Query: Electrical shorting propensity of tin whiskers

Candidate papers:

abstract: When a tin whisker bridges two differently biased conductors, an electrical short is not guaranteed. In many instances, the voltage must exceed a threshold level in order to produce current flow due to weak physical contact and the presence of a non conductive film such as an oxide layer. This paper presents a study that examines the breakdown voltage of tin whiskers and its relation to contact force. Whisker contact force studies were conducted using gold and tin coated tungsten probes, and the breakdown voltage was measured using a semiconductor parameter analyzer. It was verified that contact force is a critical factor in determining the type of current voltage transition and level of breakdown voltage. Lower contact force between the probe and the whiskers caused the multiple transitions in current voltage characteristics. The tin oxide layers on whiskers were analyzed using field emission transmission electron microscopy (FE TEM)
author_names: Sungwon Han

Michael D Osterman

Michael G Pecht
title: Electrical Shorting Propensity of Tin Whiskers
venue: IEEE Transactions on Electronics Packaging Manufacturing
year: 2010

abstract: Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that has a currently unknown probability associated with it. Due to contact resistance electrical shorts may not occur at lower voltage levels. In this experiment, we study the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From this data we can estimate the probability of an electrical short, as a function of voltage, given that a free tin whisker has bridged two adjacent exposed electrical conductors. Also, three tin whiskers grown from the same Space Shuttle Orbiter card guide used in the aforementioned experiment were cross sectioned and studied using a focused ion beam (FIB) The rare polycrystalline structure seen in the FIB cross section was confirmed using transmission electron microscopy (TEM) The FIB was also used to cross section two card guides to facilitate the measurement of the grain size to determine that the tin plating on the card guides had a bright finish.
author_names: Karim J Courey
title: An Investigation of the Electrical Short Circuit Characteristics of Tin Whiskers
year: 2013

abstract: The Third Annual International Symposium on Tin Whiskers was held on 23 24 June 2009 at the Technical University of Denmark, Lyngby, Copenhagen, Denmark. The event contained two presentations that included non tin growths and zinc whisker formation being investigated by the author. The propensity for electroplated tin fi nishes to produce, among other morphologies, fi lamentous growths, thereby producing the very real danger of short circuiting in microelectronics, is a very real threat. The move by the European Union (EU) to ban lead (the only real mitigating agent for tin whiskers) through the well known RoHS and WEEE initiatives has made the reappearance of tin whiskers a real possibility. One organisation that is actively disseminating knowledge through annual symposia is the Centre for Advanced Life Cycle Engineering (CALCE) at the University of Maryland in the USA. Previous symposia have been held at the university in 2007 and in Japan in 2008. This year's symposium moved to Europe and was also co organised by the US Surface Mount Technology Association (SMTA) and the Technical University of Denmark (DTU) The writer had presented at the fi rst symposium, which was an excellent meeting, and was keen to attend and present at this third symposium. Michael Osterman (CALCE) welcomed delegates to the meeting and began by outlining the format of CALCE, its raison d'etre and its previous studies on the phenomenon of tin whiskers. He then handed over to Rajan Ambat, who is the Director of the Centre for Electronic Corrosion (CELCORR) at DTU. He welcomed delegates and explained a little more about the university and his centre which researches corrosion phenomena in electronic devices. The fi rst presentation was given by Katsuaki Suganuma of Osaka University. He outlined the research fi ndings on whiskers made by the Japanese Electronics and Information Technology Industries Association (JEITA) He presented data on whisker growth on a number of electroplated tin alloys including Sn Bi, Sn Ag and Sn Cu. The second presentation of the morning was by Barrie Dunn of the European Space Agency (ESA) Non tin whisker growths were discussed as were historical examples of more conventional tin whisker problems that had been highlighted by the ESA. More detailed examples were highlighted from 1995. The third presentation of the morning was given by Norio Nemoto of the Japanese Aerospace Exploration Agency (JAXA) Many Japanese parts manufacturers have shifted to lead free products although the equivalent Japanese directive to RoHS, JIS C 0950 (J MOSS) does not prohibit the use of lead only that its usage is indicated. JAXA have carried out much research into the effects of combined temperature cycling in vacuo on tin whisker growth. High vacuum levels promoted whisker growths of a similar length to those from surfaces cycled in a normal air atmosphere up to 500 times. Whiskers were longer and thinner in a vacuum after 1500 cycles. Tadahiro Shibutani from Yokohama National University in Japan examined the effect of grain size on pressure induced tin whisker formation. Three surface fi nishes were examined (SnCu, bright and matte tin) along with their propensity to form whiskers following nanoindentation. The threshold stress to form a whisker depended on the whisker diameter which in turn depended on the grain size of the tin based fi nish. Also from Japan, Minoru Ueshima from Senju Metal Industry Co. Ltd, examined the infl uence of fl ux type on whisker growth at a solder fi llet. Dimethyl amine HBr salt appeared to be associated with the most prevalent whisker growth and coincidentally the most marked corrosion. It was suggested that the two were interrelated. After lunch, Carol Handwerker from Purdue University in the USA examined tin whisker and surface defect formation on electroplated fi lms and refl owed joints. It was emphasised that stress was a driving force for tin whisker growth, but not a mechanism. The different whisker/defect morphologies were also discussed, along with other whisker formation driving forces including oxidation and corrosion and the occurrence of whisker growth on lead free solders (Sn Ag Cu) containing rare earth additions. Following Professor Handwerker was a presentation by Matthias Sobiech of Max Planck Institute for Metals Research (Stuttgart, Germany) and Robert Bosch GmbH (Stuttgart, Germany) His presentation described investigations into the driving force for whisker formation on Sn thin fi lms deposited on Cu. A conclusion was that the presence of compressive macrostresses in the Sn layer on its own is not a suffi cient condition for tin whisker growth. The stress distribution in the coating is more important. A negative out of plane stress gradient near the coating surface and a negative in plane stress gradient around the whisker root were said to be driving forces. Nils Arendt of Cookson Electronics gave the next presentation about a whisker reducing immersion tin technology with an 'organic nanometal' The organic metal (Ormecon, Ammersbek, Germany) is a conductive polymer behaving as a noble metal electrochemically with nanosized primary particles and catalytic properties. In its most effective whisker reducing form, it also contains Ag. With immersion tin coating processes, it is used as a predip before tin deposition and has been found to lessen subsequent tin whisker formation. The running theme of stress and tin whisker growth was continued by Maureen Williams of National Institute of Standards and Technology (Gaithersburg, MD, USA) Simultaneous and time resolved measurements of stress in Sn and SnCu electrodeposits were made using cantilever defl ection and X ray diffraction techniques. SnCu coatings had a more compressive stress than pure Sn and produced more whiskers, eruptions and hillocks. The defl ection and X ray diffraction techniques showed different results and it was unclear, which was a better engineering predictor of whiskering. Geoff Wilcox (Loughborough University) made the penultimate presentation of the day looking at zinc whisker growth from electrodeposited coatings. Zinc whiskers are often
author_names: Geoffrey D Wilcox
title: Whiskers
year: 2010

