Effect of the Formation of CuO Flowers and SnO 2 on the Growth of Tin Whiskers on Immersion Tin Surface Finish

Transcription

1 WVCARMEA 2014 ID-119 Effect of the Formation of CuO Flowers and SnO 2 on the Growth of Tin Whiskers on Immersion Tin Surface WVCARMEA th SEPT 2014

2 Tin surface 2

3 Outlines 1. Introduction 2. Objective & scope 3. Definition of tin whiskers 4. Problem statement 5. Methodology 6. Results and discussions 7. Conclusions 8. References 3

4 Since 1960 s, Sn-Pb alloys were used for soldering and surface finishes in electronic industry. Move towards lead-free in electronic products, leaving pure Sn and Sn-based alloys to replace these Sn-Pb alloys. Tin surface finish has received greater attention in the industry due to its excellent solderability and availability. However, issues with tin whiskers on tin surface finish are also a reliability concern. Introduction The formation of intermetallics compound, IMCs between tin layer and substrate, namely Cu 6 Sn 5 & Cu 3 Sn which influenced by coating thickness, is the main factor driving the tin whisker formation. 4

5 Objectives & Scopes OBJECTIVE: to investigate and understand the effects of oxidation of copper and tin with externally applied stress under controlled environmental storage conditions of temperature and humidity on tin whisker formation and growth SCOPE: Phase 1: Surface finish deposition Deposition of tin layer on Cu substrate by using immersion process. Phase 2: Tin whisker testing and characterization Indentation on the surface using 2N load. Storage under temperature/ relative humidity of 30ºC/60%RH condition for duration up to 3 months. Optical and scanning electron microscopy are used to characterize the tin whisker growth. 5

6 Tin Whiskers Definition; A spontaneous columnar or cylindrical filament usually of monocrystalline metal, emanating from the surface of a finish. (JEDEC) Straight Kinked Tin whiskers on Cu leadframe 6

7 Whiskers failure Tin-plated copper lead frame after 3 years of ambient storage Tin whiskers on the edge of the acceleration position sensor board connection terminal of the 2002 Toyota Camry. 7

8 Mechanism of Whiskers Growth GRAIN BOUNDARY DIFFUSION & STRESS Cu atoms diffuse from the substrate to surface finish to form intermetallic compound (Cu 6 Sn 5 ). Cu 6 Sn 5 will grow over time and induce compressive stresses on Sn deposit. In order to relieve the stress, Sn atoms diffuses away to grow a spontaneous Sn whisker which is stress-free. Schematic diagram of Sn whiskers mechanism. 8

9 Problem Statement Since tin whiskers tend to form randomly, therefore the effect of oxidation process should be investigated on the tin surface finish. 9

10 Methodology Substrate preparation (pretreatment Processes) Immersion tin plating Indentation (externally applied stress) Exposure test under 30ºC/60%RH condition Characterisation (whiskers study SEM) 10

11 Methodology Immersion Tin Plating Heating coil Al hanger Chemical Stanneous chloride Hydrochloric acid (37%) Sulfuric acid (50%) Composition 20 g/l 37 ml/l 50 ml/l Sodium Hypophosphite 16 g/l Al hook Thiourea Phenolsufonic acid 200 g/l 5 ml/l Water Tin solution Cu substrate Temperature 75 o C 11

12 Thickness of Tin Coating Average thickness of tin coating for three different plating times. Plating time Thickness (µm) 8 min min min 2.3 Tin layer Cu substrate Tin layer deposits on copper based substrate sample 12

13 Indentation Process Externally applied stress by micro hardness indentation machine Load : 2N HMV SHIMADZU micro hardness tester 13

14 Results & Discussions Whiskers length is directly proportional to the storage time for nonindented surfaces. For the indented samples, the whiskers length slightly increased after 1 week to 4 weeks and then keeps constant up to 12 weeks 14

15 Results & Discussions Types of tin whiskers formed Straight Bent Twist Striations 15

16 Results & Discussions Samples: Indented Tin plating thickness: 1.2 µm Storage time:12 weeks kinked-type whisker 16

17 Results & Discussions The relationship between the indentation and kinked-type whisker (a) Cu substrate (b) Cu substrate 2µm 2µm (c) (d) CuO CuO 2µm 2µm 17

18 Results & Discussions Chemical Composition Oxidation of Cu substrates CuO Flowers (a) CuO CuO Elements Wt% Cu 79.5 O μm 52 weeks 18

