Tin Whisker White Paper

Transcription

Tin Whisker White Paper
JESD201 Approach
September 2012
by Loren R. Zapanta(ILZ)
Cypress Reliability Engineering
Tin Whisker White Paper, Sept. 2012
Page 1
CONTENTS
Abstract
3
Introduction
5
Thin whisker Theory
6
Thin Whisker Mitigation
9
Thin Whisker Growth Testing
10
Test Results
17
Reference Documents / Articles
30
Page 2
ABSTRACT
Tin(Sn) plating was known to whisker formation and growth that has resulted to reliability
problems and failures for various types of electronic equipment. Tin whisker was
identified as electroplated cadmium components during the late 1940’s, appear to
essentially be an extrusion, or hairlike growth emanating from the surface of the tin plate.
As illustrated in Figure 1.0, there are several types of whisker. Filament whiskers are
needle-like in as much as the length is at least ten times the diameter. Column and spiral
whiskers are quite self explanatory. Nodule whiskers are normally defined by their
diameter and are often referred to as mounds or hillocks.
Pure tin (Sn) present a high risk of tin-whisker failures in electronics, particularly those
demanding high level of reliability.
A) Filament whisker with striations
C) spiral Whisker
B) Column whisker with a consistent cross section
D) Spiral whisker growing from a nodule
FIGURE 1. Types of Whisker
Page 3
Examples of Whisker & Non-whisker
Reference : JESD201/JESD22-A121
Page 4
INTRODUCTION
With the elimination of lead (Pb) in electronic components in 2006 as a result of two
European Union directives, 1) RoHS (Restriction on use of certain Hazardous
Substances) and 2) WEEE (Waste from Electrical and Electronic Equipment), all
manufacturers of IC’s have been working the problems associated with tin (Sn) plated
leads, specifically the creation of “tin whiskers. This directive restricts the use of six
hazardous substances in electrical and electronic equipment at a maximum concentration
value (MCV).
Table 1 shows the MCV’s of the six hazardous substances by weight of a homogeneous
material.
Substance
RoHS Limit
Mercury (Hg)
Lead (Pb)
Cadmium (Cd)
Hexavalent Chromium (Cr+6)
Polybrominated Biphenyls (PBB
Polybrominated Diphenyl Ethers (PBDE)
< 0.1% (< 1000ppm)
< 0.1% (< 1000ppm)
< 0.01% (< 100ppm)
< 0.1% (< 1000ppm)
< 0.1% (< 1000ppm)
< 0.1% (< 1000ppm)
Cypress is committed to provide environmentally safe products. It has implemented the
use of NiPdAu pre-plated lead-frames and pure tin (Sn) leadfinish to follow the Pb-free
roadmap. Most of the packages using NiPdAu leadframes are being assembled in
Cypress Philippines while all of its pure Sn lead coated packages are produced by its
qualified subcontractor assemblies.
Although semiconductor industry and Cypress have aligned with the Pb-free directive,
the use of Pure Sn as alternative leadfinish posess risk due to tin whiskers. Since the
European Union (EU) directive came, the industry is constantly looking for alternative
leadfinish materials and ways to mitigate tin whisker growth. Currently, Cypress’
subcontractor assemblies are annealing the matte Sn finish for 1 hour at 150oC and
sustain the allowable plating thickness to minimize whisker propensity. See Figure 2.
Figure 2. Thickness of the pure Sn plating
Whiskers are essentially filaments or fiber like single crystals of tin which can grow long
enough to cause a short circuit between leads, or may break off and cause damage
elsewhere in the device. Even though JEDEC have provided some recommended
accelerated tests,understanding the growth of whisker is still unknown.
Page 5
THIN WHISKER THEORY
Tin whiskers are electrically conductive, crystalline structures that sometimes grow from
surfaces with tin being the final finish. Whisker growth starts from an incubation period
with varying growth rates. Over the past 60 years, many works on understanding tin
whisker growth were published. The different experimenters focused their own studies on
specific aspects of metal growth according to the concepts of material science and
metallurgy. Their works were not only concentrated on the growth mechanism but also to
its tin whisker mitigation.
As published by George T. Gaylon on his paper entitled “A History of Tin Whisker Theory:
1946 to 2004”, he cited the Integrated Theory for Whisker Formation and Growth. Here
