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1 National Physical Laboratory Webinar 6th March 2013 The Attraction of Sn Whiskers Martin Wickham, National Physical Laboratory, Teddington UK Your Delegate Webinar Control Panel Open and close your panel Full screen view Raise hand for Q&A at the end Submit text questions during or at the end NPL Management Ltd - Commercial 2 1

2 NPL EI Free Technology Webinars BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off Height Tuesday 30 April Reuse of Electronic Products Wednesday 15 May Through Hole Reliability for High Aspect via Holes Tuesday 11 June Functionalisiation and Applications of Nanomaterials for Electronic Applications Monday 18 July Environmental Robustness: Performance Coatings & High Temperature Interconnects Tuesday 3 September National Physical Laboratory Webinar 6th March 2013 The Attraction of Sn Whiskers Martin Wickham, National Physical Laboratory, Teddington UK martin.wickham@npl.co.uk 2

3 The Attraction of Sn Whiskers - Summary Sn whiskers and NPL Whisker resistance and current carrying capacity Electrostatic attraction of whiskers Contact pressure and voltage breakdown Component whisker project 5 National Physical Laboratory Webinar 6th March 2013 Sn Whiskers and NPL The Attraction of Sn Whiskers martin.wickham@npl.co.uk 3

4 About NPL The UK s national standards laboratory 36,000 m 2 national laboratory Founded in 1900 World leading National Measurement Institute 600+ specialists in Measurement Science State-of-the-art standards facilities The heart of the UK s National Measurement System to support business and society Experts in Know ledge Transfer W orld leading measurement science building 7 Our Technical Breadth Electronics Interconnection Group Multidisciplinary R&D and technical services for public and private sector Acoustics Advanced Materials Air Quality Biometrics Biotechnology Corrosion Dimensional metrology Environmental measurement Lasers Mass and force Micro/Nanometrology Neutron measurements Photonics Photometry/Colour Pressure RF/Microwaves Radiation Dosimetry Radioactivity Radiometry Scientific Software Sensory metrology Statistics Surface analysis Thermal measurements Time & Frequency Electrical Standards 8 4

5 Electronics Interconnection Group Research Interconnect Reliability Plastic electronics & WEEE SIR testing MWCNT Interconnect Tin Whi skers CAF Printing dispensing Instrument benchmarking X-ray inspection XRF & RoHS Prognostics Tin Pest PCB Delamination Conformal Coatings PCB Reliability Tin Whiskers Examples Since 1998, 3 satellites have been completely lost due to Sn whi sker Failure of relays used on a military airplane used by the air forces of several countries Source: Boeing Source: Defence Update Source: US DoD Patriot missiles, phoenix missiles, military radar, heart pacemakers, medical monitors, power station reactor shutdown, watch oscillators Source: Source: Swatch Source: health.yahoo.com 10 Source: Northrop Grumman Electronic Systems Source: wikipedia.org 5

6 Current whisker work at NPL Sn whisker mitigation using conformal coatings Databank of Sn whiskers in commercial components Electrostatic attraction of whiskers Whisker contact resistance measurement Whisker oxide thickness characterisation Current carrying capacity of whiskers Whisker oxide breakdown Electrical resistance of whiskers Whisker growth under electric fields Whisker mitigation by coatings using an SOIC component test vehicle eliability-webinarseries-download-videos 11 Current whisker work at NPL Sn whisker mitigation using conformal coatings Databank of Sn whiskers in commercial components Electrostatic attraction of whiskers Whisker contact resistance measurement Whisker oxide thickness characterisation Current carrying capacity of whiskers Whisker oxide breakdown Electrical resistance of whiskers Whisker growth under electric fields Whisker mitigation by coatings using an SOIC component test vehicle 12 6

7 National Physical Laboratory Webinar 6th March 2013 Sn Whisker Databank The Attraction of Sn Whiskers Tin Whisker Databank Commercial components acquired on the open market or from partners stores Total 78 different components 63 SOIC/SSOP Optocouplers, tantalums, TQFP, DIP, SOT23/ different manufacturers 14 countries of origin Visually inspect for Sn whisker every 12 months Partners Aero Engine Controls MBDA (UK) Ltd BAE Systems Thales Missile Systems HMGCC 14 7

8 Databank Examples Whiskers on 5 terminations 10 components total Databank Examples Whiskers on 8 terminations 10 components total

