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Contributions of Stress and Oxidation on the Formation of Whiskers in Lead-Free Solders
Dr. Elizabeth Hoffman | Savannah River National Laboratory
This study focused on the environmental factors influencing formation of tin whiskers on electrodeposited lead free, tin coatings over copper (or copper containing) substrates. The objective was to increase the understanding of the role of stress and oxide formation on whisker growth in lead-free solders. This understanding will support the future development of a predictive model, enabling engineers to design solder alloys for whisker growth resistance.
Metallic whisker formation has been observed in lead free films of tin, cadmium and zinc since the 1940s. Increasingly, the use of lead free solder and coatings in military applications has led to higher failure rates and lower reliability of electronic components due to short circuits and electrical arcing. As technology advances reduce the scale or size of electronic devices and environmental legislation continues to increase the life-cycle cost of lead in electronics, the instances of whisker driven failures in these devices is expected to grow. To produce solder alloy forms that show enhanced resistance toward whisker growth, this study has increased the understanding of the role of certain environmental factors on whisker growth in lead‐free solders. These environmental factors include external stress and formation of intermetallic compounds in the tin film or at the tin/substrate interface. The results of this study have increased the understanding that will support the development of a predictive model, enabling engineers to design solder alloys for whisker growth resistance.
Sn thin films on Cu substrates were fabricated in an electrochemical workstation with a three‐electrode system. A titanium‐mesh electrode with platinum film was used as the counter electrode and the reference electrode was Ag/AgCl. For Sn deposition, sodium citrate tri‐basic di‐hydrate (C6H5Na3O7∙2H2O) was added into a tin(II) chloride (SnCl2∙2H2O) aqueous solution to achieve a stable stannous solution. A potential was applied to deposit a homogenous and continuous thin film of metallic tin. Such thin thickness is stated to be a precondition for observable whisker growth.
Static load bend testing samples were prepared by electroplating tin films on substrates of copper and glass. For copper the substrates were dipped in sulfuric acid to remove the native copper oxide and then rinsed using ultrapure water immediately before electroplating. Copper substrates were electroplated with pure tin films using a commercial Sn plating solution and a pure tin anode.
Tin films on glass substrates were achieved by e-beam deposition of a Ti adhesion layer onto the glass, followed by a Cu layer. Next, the substrates were dipped in acid as above followed by electroplating with the same conditions as the copper substrates. Microstructures of the films were characterized using SEM and FIB milling.
Four-point bend fixtures were configured to ASTM E 855 and C 1161. Two types of samples were evaluated: (1) tin films electroplated on copper substrate and (2) tin films electrodeposited on glass with an evaporative copper coating on its surface. The stress response of the substrate and film were estimated based on the bi-material strip model, modifying the relation for the bending configuration.
Parallel samples of 100% air and 100% N2 atmosphere were used to study the role of oxidation in whisker formation. Immediately after opening, samples were evaluated using the SEM for whisker growth. Substrates were examined using scanning electron microscopy; counting the number of imaged features within the area.
From the initial results, two main areas were chosen to be the focus of additional research: the demonstration of effects of stress state in the nucleation of whiskers and the confirmation of the effect of oxygen content in the formation of whiskers. The preliminary observations indicate that whisker formation is proportional to compressive stress up to the elastic limit of the film. Different levels of elastic strain were induced with the incorporation of a custom designed fixture that loaded the samples in a four-point bending configuration and were maintained in an environmental chamber under conditions deemed favorable for whisker growth. The results show that induced elastic stress slightly increased the concentration of nucleation sites of whiskers.
Initial studies also showed enhanced whisker formation in dry nitrogen atmospheres. The effects of oxygen content were studied by aging substrates in gas vials of varying absolute pressure and different oxygen partial pressure. The concentration of whiskers were measured and did not appear to be sensitive to oxygen content, as previously observed, but did exhibit higher concentrations under vacuum.
The environmental factors influencing formation of tin whiskers on electrodeposited tin coatings over copper substrates was examined in this study. The effect of stress on the nucleation and growth of whiskers was studied. The results showed that induced compressive stresses increased the concentration of whiskers, slightly. However, the residual stress state in the film was not directly measured and could vary due to several factors. It is recommended that further studies on this subject measure the residual stress in the film prior to testing.
The effect of oxygen content was also studied by aging substrates in gas vials of varying absolute pressures and different levels of oxygen. The growth of whiskers/hillocks appeared to be inversely proportional to absolute pressure but is less sensitive to oxygen partial pressure. This observation underscores the stochastic nature of these experiments. It is recommended that further studies characterize the oxide layer prior to exposure. This is vital to discern the influence environmental factors on the initiation and growth of whiskers.