Tin Whisker Prevention with SnBi Plating
Issei Fujimura, Manager of R&D Section No. 2,Ishihara Chemical Co., Ltd.
5-26 Nishiyanagiwara-cho, Hyogo-ku, Kobe, Hyogo 652-0806, Japan
E. Jan Vardaman and Dr. Timothy G. Lenihan
TechSearch International, Inc.
4801 Spicewood Springs Road Suite 150
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jan@techsearchinc.com

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1Overview•Abstract•

Introduction•

Whisker Growth•Experimental Procedures and Results•Generation of Voids and Cracks after Annealing•Conclusions

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Abstract
Tin whiskers are “needle-like” crystalline structures of tin that form and grow on surfaces that use
pure or nearly pure tin (Sn) as a final finish. This reliability issue has been a significant concern tothe electronics industry with the introduction of Pb-free solders with higher Sn content. The failure
mechanism is caused by the tin whisker growing and shorting leads due to the conductivity of theSn. This problem was first documented more than 50 years ago. Today, many believe the tin
whisker problem can be mitigated by reducing the stress in the plated films during plating or bybaking parts for 1 hour at 150oC. All of the causes for tin whiskers have yet to be determinedbased on conflicting data that has been published in the literature. Vigilance is needed to insurethat an “old” problem doesn’t become a “new” problem with the introduction of new Sn materials.
Groups such as iNEMI, CALCE, and a number of electronics manufacturers continue research inthis area to understand the causes for tin whiskers and to find new ways mitigate the problem with
the introduction of new finishes for materials such as lead-free solders that contain higher
concentrations of Sn.
This presentation examines contributors to tin whisker growth such as internal and external
stress. The effect of plating treatments and the relationship between thickness and the observed
whisker length are examined. The plating samples were manufactured on copper and Alloy42
surfaces. The resulting differences in void size, after annealing, using Tin (Sn) plating and TinBismuth (SnBi) plating will be discussed using plating cross-sections. Additional work will bereported on Sn and SnBi plating, with and without annealing, after bend testing.
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Introduction
Ishihara Chemical. Ltd.
Founding Date - March 3, 1939
Capital - 1,447 million yen
Executives
President : Kanji Takemori
Senior Managing Director : Motoichi Tokizawa
Number of Employees -199
Head Office/Factory
5-26 Nishiyanagiwara-cho, Hyogo-ku,
Kobe, Hyogo 652-0806, Japan
TEL: +81-78-681-4801
FAX: +81-78-651-6784
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7/22/0942Introduction• The introduction of Pb-free solder requirements in the electronics industry isincreasing the use of Sn coatings which can result in the potential for whisker
growth.
• There have been are few reported incidents of tin whisker reliability failures
outside of the military.
• There may be more unreported incidents as companies are reluctant to advertisethey have a reliability problem.
• Numerous studies have been conducted to show the electrical short circuit
characteristics for tin whiskers. [1]• Internal stress is accepted as one of the major mechanisms for tin whisker growth.
• The January 2005, October 2006, and January 2007 issues of the IEEE
Transactions on Electronics Packaging and Manufacturing provide an excellent setof reference material on tin whiskers.
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5Whisker Growth• Tin whisker growth on leadframes in IC packages in generally agreed to be dueinternal stress in the plated film.
• The whisker growth periods reported for internal stress show a large variation fromseveral hundred hours to several thousand hours.
• This variation is known to be dependent upon environmental conditions such astemperature and humidity.
• External stress whisker growth is another mechanism (versus internal film stress)and can be found on connectors and flex circuits. This mechanism has beenreported to have growth periods as low as tens of hours.
• Growth causes for external stress induced whiskers have been reported to be due
to mechanical pressure such as contact pressure or initiated from scratches.
• The tendency for whisker growth has also been previously reported for Sn-plated
copper and Sn-plated Alloy42 leadframes.
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Whisker Growth• One key question to be answered “Is the IMC (Intermetalic compound) growth thecause of whisker growth on copper substrates under ambient conditions?”• Observations on leadframes under ambient conditions of up to 12,240 hours show
there is very little IMC growth and subsequently no whisker growth.

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7Experimental Procedures and Results• Copper(194) and Alloy42 leadframes were plated with Sn2BiSn3.5Ag and Sn1.5Cu(with and without annealing). Whisker observations were conducted for each platingtype under ambient conditions past 4,000 hours; damp testing parts past 4,000 hours;
and thermal cycling testing parts past 1,500 cycles.

Test

Test condition

IntervalTerms

AmbientTest

30±2℃, 60 ±3%RH

1000h

4000h

500cyc.

1500cyc.