abstract: Introduction 1 Risks Related To Tin Whiskers 2 A Brief Description Of Tin Whiskers 2 Tin Whisker Growth Mechanism 2 Mitigation Strategies 3 Avoid Tin Plated Parts 4 Strip and Replate 5 Solder Dip the Plated Surfaces Sufficiently Using a Tin Lead Solder 5 Select a Matte or Low Stress Tin Finish 6 Select Underplating (Barrier layer) to Reduce Intermetallic Formation 6 Vary Thickness of Tin Plating 6 Annealing 7 Avoid Applying Compressive Loads on Plated Surfaces 8 Apply a Conformal Coat 8 Evaluate Application Specific Risks 10 Summary and Conclusions 10 References 10 Document Development Details 11
author_names: M Osterman Calce-Epsc
title: Mitigation Strategies for Tin Whiskers
year: 2002

abstract: Spectacular damage of several relays from high current flowing to the metal case was very likely caused in each instance by a tin whisker growing from the armature, which is bright tin plated, toward a terminal stud. This plating is a nonconformance to the applicable military specification that was in effect at the time of manufacture. The other possible causes, wearout (arcing deposits creating a conductive path between phases) and particles are considered and shown to be unlikely.
author_names: Gordon Davy
title: Relay Failure Caused by Tin Whiskers
year: 2003

abstract: The movement to eliminate lead (Pb) especially active in Japan and the European Union, has resulted in an increasing use of pure tin (Sn) coatings on leads and other external and internal surfaces of capacitors, resistors, and other passive components. This paper discusses the issues of tin whisker growth with respect to passive components. It also presents both a critical analysis of existing published documents on tin whisker nucleation and growth and a summary of very recent experiments that provide further understanding of the potential means of whisker formation mitigation. Many of the proposed mechanisms for mitigation, including control of the immediate underplating material, use of conformal coating, regulating the thickness of the tin coating, use of matte tin electroplating, and annealing or fusing of the tin layer, are inadequate. They likely reduce the incidence of nucleation or growth but do not provide an absolute guarantee of lack of whisker formation.
author_names: Jay Brusse

