Risk labeling on Pb-free parts under debate
• Majority going with JEDEC for testing requirements
– Telecom pushing for new Class 2A

• Periodic recertification may be dropped
• Ambient conditions storage may be dropped
• Sn Pb component price increasing significantl
Sn-Pb significantly

Consumer Electronics (IPC, Dec06)

– Tin Whiskers viewed as small source of problems

– Largest Pb-Free concern is solder-joint reliability

– OEM manufacturers concerned as devices grow smaller

– Old plating techniques now known to be a problem

– Cleanliness an issue for many (smaller?) shops

– Confusion in general about best methods


Industry Perspective - Suppliers
Joint Fi di
J i t Findings:
Philips, Infineon, STMicroelectronics, Freescale Semiconductor
• Reflow Mitigates Tin Whiskering
– matches RCP initial findings
• Plating improvement efforts ceased / very limited
– JEDEC: any change to bath requires re testing
re-testing
• JEDEC inspection procedures seen as impractical
• Corrosion is found to be a significant cause of whiskering in
practical use, but is a JEDEC exemption
• Preconditioning does not cause increased whiskering,
but adds 66% workload
• Grandfathered parts fail JESD201 testing
– Surface finishes with long field history of reliability exempt


Industry Perspective - Customers
Joint Fi di
J i t Findings:
Philips, Infineon, STMicroelectronics, Freescale Semiconductor
• Very Few customers request Tin Whisker report
– Mostly, no more questions after report submitted
• Some customers ADD demands to JESD201
• Some concern about JEDEC exception for terminations
covered in solder during assembly
• Customers are accepting the g
p g grandfather clause on long-
g
established tin surface finish processes.
– from JESD201 Section 6 (Acceptance Criteria):
“data on long established tin surface finish manufacturing processes with
data
reliable field histories can be substituted with agreement between supplier
and user”.
– BUT, many of these components fail JESD201 testing
y p g


Outline
•Ti
TinWhiskers F d
Whi k Fundamentals
t l
•Risk Factors
•Mitigation
t gat o
•Reference List

JEDEC Testing Requirements
• Temp Cycle:
– +85 C Tmax, -55/-40 C Tmin, 5-10 min soak, ~3 cyc/hr
– Whi k
Whiskers typically f
t i ll form after 500 cycles
ft l

• High Temp/Hum Conditions:
– 60 C / 87% RH for 4000 hours
– Whiskers typically form after 1000-2000 hours

• Room Conditions:
– 30 C / 60% RH for 4000 hours
– AEC and iNEMI driving to eliminate this condition from JEDEC

• Calibration Standard and Equipment Certification required


Sample Requirements for 2-Leads
• Fi l as-mounted product. L
Final t d d t Loose and R fl
d Reflowed
d
– For parts without top-bottom symmetry: 3 total pre-conditions
• Minimum 3 plating lots, at least one week apart, no more than ~1-2
months old at time of test
– the older the samples, the more likely Whiskers will grow
• Minimum 2 samples per lot for pre-condition for each test
• Minimum 9 samples total per test
– Due to minimum 18 measurable terminations
– Measurable termination = able to measure top and 2 sides
• Minimum 75 mm2 total inspection area per test
• Minimum 25 mm2 total inspection area each sample
– Including at least 2 edges with 3 mm total length
• Minimum 96 total inspection areas
– Must increase number of samples if cannot achieve 96 areas
– Each inspection area should be > 0.85 mm2 (not including solder fillet)

JEDEC Allowable Whisker Length
Military Telecom Industrial Consumer
Aerospace Automotive Electronics Electronics


JEDEC Testing Requirements
• Whisker length chosen by established
lead spacing requirements. spacing

• Saturation debate (Telecom):
– Original allowable length was 65 micron
– R d
Reduced t 40 micron since classified with
d to i i l ifi d ith
automotive in JEDEC
– Move to separate infrastructure from human
safety – form Class 2A


Outline
•Ti
TinWhiskers F d
Whi k Fundamentals
t l
•Risk Factors
•Mitigation
t gat o
•Reference List

Test Flow Example

Tin Whisker Test Flow for Ni-Underplated Passive Components with Four or Less Terminals

Plating Lot C
Plating Lot A Plating Lot B
Lot (plated >1 week after lot B)
(test finished parts) (plated >1 week after lot A)
(plated <4? weeks after lot A)

3-5 per
condition for
each test

No Precondition: No Precondition:
Cond Precondition:
Glued in Normal Glued in “Dead
ition Reflow
Configuration Bug” Configuration

27-45 parts goto
::: each test :::
~300-500 parts total
(80-140 if re-use)


Test Flow Example, stress tests


Test Flow Example, examination


Reference List
• Annotated Tin Whisker Bibliography and Anthology George T Galyon IEEE Trans
Anthology, T. Galyon, Trans.
Elec. Pack., 28 (1), 94-122 (January 2005)
• Role of Intrinsic Stresses in the Phenomena of Tin Whiskers in Electrical Connectors,
Sudarshan Lal and Thomas D. Moyer, IEEE Trans. Elec. Pack., 28 (1), 63-74 (January
2005)
• An Integrated Theory of Whisker Formation: The Physical Metallurgy of Whisker
Formation and the Role of Internal Stresses, George T. Galyon and L. Palmer, IEEE
Trans. Elec. Pack., 28 (1), 17-30 (January 2005)
( ) ( y )
• An Electrical Characterization of Tin Whisker, Robert Hilty, Ned Corman, MRS Spring
Meeting, San Francisco (2007)
• Tin Whisker Reliability Assessment by Monte Carlo Simulation, Robert Hilty, Ned
Corman,
Corman IPC/JEDEC Lead Free Symposium San Jose (2005)
Symposium,
• Accelerated Tin Whisker Test Committee Update Phase 5, Heidi L. Reynolds, ECTC
Tin Whisker Workshop, Reno, NV, (May 29, 2007)
• Matte Sn Electroplating in Mass Production and Testing According to JESD201, P
Oberndorff, Ditt
Ob d ff M Dittes, P Crema, H Si
C Siegel, P S ECTC Ti Whi k W k h
l Su, Tin Whisker Workshop, R Reno,
NV, (May 29, 2007)
• Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, Part II, Tom
Woodrow, , Reno, NV, (May 29, 2007)


Reference List
• JESD22A121: Measuring Whisker Growth on Tin and Tin Alloy Surface Finishes (May
2005)
• JP002: Current Tin Whiskers Theory and Mitigation Practices Guideline (March 2006)
• JESD201: Environmental Acceptance Requirements for Tin Whisker Susceptibility of
Tin and Tin Alloy Surface Finishes (March 2006)


Tin Whisker Phenomenon, Mitigation And Testing Overview V2

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Tin Whisker Phenomenon, Mitigation And Testing Overview V2