Factors Governing Tin Whisker Growth

Abstract

Tin (Sn) whiskers are electrically conductive, single crystal eruptions that can grow from surfaces where tin is deposited on a substrate surface. They present reliability problems for the electronics industry due to the formation of stable, bridging shorts in low voltage, high impedance circuits. Due to legislation in the EU, Japan, and the U.S. which mandates a gradual shift from lead (Pb)-based to lead-free solders and board finishes, there has been a re-emergence of Sn whiskers. Continuing reports of Sn whisker induced failures coupled with the lack of an industry-accepted understanding of tin whisker growth and/or test methods to identify whisker-prone products has made blanket acceptance of pure tin plating a risky proposition in high reliability systems. This research is designed to clarify and control the mechanisms that govern whisker formation. An ultimate objective is to discover how to impede and/or prevent whisker growth, either by surface coatings or by modifications of the thin film properties. While tin whisker growth is believed to be largely mechanical, there is currently no general agreement on the mechanism governing the growth of tin whiskers. In whiskering, multiple material and processing variables interact to create whiskers which makes it difficult to produce effective mitigation schemes and develop a comprehensive picture of whisker growth. While there are some commonly accepted factors that impact whisker growth (residual stress, externally-imposed stress, intermetallic formation, Sn diffusion, scratches, corrosion, CTE mismatches, etc.), controlled laboratory experiments demonstrating which ones are most important are lacking. Further, many previous investigations of whiskers involve electroplated thin film systems on brass or copper which, although complying with industry practice, introduce several uncontrolled variables into the whiskering event. A more optimum model system to study whiskering is needed. A final motivation for this work is the existence of considerable “common lore” in the whisker community that is contradicted by recent experiments in several laboratories (including ours). Tightly designed and controlled experiments whose goal is to generate a broad experimental whisker database is necessary to aid evolving mechanistic and theoretical efforts that describe whiskering. Progress has recently been made by use of high-performance analytical tools such as focused ion beam (FIB) microscopy and EBDP (electron backscatter diffraction), which allows for whisker morphology and whisker root/grain orientation examinations respectively. However, considerably more fundamental work is needed to develop a detailed understanding of the physical, chemical, and materials mechanisms leading to initiation and growth of tin whiskers and to reduce and/or eliminate it in Pb-free electronic components. This thesis seeks to clarify some of the key questions by studying several important factors involved with whisker incubation and growth. Our work takes several novel approaches to the whisker problem: 1) We focus on a limited set of focused research objectives using “laboratory” created whiskers, as opposed to archival, industrial, and/or anecdotal specimens; 2) We use a reproducible method of growing whiskers in a reasonable (weeks) time by using magnetron sputtering techniques rather than electrochemical deposition; 3) We produce tailor-made films with known “dialed-in” degrees of intrinsic thin film stress (tensile, none, compressive) to investigate the role of net film stress; 4) We eliminate the role of interfacial stress by growing whiskers on substrates that do not form intermetallic compounds with Sn; 5) We examine whisker growth in near-real time using field-enhanced, high current density methods that grow whiskers in hours rather than weeks and months; 6) We study whisker growth from considerably thinner (submicron) films than most other researchers, which has enabled us to observe the uniform depletion of the Sn feedstock during whisker growth, which provides further evidence for the importance of long-range Sn diffusion during whisker growth; 7) We address the question of whisker prevention by studying why certain topside metal films (Ni, Pt) appear to prevent whisker growth while others (Cu, Pb) do not. Special attention has been devoted to measurements of whiskering under a variety of rigorously controlled environmental factors such as substrate roughness, gas environment, and humidity, which are known to play a significant role in whisker production. Generally, we find that whisker densities are higher for smoother substrates (such as semiconductors) and, by using accurate humidity exposures (generated by calibrated vapor pressure solutions), we show that Sn whiskers favor relative humidity values ~ 85%. Generating whisker growth from films which contain no native and/or surface oxides (such as Au) shows that a surface Sn oxide layer is not a necessary requisite for whisker production. Sluggish whisker growth from small, micron-dimensional patterned Sn deposits implies that large lateral films of tin are optimum for whisker growth. Ideally, it is desirable to mitigate and/or prevent whisker growth failures before they occur. We show that whisker prevention is possible by a variety of impenetrable topside hard metal films, which prevent Sn whiskers from penetrating through the capping barriers. In particular, Ni (700 Å or thicker) was found to successfully block all Sn whiskers for time periods of greater than one year. Pt films (325-1360 Å) also appear to be successful, preventing whisker penetration for over three months. In contrast, Au (875-3000 Å) and Cr (250-1400 Å) cap films are penetrated by Sn whiskers within a couple of months. Penetrating whiskers have been observed to carry up a fractured piece of the metal cap layer during the puncture process, which helps explain why only certain metal caps block whiskers while others do not. Cap metals with high shear moduli are likely to block whiskers since cap penetration appears to be a metal punching process. Shear modulus values for the pure elemental cap films approximately follows the trend for whisker prevention by metals, with the exception of Cr, which oxidizes considerably during thin film formation. The true situation is more complex than this simple mechanical picture, however, owing to the formation of intermetallic compounds and/or diffusion between the Sn and cap film layers.