9 Data after 12 months study No whisker greater than 50% of lead gap spacing Country of Manufacture 68 Component types 9 5 No whiskers Whiskers 10 No whiskers Whiskers Manufacturer 20 7 No whiskers Whiskers 17 National Physical Laboratory Webinar 6th March 2013 Sn Whisker Resistance The Attraction of Sn Whiskers martin.wickham@npl.co.uk 9

10 Whiskering chemistry Schloetter has provided special chemistry utilised in previous studio project Used by NPL in benchtop electroplating rig By plating very thin coatings (<2 m) rapidly Whiskers up to 1mm in length in less than 4 weeks Source: NPL 19 Source: Balco Whisker Resistance FIB was utilised to mount the whisker across four aluminium bond pads on substrate Supply voltage was swept from 0 to 1.6mV to maintain the current below 20 A to prevent damage to the whisker Four probe resistance measurement 20 10

11 Whisker V-I Plots 21 Whisker Resistance The whisker resistance was measured to be 5.7 Ohms. Whilst the resistance is seen to be linear with up to 20 A flowing though it, we do not know for sure what the resistance is at higher current densities. Assuming the cross section of the whisker is circular, the resistivity m Close to pure Sn at Wm 22 11

12 Resistivity measurements Measurements revealed a linear response Resistance of two whiskers yielded different resistance values, but they did have different diameters 23 Resistivity values The resistivity of whisker FIB2 compares well to that of pure Sn of 1.1x10-7 Ωm Resistivity of FIB1 is over double this Further analysis of this whisker in the SEM revealed a defect Assuming the whisker to have resistivity of pure tin, then the resistance of this defect was calculated at ~4 ohms

13 Current carrying capacity For FIB2 current gradually increased. The whisker failed at 13.3 ma. The resistance increased by 22% up to the final current SEM suggest the whisker failed twice. The first failure occurred on the pad, such that when it melted, it fused to the substrate corresponding to the drop at 12mA As the current was further increased, the whisker failed near the pad edge, & a Sn droplet formed The current at failure is ~ 26,000A/mm² 25 Summary of current carrying of whiskers Resistivity of a sample tin w hisker w as Ωm, close to pure Sn. The resistivity of a second w hisker w as Ωm, attributed to a microstructural defect, estimated at 4Ω. If such a defect occurred in the thinner diameter w hisker of the same resistance, it w ould correspond only to a 4% increase in measured resistivity. It is probable that greater diameter w hiskers will have a greater spread in resistivity values, and the average measured resistivity w ill increase w ith diameter due to defects contributing more significantly to the resistance of larger w hiskers. Current heating w ill lead to thicker w hiskers heating more for equivalent current densities

14 National Physical Laboratory Webinar 6th March 2013 Sn Whisker Field Effects The Attraction of Sn Whiskers Whisker field effects Well known that whiskers deform in electrical fields Have high aspect ratio, and are conductors Susceptible to deformation Clearly the probability of shorting out between adjacent conductors will be increased if the whisker is pulled in the electric field Characterisation is problematic, as the tendency of the whisker to deform, is not only dependent on its integral strength, but how it is attached at the root to the substrate

15 Initial Attraction Work Whiskers close to body of TH capacitor in dual beam FIB (focused ion beam) The whisker was 0.9mm in length and 1.2 m in diameter Tungsten probe at 10V 29 Whisker Attraction Video 30 15

16 Electrostatic Attraction of Whiskers Whiskers attracted into contact with the probe from a maximum distance of 50 m (10V) This equates to a field strength of ~200V/mm 31 Electrostatic Attraction of Whiskers Increased attraction when in contact 32 16

17 Electrostatic Attraction of Whiskers 33 Further Attraction Work Since the properties of the whisker are dependent on the attachment, any study needs to characterise whiskers in-situ. NPL has a plating chemistry that permits predictable whisker growth on copper Olin194 Such samples were mounted in a rig that allowed an electric field to be applied and visual inspection, and hence characterisation of the deflection in the electric field

18 Arrangement Low pass filter 4 axis sample manipulator 35 Effect of electric field Variation of the angle of a whisker tip, & the position of the tip, to the plate as a f unction of electrical field. The measurements assume a non-bending whisker, which is not the case. Distance between plates was set to approximately 600µm 36 18

19 Similar results observed with other whiskers

20 Predicting whisker bending at the tip Assuming a unif orm f ield and constant capacitance in the gap the whisker def lection can be shown to f ollow this equation u tip, deflection of tip V, voltage between parallel plates L, is the whisker length E, Young s modulus (50GPa) s, separation of parallel plates d w is the whisker diameter k, constant (8x10-13 ) for the analytical solution shown in the figure 39 Summary of electric field effects Whiskers are attracted by electric fields Degree of attraction is a function of the electrical field strength, the angle of the whisker to the substrate from which it has grown and the dimensions of the whisker Polarity of electric field is not important. The force will be attractive in both cases. Electrostatic attraction increases likelihood of short occurring between whiskers and conducting surfaces at different electric potentials