Gary J Ewell

Jocelyn Siplon
title: TIN WHISKERS: ATTRIBUTES AND MITIGATION
year: 2002

abstract: The connector industry is at a crossroads with respect to selecting lead free coatings in response to legislation and customer demands. Component manufacturers have long used tin lead coatings because of lead's ability to inhibit tin whisker formation. Several lead free alternate finishes such as pure tin, tin bismuth, tin copper, tin zinc, Pd, and Pd Ni Au have been proposed. One simple, economical, and easy drop in replacement is pure matte tin. Unfortunately, pure tin films have a fairly high risk of whisker formation. Factors, such as substrate material, plating process variables, and assembly operations also play a role in whisker generation. The stresses imposed at each process step pose a serious threat to whiskering. We have critically evaluated three matte tin chemistries by changing bath composition and deposition parameters. We also characterized stresses in these matte tin coatings by using flexure beam and X ray diffraction techniques. This paper identifies stress types encountered in various connector applications, provides an insight to the users about practical situations, and suggests whisker mitigation strategies. The impact of environmental (e.g. temperature and humidity) stresses on tin coatings is also discussed.
author_names: Sudarshan Lal

Thomas D Moyer
title: Role of intrinsic stresses in the phenomena of tin whiskers in electrical connectors
venue: IEEE Transactions on Electronics Packaging Manufacturing
year: 2005

abstract: Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that has a currently unknown probability associated with it. Due to contact resistance, electrical shorts may not occur at lower voltage levels. In this experiment, we study the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From this data, we can estimate the probability of an electrical short, as a function of voltage, given that a free tin whisker has bridged two adjacent exposed electrical conductors. In addition, three tin whiskers grown from the same Space Shuttle Orbiter card guide used in the aforementioned experiment were cross sectioned and studied using a focused ion beam (FIB)
author_names: Karim J Courey

Shihab S Asfour

Jon A Bayliss

Larry L Ludwig

M C Zapata
title: Tin Whisker Electrical Short Circuit Characteristics Part I
venue: IEEE Transactions on Electronics Packaging Manufacturing
year: 2008

abstract: To comply with lead free legislation, many manufacturers have converted from tin lead to pure tin finishes of electronic components. However, pure tin finishes have a greater propensity to grow tin whiskers than tin lead finishes. Since tin whiskers present an electrical short circuit hazard in electronic components, simulations have been developed to quantify the risk of said short circuits occurring. Existing risk simulations make the assumption that when a free tin whisker has bridged two adjacent exposed electrical conductors, the result is an electrical short circuit. This conservative assumption is made because shorting is a random event that had an unknown probability associated with it. Note however that due to contact resistance electrical shorts may not occur at lower voltage levels. In our first article we developed an empirical probability model for tin whisker shorting. In this paper, we develop a more comprehensive empirical model using a refined experiment with a larger sample size, in which we studied the effect of varying voltage on the breakdown of the contact resistance which leads to a short circuit. From the resulting data we estimated the probability distribution of an electrical short, as a function of voltage. In addition, the unexpected polycrystalline structure seen in the focused ion beam (FIB) cross section in the first experiment was confirmed in this experiment using transmission electron microscopy (TEM) The FIB was also used to cross section two card guides to facilitate the measurement of the grain size of each card guide's tin plating to determine its finish
author_names: Karim J Courey

Shihab S Asfour

Arzu Onar

Jon A Bayliss

Larry L Ludwig

Maria C Wright
title: Estimating the Probability of Electrical Short Circuits from Tin Whiskers
year: 2013

abstract: A technique using interdigitated, specially electroplated discs has been developed at the National Physical Laboratory for determining the relative ability of conformal coatings to mitigate against Sn whisker growth. All the control samples without conformal coatings developed whiskers of sufficient length >250mm) to cause electrical shorts between the discs. The electrical monitoring system was able to detect these shorts and the current flowing in the detection circuit was restricted to a level sufficient to prevent destruction of the whiskers. The control samples all exhibited electrical shorts within 14 days, some within as few as 3 days, allowing for a relatively rapid i.e. less than 12 week evaluation of the Sn whisker mitigation benefits of conformal coatings.
author_names: Martin Wickham

Christopher Hunt
title: Test method for measurement of the propensity for conformal coatings to inhibit tin whiskering.
year: 2008