Examensarbete Study of Tin Whisker Growth and their Mechanical and Electrical Properties Moheb Nayeri Hashemzadeh LITH - IFM - EX - - 05 / 1499 - - SE Study of Tin Whisker Growth and their Mechanical and Electrical Properties IFM, Link¨opingsUniversitet Moheb Nayeri Hashemzadeh LITH - IFM - EX - - 05 / 1499 - - SE Examensarbete: 20 p Level: D Supervisor: Dr. Werner H¨ugel, Dr. Verena Kirchner Robert Bosch GmbH-AE/QMM-S5 Examiner: Prof. Ulf Helmersson, IFM, Link¨opingsUniversitet Link¨oping: September 2005 Avdelning, Institution Datum Division, Department Date Department of Physics and Measurement Technology, IFM Link¨oping University September 2005 SE-581 83 Link¨oping Spr˚ak Rapporttyp Language Report category ISRN NUMMER: Svenska/Swedish Licentiatavhandling x Engelska/English x Examensarbete LITH - IFM - EX - - 05 / 1499 - - SE C-uppsats D-uppsats Ovrig¨ rapport URL f¨orelektronisk version Titel Study of Tin Whisker Growth and their Mechanical and Electrical Title Properties F¨orfattare Moheb Nayeri Hashemzadeh Author Sammanfattning Abstract The phenomenon of spontaneous growth of metallic filaments, known as whisker growth has been studied. Until now the problem that Sn whisker growth could cause in electronics by making shorts has been partially prohib- ited as Pb and Sn have been used together in solders and coating. Regulations restricting Pb use in electronics has made the need to understand Sn whisker growth more current. It is shown that whiskers are highly resilient towards vibrations and shocks. A Sn whisker is shown to withstand 55 mA. Results show that reflowing of the Sn plated surface does not prevent exten- sive whisker growth. Results show that intermetallic compound growth can not be the sole reason behind whisker growth. Nickel and silver underlayer have been shown not to prevent whisker growth, but perhaps restrain whisker growth. Heat treatment damped whisker growth considerably. It is judged that base material CuSn6 is less prone to show whisker growth than CuSn0.15 and E-Cu58. Nyckelord Tin whisker Keyword Abstract Purpose The phenomenon of spontaneous growth of metallic filaments, known as whiskers, has been studied and compared with existing theories. Until now the problem that tin whisker growth could cause in electronics by making shorts has been partially prohibited as lead and tin have been used together in solders and coating. Due to a more environment aware society, where use of lead in solder and coating will be restricted, the need of understanding the phenomenon of tin whisker growth has become more current. A study of the mechanical and electrical durability of tin whiskers has also been carried out. Procedure Tin whiskers current carrying capacities was tested and based on the experimental results finite element method calculations were made in the Ansys program. Tin whiskers endurance against vibrations and shocks have been calculated and tested. Whisker growth on different metallic systems, undergoing different conditions have been observed. The metallic systems have been three different sorts of base materi- als (all consist mainly of copper) with a layer of dull or bright tin on it and in some cases nickel or silver underlayer between the base material and the tin layer. The test conditions the samples underwent with different combinations were: soldering simulation, temperature cycling between −60 oC to 60 oC, humidity storage, high temperature storage and room temperature storage. Some samples were also heat treated post plating, for 5 minutes at 180 oC. Some of the samples were studied with help of Focused Ion Beam systems. Nayeri Hashemzadeh, 2005. 6 7 Results and Conclusions Both calculations and tests showed that whiskers are highly resilient towards vi- brations common in vehicles. Tests showed also that whiskers can withstand very high shocks, not breaking when subjected to shocks of 1000 m/s2 during 6 ms. Tests have shown whiskers withstanding currents up to 55 mA. The longer the whiskers the lower current carrying ability, while thicker whiskers can carry higher currents. The whisker that could withstand 55 mA was 120 µm long and had a diameter of 5.5 µm. Due to the shifting quality of the plating of the samples clear conclusions regarding storage conditions that are favorable for whisker growth can not be made. This points to the importance of process stability when plating, as to get homogenous plating quality. Results show that soldering simulation, which is melting the tin plated surface does not prevent extensive whisker growth. Results show that intermetallic com- pound growth can not be the sole reason for whisker growth. Nickel and silver underlayer have been shown not to prevent whisker growth, but perhaps restrain it. Heat