21 Summary of electric field effects Whiskers show deflection of >10 µm at 50V/mm. Comparable with other work: Kadesch et al The V² dependence significantly increases the probability of faster deflection as the whisker approaches the attracting electrode Whisker can suddenly snap across to the other electrode bridging the gap. Opposing whiskers in the same vicinity are more likely to short because of this flexibility. 41 National Physical Laboratory Webinar 6th March 2013 Sn Whisker Contact Effects The Attraction of Sn Whiskers 21

22 Conduction and contact pressure Contact resistance in whiskers is highly variable being a function of: contact angle contact area variable voltage breakdown of whisker oxide This work focuses on determining if increasing whisker pressure can cause oxide breakdown 43 Voltage Breakdown of Growing Whiskers Plates of whiskering plated Sn with 0.5mm separation from SnPb or Au detector plates Whiskers allowed to grow into contact Voltage periodically swept from 0 to 20V in 1 v steps to detector whisker contact Sn Whisker Plate SnPb or Au detector plate 44 0 to 20V 22

23 SnPb vs Au Detector Plates Percentage of Whisker Conducting SnPb detector plates Au detector plat es Breakdown voltage for first whisker (V) 45 Arrangement Low pass filter 4 axis sample manipulator 46 23

24 Procedure The probe was initially set at approximately 30 µm from the whisker tip and then moved towards the whisker in steps of 10 µm until the whisker tip is pushed back by 40 µm to ensure good contact. The procedure was repeated with increased voltage steps of 0.5V. 47 Typical conductance response 9V 7V 8V 6V 5V 4V Start at 2V, increased ev ery 0.5V up to 9V Intermittent conduction only occurred below 4V in the +50µm position. At 4V, the whisker was only conducting when it was pushed back 50µm. At higher v oltages the whisker conducted with less pressure on the tip. 48 The degree of bend in the whisker required to produce conduction at 4V 24

25 Summary of contact pressure effect on conduction Pressure on the whisker can reduce the breakdown voltage. Whilst it is not the case for all whiskers, there are clearly examples where increased pressure due to bending of the whisker causes conduction to occur at a lower voltage than would be the case with minimal pressure on the whisker tip. Shock, vibration, or thermal expansion could all affect the contact pressure on a whisker and thus be a secondary influence that triggers a failure from a tin whisker more easily than seen here. 49 National Physical Laboratory Webinar 6th March 2013 Sn Whisker Component Test Vehicle The Attraction of Sn Whiskers martin.wickham@npl.co.uk 25

26 Component Whisker Project Development of whiskering assembly and monitoring equipment 16 industrial partners NPL have collaborated with CML Microcircuits (UK) Ltd to produce a SOIC test vehicle. This will allow the coating mitigation properties to be tested, in conjunction with its coverage capability. 51 Acknowledgements NPL Chris Hunt Owen Thomas Buf a Zhang David Cox Barrie Dunn, ESTEC Bob Gregory, RR Department for Business Innovation & Skills AEC CML Microcircuits Ltd Cookson Electronics Elantas Beck GmbH Handy & Harman Henkel Loctite Adhesives H K Wentworth Ltd Humiseal Europe Enthone European Space Agency General Dynamics HMGCC Lockheed Martin Lucent 52 MBDA (UK) Ltd Motorola NSWC Crane Philips CFT Raytheon Integrated Defense Systems Rockwell Collins Rohm and Haas Electronic Materials Europe Limited Rolls Royce Marine Sagem Defense Schloetter Company Ltd Selex Galileo Semblant Ltd Surrey Satellite Technology Ltd Thales 26

27 National Physical Laboratory Webinar 6th March 2013 The Attraction of Sn Whiskers Martin Wickham, National Physical Laboratory, Teddington UK NPL EI Free Technology Webinars BGA Reliability - The Effects of Solder Joint Voiding and Uneven Stand-Off Height Tuesday 30 April Reuse of Electronic Products Wednesday 15 May Through Hole Reliability for High Aspect via Holes Tuesday 11 June Functionalisiation and Applications of Nanomaterials for Electronic Applications Monday 18 July Environmental Robustness: Performance Coatings & High Temperature Interconnects Tuesday 3 September 27