are the salient points of his review on the works done by previous authors from the late
1940’s to early 21st century:
1. Whiskers do not grow from as-plated microstructures – this means that existing
microstructure of a plated surface will not grow whisker on the same microstructure
(e.g. one grain of tin of the deposited finish). A whisker will be formed from a
“different” grain.
2. Whisker grains are formed by recrystallization events. Macro- and/or microstresses
within the film (finish) drive recrystallization.
3. Tin atoms are transported to the whisker grain through a grain boundary network that
connects the whisker grain with the film / substrate interface region. – This statement
supports #1. There is some sort of diffusion events that influence the growth of the
whisker grain.
4. The driving force for tin transport is a positive-stress gradient, not a compressive stress
state.
5. Intermetallic formation at the substrate interface generates very high compressive
stresses in the intermetallic region. This region is always a combination of intermetallic
and unreacted/displaced tin atoms.
6. The unbalanced inter-diffusion of copper and tin results in a Kirkendall effect with a
vacancy-rich zone within the copper substrate in the vicinity of the film/substrate
interface.
7. The Kirkendall zone within the copper substrate results in a shrinkage effect that
establishes a tensile stress state in the Kirkendall zone. – this can be the source of the
positive-stress gradient stated in #4.
As opposed to the late 1940’s theory, dislocation mechanisms are probably not relevant
to whisker growth. The above points are the ones that have been corroborated to happen
during whisker growth. But even with these pieces of information, whisker growth is still
not yet fully understood.
Page 6
Below is a rough illustration of how tin whiskers grow according to what has been
known from past experimentations & studies:
Figure 2. Formation of Tin whisker by the Integrated Theory (taken from iNEMI
report “Tin Deposit Ripening and Whisker Growth” by Andre Egli)
Page 7
The Integrated Theory predicts that both the tin and the intermetallic from the
“intermetallic region” are mutually compressed (there is a large amount of force/stress
due to the reaction of Cu and Sn atoms to form intermetallic compound).
The tendency of the highly stressed intermetallic region is to relax by diffusing tin atoms
into the overlying tin film. This will eventually stop when a maximum compressive stress
was achieved within the tin film. In effect of diffusion, there will also be an increase in
surface area and volume. Whisker growth is a more efficient way to relax the internal
stresses within the intermetallic region.
The highly compressed tin atoms diffuse through the grain boundary network to the
whisker grain. Unlike intermetallic compound formation, whisker formation does not
generate a back stress.
Diffusion of tin atoms to the whisker grain continues until all available strain
energy/positive gradients have been utilized/neutralized to produce the new surface area
of the whisker structure.
Page 8
THIN WHISKER MITIGATION
Tin Whisker Mitigation strategies is recommended due to whisker growth phenomenon
associated with use tin (Sn) during soldering process. This manifestation was
acknowledged by Cypress Semiconductor and the problem was minimized by the
addition of Lead (Pb) to the soldering process.
Even with a lot of studies and experiments done globally on tin whisker growth, no
definite conclusion to how or why tin whiskers form. Established method performed at
Cypress to relief the stress of Tin Plating was adding Annealing or Post Bake during
solder plate process with one (1) Hour, 150C condition. It is adequate to confirmed that
growth was significantly reduced but appropriate prevention and suitable solution of tin
whisker growth is still unresolved & unknown.
For health and environmental reasons, Lead (Pb) was identified as a hazardous
substance, and soldering process has migrated to lead-free. To maintain reliability and
quality of Cypress products, Tin Whisker Mitigation Program was implemented on all Pure
Tin(Sn) products. The program will be described on this document.