1000h

4000h

ThermalCycling Test
Damp Test- 40+0/-10℃-55+0/-10℃⇔85+10/0℃⇔ 85+10/-0℃
Soaking
Soaking10min,10min,3cyc./h3cyc./h_55±3℃, 85±3%RH

Figure 1: Stress matrix used for plating and annealing experiments.
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Experimental Procedures and Results• Tin-plated Alloy42 ambient and damp test results were found to be acceptable.
• Thermal cycling testing was conducted from -40oC to 85oC. After 500 cycles, a 20
micron whisker growth was observed; after 1,000 cycles, a 51 micron whisker growth
was observed; and after 1,500 cycles, a 65 micron whisker growth was observed.
• One cause for tin whisker growth, during thermal cycling, is the Cu diffusion rate isgreater than the Sn diffusion rate. The CTE of Sn is 23.5 (x 10-6/oK) versus the CTEof 42 Alloy which is 4.6 (x 10-6/oK). This can contribute to increased internal stress inthe plated films.

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9Experimental Procedures and Results>

Copper diffusion rate
Tin diffusion rate to• Cu下地でウィスカ発生
totin
copper Snめっき皮膜– Cu下地が影響→金属間化合物の成長が原因

Tin plating layer

IMC(Cu6Sn5)
Cu substrate
Figure 2: Copper/Tin Diffusion Stress
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Experimental Procedures and Results
Damp test
4000h

Ambient
4000h

Thermal cycling
1500cyc.

Cu（194）42alloy

30µm

Figure 3: Whisker Observations with Sn-3.5Ag plating.
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11Experimental Procedures and Results

500cyc.

Thermalcycling test(-40/85 ℃)

1000cyc.

1500cyc.

20µm
51µm

65µm

50µm

Figure 4: Whisker Observations with Sn plating.
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Experimental Procedures and Results
Ambient
4000h

Damp test
4000h

Thermal Cycling 1500cyc.

Cu
（194）

42alloy

50µm

Figure 5: SnBi plated surfaces post stress and anneal.
• The SnBi plated parts showed little whisker growth as compared to the Sn plating.
By annealing samples for the standard one hour at 150oC, tin whisker growth
prevention was demonstrated.
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Experimental Procedures and Results
Blank

Ni undercoat

Ag undercoat

100µm

100µm

100µm

100µm

Ambient
1 year
50µm

Ambient
2 years
100µmNote; Thickness of tin plating is 1mircon.

Figure 7: Affect of pretreatment with undercoating.
• Pretreatment with a 0.1 micron undercoat of Ni or Ag was also found to be asolution to prevent whisker growth. The effect undercoats of 0.2 micron of Cu, Ni,and Ag, respectively.
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Experimental Procedures and Results• One solution to reduce the potential for tin whiskers has been the adoption of SnBi
plating by many Japanese companies (NEC, Panasonic, Fujitsu, Hitachi, Sanyo, Oki,and Sony).
• This has become a popular choice in Japan where IC manufacturing tends to bevertically integrated.
• Outside of Japan, there is a perception that when Bi used with Pb-containing solders,
there is a risk of brittle intermetalic formations. This is a concern for companies thatare not vertically integrated and use many sources for process materials in theirassembly facilities.
• Researchers from the University of Toronto reported there were no issues with SnBifinishes (with a Bi content up to 6 percent) in PbSn finishes. They also reported that theSnBi lead surface finish provided the highest pull strength for both Pb-free and SnPb
solder joints. They believed this affect was due to stabilization in the SnPb joint
microstructure (during thermal cycling) which reduced grain growth and prevented
crack formation. [2], [3], [4]ISHIHARA CHEMICALInterPACK '09

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Key code here13Experimental Procedures and Results

Max Whisker Length

105 um810 um

615 um

420 um20050

100150Cycling Times
Figure 6: Effect of Plating Thickness on Whisker Growth.
• The effect of plating thickness in the - 45oC to 85oC thermal cycling data is shown inabove. As thicknesses increased the propensity for tin whisker growth was reduced.
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Experimental Procedures and Results• Because alloying Sn with a second metal reduces the propensity for whisker growth,some companies have investigated the use a number of coatings to mitigate thisproblem.
• Researchers from Hitachi’s Materials Research Lab and Renesas Technology
investigated the effect of adding minor elements in copper leadframes on whisker
initiation such as electrodeposited SnCu coatings on two different copper leadframe
materials. CuFe and CuCr were investigated by FE-STEM, FE-TEM, EDX analysis,and X-ray diffraction.
• Drastically different results in whisker initiation tendencies were reported for thesame SnCu coatings after long-term storage at room temperature. They also
reported tin whisker growth was completely absent on the SnCu coating for CuCrleadframes after a storage time of 52 months. They did observe 200 µm longwhiskers forming on the SnCu coated CuFe leadframes after a storage period of 34months. [5]ISHIHARA CHEMICALInterPACK '09

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Experimental Procedures and Results
Blank

Ni undercoat

Ag undercoat

100µm

100µm

100µm

100µm

Ambient
1 year
50µm

Ambient
2 years
100µm

Figure 7: Affect of pretreatment with undercoating.
• Pretreatment with a 0.1 micron undercoat of Ni or Ag was also found to be asolution to prevent whisker growth. The effect undercoats of 0.2 micron of Cu, Ni,and Ag, respectively.
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Experimental Procedures and Results

Maximum whisker（μm）

100Blank

80

Ag0.260

Ni0.2

40

Cu0.2

2000500

1000
Cycling times

1500

Figure 8: Affect of pretreatment with undercoating (- 45〜85℃).
Note; Thickness of tin plating is 10mircons.
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Experimental Procedures and Results
Apparatus for Acrylic Pressure Test
Operational Procedure :The sample is placed within the acrylic sheets and pressure is applied bytightening the nuts.