treatment damped whisker growth considerably but did not prevent it. It is judged that base material CuSn6 is less prone to show whisker growth than CuSn0.15 and E-Cu58. Keywords: Tin whisker Acknowledgements I would like to thank Prof. Ulf Helmersson for his help and support, making it possible for me to do my master thesis in Germany at Robert Bosch GmbH, Reut- lingen. I would like to thank both my supervisors at Robert Bosch GmbH, Dr. Werner H¨ugeland Dr. Verena Kirchner, for the help and the support I have received with my work. Dr. Werner H¨ugel had a great part in helping me in my work, from having ordered the samples that were studied during this work, to booking test sessions as well as helping with the theoretical part of the work. Dr. Verena Kirch- ner was very helpful in helping with practical issues and giving advise in theoretical matters as well as helping me write this report. I would also like to thank all the other people at Robert Bosch GmbH that were very forthcoming in helping me, specially following names come to mind, even though there were many more: Joachim Gugeler for the tests concerning mechanical durability. Thomas von Bargen for the FEM simulations. Markus Schill for the FIB sessions. S¨ukr¨uTavasligolu for saving my computer several times. Markus Guber for providing time to discuss his results and views regarding whisker growth. Fabian Bez for helping me looking through some of the samples Pravin Sinha for helping me in searching for articles. David Nerz for his help with some of the samples. Nathalie Becker for her help in practical and scientific matters. Parviz Kamvar for his advices in scientific matters. Moreover I would like to thank my brother Mohit Nayeri, for being supportive both on a scientific sense as well as on a personal level. I would like to dedicate this work to my mother, Mary Berari. Nayeri Hashemzadeh, 2005. 8 Frequently Used Abbreviations and Symbols EDX Energy Dispersive X-ray spectroscope FEM Finite Element Method FIB Focused Ion Beam IMC Intermetallic Compound LM Light Microscope PSD Power Spectral Density RoHS Restriction of Hazardous Substances Directives SEM Scanning Electron Microscope 0 η -phase Cu6Sn5 Icrit The critical current at which the whiskers fuse Tm The melting point temperature Ag Silver Cu Copper Ni Nickel Pb Lead Sn Tin Zn Zink Nayeri Hashemzadeh, 2005. 9 Contents 1 Introduction 12 1.1 Purpose . 14 1.2 Outline . 14 2 Theoretical Background 16 2.1 Fundamentals . 16 2.1.1 General Properties of Whiskers . 16 2.1.2 Cu-Sn Properties . 19 2.2 Tin Whisker Growth Models . 19 2.2.1 Surface Energy Effects . 20 2.2.2 Stored Strain Energy . 22 2.2.3 Internal Mechanical Stresses . 25 2.3 Mitigation Strategies . 29 3 Procedure 31 3.1 Instruments Used . 31 3.1.1 Light Microscope . 31 3.1.2 Scanning Electron Microscope . 31 3.2 Measuring Mechanical Durability . 33 3.2.1 Stereo Microscope . 33 3.2.2 Vibration and Shock Table . 33 3.2.3 Vibration and Shock Tests . 33 3.3 Measuring Electrical Durability . 35 3.3.1 Microprobe . 35 3.3.2 Electrical Conductivity Tests . 36 3.3.3 Finite Element Method . 37 3.4 Studying Whisker Growth . 38 3.4.1 Focused Ion Beam . 38 3.4.2 Sample Specifics . 39 3.4.3 Sample Study . 40 Nayeri Hashemzadeh, 2005. 10 Contents 11 4 Results 42 4.1 Mechanical Durability . 42 4.1.1 Natural Frequency of Whiskers . 42 4.1.2 Results from Vibration Tests . 43 4.1.3 Results from Shock Tests . 44 4.2 Electrical Durability . 45 4.2.1 Electrical Conductivity of Whiskers . 45 4.2.2 FEM Calculations . 46 4.3 Whisker Growth Study . 47 4.3.1 FIB Results . 48 5 Discussion of the Results 56 5.1 Mechanical Properties . 56 5.2 Electrical Properties . 56 5.3 Tin Whisker Growth . 57 5.3.1 Heat Treatment . 57 5.3.2 Soldering Simulation . 57 5.3.3 Storage Conditions . 58 5.3.4 Base Material . 58 5.3.5 Underlayer . 59 5.3.6 Dull versus Bright Sn . 60 5.3.7 Tin Layer Thickness . 60 5.3.8 Conclusions on Whisker Growth Models . 61 6 Future Work 62 A Methods 67 B Results 70 B.1 Natural Frequency Calculations . 70 B.1.1 Connected at Both Ends . 73 Chapter 1 Introduction Whiskers are metal filaments that have been shown to grow spontaneously on met- als such as cadmium (Cd), zinc (Zn), aluminum (Al), silver (Ag), molybdenum (Mo), tungsten (W) and tin (Sn) [1, 2]. In everyday language the word whisker usually refers to cat bristle, which is quite describing for most whiskers, which grow straight and stiff, as can be seen in figure 1.1 a), however kinked whiskers, as e.g. shown in figure 1.1 b), are quite usual as well. Figure 1.1: a) Most commonly whiskers grow quite straight out of the surface. This picture was taken with scanning electron microscopy. b) A kinked whisker in a M-form. This picture was taken by a focused ion beam microscopy.