There are accelerated methods to test lead-free devices for tin whisker growth.
JEDEC and iNEMI established a set of accelerated stresses to grow whiskers and came
up with recommendations that pertains to tin whisker testing requirements along with
acceptance criteria for the evaluation of devices with tin finishes.
JESD201A
:
Environmental Acceptance Requirements for Tin
Susceptibility of Tin and Tin Alloy Surface Finishes
Whisker
JESD22-A121 : Test Method for Measuring Whisker Growth on Tin and Tin Alloy
Surface Finishes
These standards were generated to provide a uniform environmental acceptance
testing and reporting methodology for tin whisker susceptibility of tin and tin alloy
surface finishes used in the Electronics Industry.
Test conditions stated on these standards have not been correlated with longer
environmental exposures of components in service. Thus, there is at present no way to
quantitatively predict whisker lengths over long time periods based on the lengths
measured in the short-term tests described in this document. At the time of writing, the
fundamental mechanisms of tin whisker growth are not fully understood and acceleration
factors have not been established. Therefore, the testing described in this document does
not guarantee that whiskers will or will not grow under field life conditions.
Page 9
TIN WHISKER GROWTH TESTING
A complete report is provided upon completion of the program. Four(4) assembly
subcontractors (Site A/B/C&D) were selected and subjected for tin whisker growth
testing using JESD201 approach.
Test Frequency & Sample Preparation
Individual production samples were used for the tin whisker growth test. Samples
manufactured per assembly site were taken as being representative of a production
batch or monitor.
Table below describe sampling plan and frequency per site.
Assy Site
Site A
Frequency
Each base metal / plating chemistry /
lead frame type combination must be
tested 2x per year
Sample Size
Using optical microscope
Minimum 32 terminations per lot, 3 separately
plated lots
Site B
Per package per year
Detailed Inspection
minimum 6 terminations per lot, only if whiskers
are detected during screening inspection
5units/ package/condition
1st view : 50-1000x all leads optical inspection
Final view : 1000-5000x measure longer whisker
per package per condition
Site C
Per package per year
30units per package
Site D
1 lot per month
18 units per lot; 6 units per test condition
Page 10
Tin Whisker Stress Acceptance Criteria (JESD201)
JEDEC and iNEMI established a set of accelerated stresses to grow whiskers.
JEDEC, JESD201, has referenced iNEMI standard on the acceptance criteria and on the
stresses. Table 1 shows the sampling size per stress condition and Table 2 shows the
preconditioning treatment as recommended by the two groups:
Minimum Sampling and Inspection Requirements per Precondition
Treatment
Stress Type
Lots Sample
per
s per Components Inspected
Stress Lot
per readpoint
Note 1
Temperature Cycling 3
Temperature /
Humidity
Storage
3
High Temperature
/ Humidity Storage
3
Screening Inspection
Terminations per
Readpoint
Note 2
Detailed Inspection
Terminations per
Readpoint
Note 3
2
6
96
18
2
6
96
18
2
6
96
18
Note 1 Components should be drawn equally from the Manufacturing lots, as much as is practicable.
Note 2 Minimum number of terminations inspected during Screening Inspection per readpoint (See JESD22A121).
If whiskers are detected in the Screening Inspection, then the terminations with the longest whiskers shall be
measured in the Detailed Inspection. The longest whisker is measured and recorded for each of these
Note 3
terminations. If no whiskers are detected in the Screening Inspection, then no Detailed Inspection is required,
according to JESD22A121.
Table 1. Sampling size matrix & stress conditions
Page 11
Table 2. Precondition treatment matrix
Page 12
Acceptance criteria for whisker lengths per Class and performed
stresses
Whisker length measurements shall be made at each inspection interval and at the total
test duration listed Table 3. Reference.JESD22A121. Each of these measurements shall
be compared to the maximum allowable tin whisker length acceptance testing.
Any measurement of whisker length that exceeds the appropriate maximum allowable
