Presentations

Tin Whiskers: Mitigations

Michael OstermanCALCE, University of Maryland

Tin whiskers are fine hair-like conductive structures that form unpredictably on pure tin and tin based alloys. In electronic systems, the formation of tin whiskers can lead to serious failure risks. Tin whiskers have been responsible for failure of automotive, satellite, nuclear power plant, missile defense, and medical equipment. Alloying tin with lead has been demonstrated to significantly reduce the risk of tin whisker formation. However, the ban on lead in electronics based on regulations that have been imposed by many countries has result in the increased use of lead-free tin. As a result, concern of tin whisker induced failures has risen. This presentation will review strategies for assessing and mitigating tin whisker failure risk.

Electrostatic theory of nucleation and growth of metal whiskerInvestigation of zinc whisker growth from electrodeposits produced using an alkaline non-cyanide electroplating bath

Liang Wu, Ph.D. student, Dr Geoffrey Wilcox and Dr Mark Ashworth, Loughborough University, Loughborough, England Liang Wu, Loughborough University, UK

During the past 70 years, many electrical and electronic failures have been attributed to the presence of metallic whiskers. Researchers have been striving to address the problems induced by metal whisker growth. However, most research effort has been focused on tin whiskers, whilst very little attention was paid to reliability issues associated with zinc whiskers. Only a few previous studies have been carried out to investigate the growth mechanisms of zinc whiskers, but the results of such studies are often conflicting and there is, at present, no widely accepted growth mechanism in place.

The current study focuses on whisker growth from zinc coatings on mild steel substrates electrodeposited from a commercial alkaline non-cyanide electroplating bath. Experimental work has been undertaken to investigate the effect of deposition parameters and storage conditions on whisker growth, and also to understand the formation and growth mechanism. Scanning electron microscope (SEM), focused ion beam (FIB), energy dispersive x-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) have been used to characterise the as-deposited structure of the zinc coatings and to evaluate whisker growth.

Analysis showed that, for samples stored at room temperature, whiskers were present on surfaces deposited at current densities in the range of 20-50 mA cm^-2 within 4 weeks of electroplating. However, electroplating at low current densities significantly reduced whisker growth and no whiskers were observed on samples deposited at 5 mA cm^-2after 16 months. It was also found that a short period of post-electroplating heat treatment at high temperatures markedly reduces zinc whisker growth. In terms of the mechanisms for zinc whisker growth, our experimental data indicated that recrystallisation of the as-deposited columnar structure occurred with whisker growth. In addition, there was no evidence of Fe-Zn intermetallic compounds at the coating-substrate interface or beneath whiskers.

A Microscale Material Characterization of Tin Whiskers

Mark D. Peterson, Miles J. Brodie, Zachary R. Lingley, Scott D. Sitzman, John A. Chaney, Brendan J. Foran, Daniel A. Gutierrez, and Maribeth S. Mason, Microelectronics Technology Department, The Aerospace Corporation

Despite decades of study, our ability to predict the location at which a tin whisker will grow remains extremely primitive. This difficulty endures, in part, due to the large number of variables influencing the formation and growth processes, such as film thickness, substrate, impurities, grain size, texture, and environmental factors. In order to identify the most important variables influencing whisker formation and growth, it is important to perform a thorough characterization of whisker-prone thin films. In this study, we focus on 3-5 µm thick thin films electrodeposited on copper coupons, and perform a microscale material characterization from both a physical and chemical perspective. Physical characterization includes TEM, SEM, and EBSD, and suggests that whiskers are not merely extensions of the grain immediately beneath the base. Instead, crystallographic analysis shows that whiskers have a distinct orientation, different from their base grain. We also observe that whisker material largely originates near grain boundaries, and large voids are frequently found near the whisker base. Our chemical analysis, which includes EDX, ToF-SIMS, and XPS, is used to identify impurities within the general tin film and specifically at the whisker base. Importantly, we find differences in the chemical composition of whisker-prone and whisker-free samples, though we have not detected spuriously high contaminant concentrations near the whisker base. Our findings also suggest that contaminants are not always electrodeposited in concentrations equal to their relative values in the anode. Finally, we apply static and dynamic temperature stresses to our thin films, and find heat to be a reasonable inhibitor of whisker formation. When exposed to temperature cycling from -55 ^oC to 85 ^oC, samples were less prone to whisker formation than those left in ambient. Our results are discussed within the framework of the existing literature.

Stimulated Whisker Growth under a Range of Conditions

V. G. Karpov, University of Toledo, Toledo, Ohio, USA

We present SEM data demonstrating Sn and Zn whisker growth stimulated by electric fields of various origins as specified below.

