Mark A. Ashworth

Zinc whisker growth from electroplated finishes – a review

Abstract Electroplated zinc finishes have been associated with the electronics industry for many years as a result of their excellent corrosion resistance and relatively low cost. They are normally applied onto ferrous products to provide corrosion protection in a range of different environments. However, the formation of spontaneously grown whiskers on zinc-electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long life electrical and electronic equipment. The growth of zinc whiskers has been identified as the cause of some electrical and electronic failures in telecommunications and aerospace-based applications, with consequences ranging from mild inconvenience to complete system failures. Investigators have been striving to address the problems induced by whisker growth since 1940s. However, most research effort has been focused on tin whiskers, especially following European Union environmental legislation that restricted the use of lead (Pb), which when alloyed with tin (3–10% by weight) provided effective tin whisker mitigation. Compared with tin whisker research, much less attention has been paid to zinc whiskers. A number of mechanisms to explain zinc whisker growth have been proposed, but none of them are widely accepted and some are in conflict with each other. The aim of this paper is to review the available literature in regard to zinc whiskers, to discuss the reported growth mechanisms, to evaluate the effect of deposition parameters and to explore potential mitigation methods. This paper presents a chronologically ordered review of zinc whisker-related studies from 1946 to 2013. Some important early research, which investigated whisker growth in tin and cadmium, as well as zinc, has also been included.

An Investigation Into the Role of Lead as a Suppressant for Tin Whisker Growth in Electronics

The effect on Sn electrodeposits of up to 10 wt.% codeposited Pb was investigated. The influence of Pb on cathodic polarization of the electrodeposition process, the microstructural and crystallographic characteristics of as-deposited films, the Sn-Cu intermetallic compound (IMC) formation between the deposits and Cu substrate and surface oxidation after ambient storage were examined. During electrodeposition, the plumbous (Pb2+) ions were observed to inhibit stannous (Sn2+) ion reduction. The use of a metallographic cross sectioning approach enabled the observation of a grain structure evolution toward a more equiaxed morphology with increasing Pb content. This was accompanied by a duplex-phase structure consisting of isolated Pb-rich particles at grain boundary intersections together with a continuous Sn-rich phase. X-ray diffraction measurements corroborate the existence of the duplex-phase structure and also indicate an as-deposited texture transition from (112) through (200) and further toward (220) with an increase in Pb content from 0 to 8 wt.%. A marked topological transition of the interfacial Cu-Sn IMCs from wedge-shaped and discretely distributed to relatively even and uniformly distributed was found to occur concurrently. After tin whisker growth had occurred from low-Pb content (up to 5 wt.%) electrodeposits on brass, Pb was generally observed to be strongly associated with actual Sn whiskers themselves, thus the spontaneous redistribution of Pb after electrodeposition has been evidenced. X-ray photoelectron spectroscopy sputter profiling indicates the absence of Pb in the Sn oxide layer and the enrichment of metallic Pb beneath the Sn oxide from a 90Sn-10Pb deposit ambient stored for 270 days. The whisker-inhibiting effect of Pb codeposition is postulated to emanate from more than one source: not only those reflected in grain structure modification, grain texture transition, and changes in the growth mode of the interfacial Cu-Sn IMCs as prevailingly acknowledged, but also significantly, the universal distribution of the soft Pb-rich phase, providing simultaneous diffusion toward potential whisker sites as well as effective strain dissipation.

Effect of direct current and pulse plating parameters on tin whisker growth from tin electrodeposits on copper and brass substrates

Abstract Electroplated tin finishes are widely utilised in the electronics industry due to their advantageous properties such as excellent solderability, electrical conductivity and corrosion resistance. However, the spontaneous growth of tin whiskers during service can be highly deleterious, resulting in localised electrical shorting or other harmful effects. The formation of tin whiskers, widely accepted as resulting from the formation of compressive stresses within the electrodeposit, has been responsible for a wide range of equipment failures in consumer products, safety critical industrial and aerospace based applications. The numbers of failures associated with tin whiskers is likely to increase in the future following legislation banning the use of lead in electronics, the latter when alloyed with tin, being an acknowledged tin whisker mitigator. Using a bright tin electroplating bath, the effect of process parameters on the characteristic structure of the deposit has been evaluated for deposition onto both brass and copper substrates. The effect on whisker growth rate of process variables, such as current density and deposit thickness, has been evaluated. In addition, the effect of pulse plating on subsequent whisker growth rates has also been investigated, particularly by varying duty cycle and pulse frequency. Whisker growth has been investigated under both ambient conditions and also using elevated temperature and humidity to accelerate the growth of whiskers. Studies have shown that whisker formation is strongly influenced by pulse plating parameters. Furthermore, increasing both current density and thickness of the deposit reduce whisker growth rates. It is also observed that whisker formation is greatly accelerated on brass substrates compared with copper. The basis for this observation is explained.