whisker length is considered failure of the surface finish that is being tested.
The appropriate failure criterion or maximum allowable whisker length depends on the
product Class determined.
Table 3. Acceptance Criteria for Maximum Allowable Tin Whisker Length
Determination of the Class level for testing
The Class level indicates the test program (test duration and whisker length criteria) used for the
surface finish technology acceptance testing. Product Classes shall be agreed to between
supplier and user.
General guidelines for product Classes follow but may not apply in all cases:
Class 3 : Mission/Life critical applications such as military, aerospace and medical applications
Pure tin and high tin content alloys are not typically acceptable
Class 2 : Business critical applications such as Telecom Infrastructure equipment, High-end
Servers, Automotive, etc.
A whisker mitigation practice is expected, see section 5 of JP002 for examples,
unless otherwise agreed between supplier and user.
Long product lifetimes and minimal downtime
Products such as disc drives typically fall into this category
Breaking off of a tin whisker is a concern
Class 1 : Industrial / consumer products
Medium product lifetimes
No major concern with tin whiskers breaking off
Class 1A : Consumer products
Short product lifetimes
Minimal concern with tin whiskers
Page 13
Page 14
Tin Whisker Preconditioning Reflow Profile (JESD22-A121A)
Table 4. Optional Preconditioning Reflow Profile
Page 15
Tin Whisker Test Conditions (JESD201)
These test conditions represent a minimum set of conditions that shall be used to assess
the propensity for tin whisker growth on any given tin finish under study.
Table 5. Acceptance Tests and Durations
Page 16
TEST RESULTS (Page 1 of 3):
Packages assembled at Cypress Sub Assembly Sites using with MATTE / PURE TIN(Sn) Surface
MET TIN WHISKER Susceptibility under CLASS 2 requirements per JEDS201 & JESD22-A121A.
All results were summarized and presented on Table below. Detailed SEM images of examples from all tests are
also shown.
Tin Whisker Growth Measurement - No Preconditioning
Assembly Site
Site A
Package Profile
Pkg Type
28Ld TSSOP
LF Material
Cu Olin 7025
Leadfinish
Temperature Cyling Test
Inspection Results
(SEM)
Matte Tin(Sn)
℃/ + 85℃
-55
Sample size
500cycles
1000cycles
6 units
25.28 um
Site B
128Ld LQFP,
(14x14mm)
Cu Olin 7025
Matte Tin(Sn)
℃/ + 85℃
-55
Site C
Site D
64Ld TQFP
48L TQFP
Cu C7025
Cu C7025
Matte Tin(Sn)
℃/ + 85℃
-55
Pure Tin(Sn)
- 40C/ + 85C
18units
21.18um
18units
0.00 um
18.18 um
28.67 um
1500cycles
Temperature Humidity Storage (Ambient)
Inspection Results
(SEM)
30C/60%RH
30C/60%RH
Sample size
6 units
18units
500Hours
0.00 um
0.00 um
1000Hours
0.00 um
30C/60%RH
30C/60%RH
18units
0.00 um
0.00 um
1500Hours
2000Hours
0.00 um
3000Hours
0.00 um
0.00 um
4000Hours
Temperature Humidity Storage (No Bias)
Inspection Results
(SEM)
55C/ 85%RH
55C/ 85%RH
Sample size
6 units
18units
500 Hours
0.00 um
0.00 um
1000Hours
0.00 um
55C/ 85%RH
60C/ 85%RH
18units
0.00 um
0.00 um
1500Hours
2000Hours
0.00 um
3000Hours
0.00 um
5.66 um
4000Hours
Classification
Class 2
No data
Class 2
Class 2
Page 17
Class 2
Tin Whisker Growth Measurement
With Preconditioning(220C +/-5C)
Assembly Site
Pkg Type
Package Profile
Site D
28Ld TSSOP
128Ld LQFP, (14x14mm)
64Ld TQFP
28Ld SSOP
Cu Olin 7025
Cu Olin 7025
Cu Olin 7025
Cu C194
Matte Tin(Sn)
Matte Tin(Sn)
Matte Tin(Sn)
Pure Tin(Sn)
℃/ + 85℃
-55
500cycles
℃/ + 85℃
-55
℃/ + 85℃
-55
6 units
18units
18units
25.31 um
11.33 um
14.14 um
21.78 um
1500cycles
31.30 um
30C/60%RH
30C/60%RH
Sample size
6 units
18units
500Hours
0.00 um
0.00 um
1000Hours
0.00 um
30C/60%RH
18units
0.00 um
2000Hours
0.00 um
3000Hours
0.00 um
4000Hours
0.00 um
55C/ 85%RH
55C/ 85%RH
Sample size
6 units
18units
500 Hours
0.00 um
0.00 um
1000Hours
0.00 um
1500Hours
0.00 um
55C/ 85%RH
60C/ 85%RH
18units
0.00 um
0.00 um
2000Hours
0.00 um
3000Hours
0.00 um
0.00 um
4000Hours
Class 2
Class 2
No data
30C/60%RH
0.00 um
1500Hours
Classification
- 40C/ + 85C
1000cycles
Inspection Results
(SEM)
Site C
Leadfinish
Sample size
Inspection Results
(SEM)
Site B
LF Material
Inspection Results
(SEM)
Site A
Page 18
Class 2
Class 2
Tin Whisker Growth Measurement
With Preconditioning(260C +/-5C)
Assembly Site
Package Profile
Site B
Site C