1. DC electric field of ~ 3000 V/cm between two parallel plates (capacitive configuration) led to rapid Sn whisker growth within 1 week while control samples did not show any whiskers. In addition, evidence of explosive whisker growth was found.
2. Field due to sample charging by 6 MeV electron beam of a medical accelerator up to 30,000 V/cm. We have observed strongly accelerated whisker growth during the time intervals of 10 and 20 hours on sputtered and evaporated Sn films and electroplated Zn films (the latter provided by NASA and preliminary whisker infested).
3. SEM 10 keV e-beam significantly increased Zn whisker concentration after 10 hours of exposure for electrically ungrounded samples, while electrically grounded samples remained practically intact.
4. A beam of 100 keV Sn ions on Zn plated steel sample infested with whiskers caused significant change in whisker concentration after just 1 hour of ion beam application.
5. Zn whisker infested samples put on the Van der Graaff electric field generator for 10 hours (under induced field of approximately 20,000 V/cm) have shown about 30 percent increase in whisker concentration.
6. Laser beam was incident on the surfaces of Zn and Sn films in the plasmon polariton mode known for its significant, by orders of magnitude, increase of the electric field amplitude in the near surface region. Upon several hours of exposure on fresh Sn and Zn samples, whiskers grown in the under-the-beam region were found, while there were no whiskers far from the laser spot.

We interpret the above observations in the framework of the electrostatic theory of metal whiskers that attributes the driving force behind whisker formation and growth to the electric fields induced by surface imperfections. That theory predicts possible effects of the external electric fields as well. We discuss in parallel a rather scarce previous work demonstrating no electric bias effects on whisker growth. We show how various observations can be reconciled and propose verifying experiments. In addition, we discuss how the observed effects can be developed to become accelerated life testing protocols against the whisker related reliability problems.

Tin Whiskers: A Practioner's Perspective - Balancing and Integrating the Technical and Business Risks

Dock Brown, DRF Solutions, LLC.

The electronics industry is well into its second decade working with increased environmental sensitivity. The risks associated with tin whiskers are well known; the risks of common mitigations are likewise well known. As a discipline, design engineering has inescapable elements of both art and science. Where tin whiskers and their mitigations are concerned, the art portion is: 'the art of the possible.' The art of astutely balancing and integrating the legitimate technical and business risks. This presentation explores some approaches that have been successfully used to balance and integrate those risks. Emphasis is placed on the hard cases; those electronic products that are deployed in field environments that are neither benign nor harsh with product reliability life expectations that are neither months nor decades.

New Research into Tin Whiskers Mitigation

Martin Wickham, Kate Clayton, Owen Thomas and Chris Hunt, National Physical Laboratory, Teddington, UK

The growth of tin whiskers leading to electrical short circuits has been an issue for a number of organisations across avionics, space and other high reliability applications. The National Physical Laboratory has led a number of collaborative projects with industry to develop test vehicles to enable measurement of Sn whisker growth on tin-plated copper. The work reported here is based on a recently developed test vehicle incorporating specially plated SOIC components mounted onto PCBs. NPL, along with 17 industrial partners has undertaken trials on different mitigation techniques to inhibit Sn whisker growth. In the course of this work, a number of control specimens were fabricated which were allowed to whisker freely and the development of short circuits was studied. The experimental set up to measure the occurrence of electrical shorts is built on a bespoke daisy chained SOIC16W, twenty-four of which are mounted on a PCB. Each SOIC is monitored for a short between adjacent terminations by a resistance measurement. By multiplexing the current experiment is looking at 2176 SOICs, and over 48,000 opportunities for failure. The system has captured the resistance state across all channels every 15 minutes for this experiment. The collected data permits the study of the incidence of short circuits, the length of time of each short and the number of intermittent short circuits. We will discuss the rapidity of whisker formation and the formation of intermittent shorts and their duration as the whisker continues to grow.

SERDP WP2213 Project: Conformal coat modeling for tin whisker mitigation

S. McKeown, S. Meschter; BAE Systems, P. Snugovsky, J. Kennedy, Z. Bagheri, J. Keeping; Celestica, J. Cho; Binghamton University, D. Edwards; Henkel LLC, K. Elsken; Covestro LLC

The whisker shorting potential can be mitigated by applying conformal coatings traditionally used for moisture protection to electronic assemblies. However, tin nodules and odd-shaped eruptions can also form, which can rupture the coating and reduce moisture protection and whisker mitigation effectiveness. The application of conformal coating to the original "tin free" surface alters the surface, changing the whisker nucleation and growth characteristics. A combination of finite element, classical plate deflection and adhesion models have been developed to examine the critical pressure that a tin nodule/whisker can exert on a coating before rupture or delamination occur. Supporting experimental results reveal that a high strength high modulus polyurethane conformal coating can inhibit nodule/whisker formation provided the coating is sufficiently thick and well adhered to the tin.