19 4 weeks Tin Whiskers Results & Discussions Oxidation of Tin Whiskers Pure Sn Elements Wt% Sn 95.9 Cu μm 52 weeks SnO 2 SnO 2 Elements Wt% Sn 68.9 O μm 19

20 Conclusions In conclusion, the whiskers have formed and grown on the immersion tin surface finish. The externally applied stress influenced the shape of whiskers form on the immersion tin surface. The whiskers formed in the straight, twist, bent, and striation shapes have grown continuously over the exposure time and they were found on the non-indented surfaces. For the indented surface, kinked-type whiskers were found and the whiskers growth were discontinued due to the distruction at the end-tip of kinked-type tin whisker, the formation and growth of CuO flowers, and the oxidation of non-kinked-type of tin whiskers to form SnO 2. 20

21 References 1. K. J., Puttlitz and G. T., Galyon, Impact of the ROHS Directive on high-performance electronic systems. J Mater Sci: Mater Electron, 18,pp. (2007) A., Vicenzo, Tin Whiskers. In: C. Z.G. GROSSMANN, (ed.) The ELFNET Book on Failure Mechanisms, Testing Methods, and Quality Issues of Lead-Free Solder Interconnects. Springer-Verlag London, A., Baated, K. S., Kim, and K., Suganuma, Effect of Intermetallic Growth Rate on Spontaneous Whisker Growth from a Tin Coating on Copper. J Mater Sci: Mater Electron, 22, (2011) S. M., Miller, U.,Sahaym, and M. G., Norton, Effect of Substrate Composition on Sn Whisker Growth in Pure Sn Films. Metallurgical and Materials Transactions A, 41 A (2010) A., Dimitrovska and R., Kovacevic, Mitigation of Sn Whisker Growth by Composite Ni/Sn Plating. Journal of Electronic Materials,vol. 38 (2009) B., Illes, B., Horvath, and G., Harsanyi, Effect Of Strongly Oxidizing Environment on Whisker Growth form Tin Coating. Suface & Coating Technology, 205 (2010) B., Horvath, B., Illes, T., Shinohara, and G., Harsanyi, Copper-oxide Whisker Growth on Tin Copper Alloy Coatings Caused by the corrosion of Cu 6 Sn 5 Intermetallics. J Mater Sci: Mater Electron, 48, (2013) J. L., Jo, K. et.al, Least Lead Addition To Mitigate Tin Whisker For Ambient Storage. J Mater Sci: Mater Electron, 24 (2013) Y.H., Chen, Y. Y., Wang, and C. C., Wan, Microstructural Characteristics of Immersion tin Coatings on Copper Circuitries in Circuit Boards. Surfac and Coatings Technology, 202 (2007)

22 References 1. R., Gedney, J., Smetana, N., Vo, and G., Galyon, NEMI Tin Whisker Projects M.A., Dudek, and N., Chawla, Mechanisms for Sn Whisker Growth in Rare Earth- Containing Pb-free Solders. Acta Materialia, 57 (2009) S.M., Miller, U., Sahaym, and M. G., Norton, Effect of Substrate Composition on Sn Whisker Growth in Pure Sn Films. Metallurgical and Materials Transactions A, 41A (2010) D., Susan, J., et.al, Morphology and Growth Kinetics of Straight and Kinked Tin Whiskers. Metallurgical and Materials Transactions A,vol. 44A (2013) J., Liang, Z. H., Xu, and X., Li, Whisker Nucleation in Indentation Residual Stress Field on Tin Plated Component Leads. J Mater Sci: Mater Electron, 18 (2007) J., Cheng, et.al., Optimum Thickness of Sn Film for Whisker Growth. Journal of Electronic Materials, 40 (2011) N.A., Fadil, S.Z., Yusof, T.A., Abu Bakar, A. Ourdjini, Effect of Externally Applied Stress on Tin Whiskers Formation and Growth for Immersion Tin Surface Finish. Proceeding of 9 th International Materials Technology Conference & Exhibition (IMTCE2014), Kuala Lumpur, Malaysia ((13-14 May 2014). 8. S.Z., Yusof, N.A., Fadil, T.A., Abu Bakar, A. Ourdjini, Effect of Thin Coating Thickness on Tin Whiskers Formation and Growth. Proceeding of 8 th SEATUC Symposium, Johor Bahru, Malaysia (4-5 March 2014). 22

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