Site D
Pkg Type
28Ld TSSOP
128Ld LQFP, (14x14mm)
64Ld TQFP
64Ld TQFP
LF Material
Cu Olin 7025
Cu Olin 7025
Cu Olin 7025
Cu Olin 7025
Leadfinish
Matte Tin(Sn)
Matte Tin(Sn)
Matte Tin(Sn)
Pure Tin(Sn)
-55
-55
Inspection Results
(SEM)
Site A
℃/ + 85℃
Sample size
6 units
18 terminations
500cycles
14.50 um
℃/ + 85℃
18units
℃/ + 85℃
-55
30units
11.48 um
0.00 um
0.00 um
2000cycles
30C/60%RH
30C/60%RH
30C/60%RH
30C/60%RH
Sample size
6 units
18 terminations
18units
30units
18units
500Hours
0.00 um
0.00 um
1000Hours
0.00 um
0.00 um
0.00 um
0.00 um
2000Hours
0.00 um
0.00 um
3000Hours
0.00 um
0.00 um
0.00 um
1500Hours
0.00 um
4000Hours
Inspection Results
(SEM)
55C/ 85%RH
55C/ 85%RH
55C/ 85%RH
60C/ 85%RH
Sample size
6 units
18 terminations
18units
30units
18units
500Hours
0.00 um
0.00 um
1000Hours
0.00 um
0 um
0.00 um
1500Hours
0.00 um
2000Hours
0 um
0.00 um
3000Hours
0 um
0.00 um
Class 2
Class 2
0 um
21.39 um
4000Hours
Classification
Class 2
Class 2
No data
19.53 um
33.41 um
1500 cycles
Inspection Results
(SEM)
18units
14.08 um
1000cycles
- 40C/ + 85C
Page 19
SEM PHOTOS:
SITE A
500cycles TCT (-55C/ +85C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
Tin whisker length: 25.28um
Tin whisker length:25.31um
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
No tin whisker growth
1500 Hrs THS (30C/ 60%RH)
Page 20
SITE A
1500 Hrs THS (55C/ 85%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
Inspection Criteria:
Item
After Reliability Testing
Axial length of longest whisker
for TCT standard
Reject, if >45um
for ambient & THT standard
Reject, if >45um
Remarks
Pass.
Whisker growth observed in all samples after 500cyc
TCT. Longest whiskers measures 25.31um.
Pass. No whisker growth noted
Page 21
SITE B
500cycles TCT(-55C/ +85%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
1500 Hrs THS (30C/ 60%RH) & (-55C/85%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
Item
Axial length of longest whisker for TCT standard
Axial length of longest whisker for ambient & THT standard
Reject, if >45um
Reject, if >20um
Remarks
Pass criteria. Max whisker measures 11.48um
Page 22
SITE C
TCT : 1000/2000CYCLES
TH (30C/60%RH) : 1000/2000/3000HRS
With Pb-Free(260C +/-5C) Precon
With Pb-Free(260C +/-5C) Precon
Item
for TCT standard
Reject, if >45um
Remarks
Reject, if >45um
Pass. No whisker growth noted.
Page 23
SITE D
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
Page 24
SITE D
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
1000 Hrs THS (30C/ 60%RH)
Page 25
SITE D
2000 Hrs THS (30C/ 60%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
3000 Hrs THS (30C/ 60%RH)
Page 26
SITE D
4000 Hrs THS (30C/ 60%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
1000 Hrs THS (60C/ 85%RH)
Page 27
SITE D
2000 Hrs THS (60C/ 85%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
3000 Hrs THS (60C/ 85%RH)
Page 28
SITE D
4000 Hrs THS (60C/ 85%RH)
NO Precon
SnPb (220C +/-5C)
Pb- Free (260C +/-5C)
Item
Reject, if >45um
for TCT standard
Reject, if >45um
Remarks
Pass.
Whisker growth observed in all samples after 1500cyc
TCT. Longest whiskers measures 33.41um.
Pass. Whisker growth measures 21.39um after
4000Hrs THS (60C/85%RH)
Page 29
Reference Documents / Articles:
JESD201A
Environmental Acceptance Requirements for Tin Whisker
Susceptibility of Tin and Tin Alloy Surface Finishes
JESD22-A121
:
Test Method for Measuring Whisker Growth on Tin and
Tin Alloy Surface Finishes
Report # 20110222
:
Site - A Tin Whisker Report for TSSOP 28L
Report # RT120626-RM04 :
Site - B Whisker Monitor Report for LQFP128LD
Report # RTWR-2012-001 :
Site - C Pb-Free Whisker Test Report for TQFP64LD
Report # 2011H1SN
:
Site - D Tin Whisker Growth Monitor Summary report for
Matte Tin Plating Process
“A History of Tin Whisker Theory: 1946 to 2004” by George T. Gaylon
“Tin Deposit Ripening and Whisker Growth” by Andre Egli
:
Page 30

Tin Whisker White Paper

Transcription

Similar documents

Tin-From ore to ingot

LARGE ESTATE AUCTION - northstarauction.com

Nescafé Espresso Instant Coffee Tin Features

Request for Taxpayer ldentification Number and

Strictly Fishing

Untitled - Tin Roof

Sha Tin College 沙田學院

Tin Oxides

Fine-scale whisker growth measurements can reveal

Contents - Sha Tin College 沙田學院

The Use of Gilded Tin in Giotto`s `Pentecost`