Figure 9: Acrylic Pressure Apparatus
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Experimental Procedures and Results

Sn-2BiSn-3.5Ag

Sn-1.5Cu

50µm

Figure 10: Acrylic pressure test （1.2Nm，280h）results.

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20Experimental Procedures and Results
Additional testing was done using an Acrylic Pressure testing technique toinduce tin whisker growth.
138

Sn-Cu
95

Sn
65

Sn-Ag
24

Sn-Bi
10Sn-Pb
0Reflow
Sn050

100

150200Figure 11: Max Tin Whisker Length (µm)
The best results were achieved with Reflow Sn and PbSn. The SnBi results werealso very good compared to SnCu, Sn, and SnAg.
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Generation of Voids and Cracks after Annealing• Research was also conducted to examine void formation in the Sn and SnBi platedleads.
• In this experiment, sample leadframes were plated with Sn and with SnBi. Bothsets of samples received a post treatment of annealing at 150oC for one hour. After
bend stressing, cross sections of the samples from both groups were polished andSEM photos were taken.
• Samples from both plated groups with the annealing treatment showed non-uniform
intermetalic compound (IMC) formation in the bent area of the plated leadframes.
• There were voids and cracks observed in the bent area which were most likely
generated by the bending mechanical stress. [6]• All of the samples showed no voiding or cracking with anneal and without anneal
when there was no mechanical stress induced on the Cu or Alloy42 leadframes.

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Generation of Voids and Cracks after Annealing• Sn and SnBi samples that received mechanical bending stress showed voiding at
the diffusion boundary and cracking.

Figure 12: Void Formation Post Anneal.
• It is believed the Cu3Sn or Cu6Sn5 boundary layer cracked during the bending
process due to high stress.
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Generation of Voids and Cracks after Annealing• With the addition of the standard 150oC, 1 hour, anneal for whisker prevention, allof the samples showed an increase in the void and crack size.

200µm

50µm

Figure 13: Whisker growth from lead frame due to mechanical stress.
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Generation of Voids and Cracks after Annealing• While the SnBi finish also showed the same cracking and voiding phenomenon,
the annealing process is not necessary as it is less susceptible to tin whisker
growth.

Observation: Bent part.

Figure 14: Post bend stress leadframes.

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Conclusions• The number of “reported” tin whisker failures remains low, vigilance is required as
new Sn materials are introduced to assembly processes along with new use
conditions for these materials.
• This presentation highlights that it is possible to have a tin whisker problem under
the right conditions.
• Efforts need to continue to reduce the risk of tin whiskers such as the adoption ofSnBi plating.
• The crack and void problem observed, in areas of high mechanical stress, can be
avoided by using SnBi to prevent tin whiskers and eliminating the standard
150oC anneal process step. Based on the data presented, the risk cannot be
avoided with annealing and mounting strength could be a major concern.
• Several tin whisker mitigation techniques are possible with copper substrates.
• No effective technology has been found yet for tin whisker prevention for Alloy42
leadframes under thermal cycling conditions.
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Thank you for your kind attention!

Questions?

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References[1] K. J. Coury, et al., “Tin Whisker Electrical Short Circuit Characteristics—Part I,” IEEETransactions on Electronics Packaging Manufacturing, Vol. 31, No. 1, January 2008, pp. 32-40.
[2] P. Snugovsky, J. McMahon, and M Romasky, L. Snugovsky, D. Perovic, and J. Rutter,“Microstructure Properties of Sn-Pb Solder Joints with Sn-Bi Finished Components,” IPC
Printed Circuits Expo, APEX, and the Designers Summit, 2006, pp. S28-02-1.
[3] I. Fujimura, “Lead free plating technology for tin whisker mitigation,” Technical Conferenceof the 37th InterNepcon Japan, January 2008.
[4] K. Tsuji, Journal of the Surface Finishing Society of Japan, vol.57;No.6, p. 451(2006)“Study on the Mechanism of Sn Whisker Growth: Part I.
[5] T. Kato, et al., “Effects of Minor Elements in Cu Leadframe on Whisker Initiation From
Electrodeposited Sn/Cu Coating, IEEE Transactions on Electronics Packaging Manufacturing,
CPMT, October 2007, Vol. 30, No.4., pp. 258-269.
[6] E.J. Vardaman, T.G. Lenihan, and I Fujimura, “Generation of Voids in Pb-Free Plating afterAnnealing,” IMAPS International Conference and Exhibition on Device Packaging, March 9-12,
2009.
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