An Investigation into the Effect of a Post-electroplating Electrochemical Oxidation Treatment on Tin Whisker Formation

Since the ‘cracked oxide theory’ was proposed by Tu in 1994,1 there has only been a limited number of studies that have sought to investigate the effect of the Sn oxide on whisker growth. The current study has used electrochemical oxidation to produce oxide films, which has enabled the effect of the surface oxide thickness on whisker growth to be established. The effect of oxide thickness on whisker growth has been investigated for tin electrodeposits on both Cu and brass substrates. The influence of applied oxidation potential on the thickness of the Sn oxide film has been investigated using x-ray photoelectron spectroscopy (XPS) for potassium bicarbonate–carbonate and borate buffer electrolyte solutions. Whisker growth from electrochemically oxidised Sn-Cu deposits on Cu and Sn deposits on brass has been investigated and compared with samples left to develop a native air-formed oxide. XPS studies show that the thickness of the electrochemically formed Sn oxide film is dependent on the applied oxidation potential and the total charge passed. Subsequent whisker growth studies demonstrate that electrochemically oxidised Sn-Cu deposits on Cu and Sn deposits on brass are significantly less susceptible to whisker growth than those having a native oxide film. For Sn deposits on brass, the electrochemically formed Sn oxide greatly reduces Zn oxide formation at the surface of the tin deposit, which results in whisker mitigation. For Sn-Cu deposits on Cu, the reduction in whisker growth must simply derive from the increased thickness of the Sn oxide, i.e. the Sn oxide film has an important role in stemming the development of whiskers.

Sn whisker evaluations in 3D microbumped structures

Abstract Sn whiskering remains a reliability concern in electronic applications. Despite extensive research on growth rates and mitigation strategies, no predictive theory is in place. Literature data are available for Cu/Sn-based films and coatings as well as for board-level and flip-chip solder bumps but data are scarce for scaled-down solder volumes and for higher intermetallic-to-solder ratios. The current work investigates whiskers in “isolated geometries” for 3D solder-capped Cu microbumps with >2 orders of magnitude smaller solder volumes compared to state-of-the-art. To the best of the authors’ knowledge, this is the first time Sn whisker growth is reported in isolated solder volumes (e.g.

Investigation of Whisker Growth from Alkaline Non-cyanide Zinc Electrodeposits

Electroplated zinc finishes have been widely used in the packaging of electronic products for many years as a result of their excellent corrosion resistance and relatively low cost. However, the spontaneous formation of whiskers on zinc electroplated components, which are capable of resulting in electrical shorting or other damaging effects, can be highly problematic for the reliability of long-life electrical and electronic equipment. This work investigated the mechanism for whisker growth from zinc electrodeposited mild steel substrates. The incubation time for whisker growth from the surface of nodules on the surface of the electrodeposit was considerably reduced compared with that from the planar deposit surface. Recrystallisation of the as-deposited columnar structure was observed at the whisker root. This result is consistent with some recent whisker growth models based on recrystallisation. There was no evidence of iron-zinc (Fe-Zn) intermetallic formation at the iron/zinc (Fe/Zn) interface or within the zinc coating beneath the whiskers.

Tin whisker mitigation by means of a post-electroplating electrochemical oxidation treatment

There are very few studies that have investigated directly the effect of an oxide film on tin whisker growth, since the ‘cracked oxide theory’ was proposed by Tu in 1994 [K.-N. Tu: Phys. Rev., 1994, 49, (3), 2030–2034]. The current study has investigated the effect of an electrochemically produced oxide on tin whisker growth, for both Sn–Cu electrodeposits on Cu and pure Sn electrodeposits on brass. X-ray photoelectron spectroscopy (XPS) has been used to investigate the effect of the applied electrochemical oxidation potential on the oxide film thickness. Focused ion beam has been used to prepare cross sections from electrodeposited samples to investigate the influence of the electrochemically formed oxide film on deposit microstructure during long-term room temperature storage. The XPS studies show that the thickness of electrochemically formed oxide film is directly influenced by the applied potential and the total charge passed. Whisker growth studies show that the electrochemical oxidation treatment mitigates whisker growth for both Sn–Cu electrodeposits on Cu and pure Sn electrodeposits on brass. For Sn electrodeposits on brass, the electrochemically formed oxide greatly reduces both the formation of zinc oxide at the surface and the formation of intermetallic compounds, which results in the mitigation of tin whisker growth. For Sn–Cu electrodeposits on Cu, the electrochemically formed oxide has no apparent effect on intermetallic compound formation and acts simply as a physical barrier to hinder tin whisker growth.

Characterisation of tin–copper intermetallic growth in electrodeposited tin coatings using electrochemical oxidation techniques

Abstract In the present investigations electrochemical oxidation has allowed for the removal of tin from coated copper substrates permitting scanning electron microscopy to be employed to assess intermetallic compound distribution and morphology. It was shown that the initial growth of intermetallic compound was not uniform, with its distribution dependent upon the grain structure of the copper substrate, and the grain orientation and distribution within the tin coating. After a period of further nucleation and coarsening, the intermetallic compound forms an almost complete layer along the interface. It is thought this uniform layer of intermetallic compound, formed soon after deposition, may be responsible for this specific electrodeposited tin system having a low whiskering propensity.

An Evaluation of Sn-Cu-Ga and Sn-Cu-Ag Solder Alloys for Applications within the Electronics Industry

Components soldered with Sn-based alloys are susceptible to the growth of whiskers. Tin whiskers have been proven to be responsible for equipment failures in a wide range of industries. In order to reduce defects in electronic components a new solder alloy is proposed based on the Sn-Cu alloys. The Sn-Cu-Ga alloy utilised in this study was fabricated as a ribbons using melt-spinning method. These ribbons were then soldered onto electroplated tin layers. Preliminary characterization of the solder alloy is presented in this paper, including scanning electron microscopy, EDX mapping and X-ray diffraction. Key words: tin whiskers, PCB, printed circuit boards, melt spinning, solder, ribbons