A New Mechanism for Hillock Formation over Electrodeposited Thin Tin Film Jing Cheng 1 , Samuel Chen 2 , Paul Vianco 3 and James C.M. Li 1 1 Materials Science Program, University of Rochester, Rochester, NY 14627 2 Eastman Kodak Company, Rochester, NY 14559 3 Sandia National Laboratories, Albuquerque, NM 87185 Email:jicheng@me.rochester.edu, Phone:1-585-489-5302 Abstract Tin film about one micron thick was electroplated over a silicon wafer pre-coated with a layer of Cr and another layer of Ni by evaporation. A special sample holder was designed to apply compressive stresses in the electroplated tin film. After incubation in a vacuum oven at 160 o C for 7 days, huge hillocks about 10-30µm diameter and 30-150µm length grew with a density about 7 per square mm. On the top of the hillocks there appeared a polycrystalline layer similar to the tin film. FIB technique was used to reveal the inner microstructure of these huge hillocks. They are single crystals of Sn with [001] direction perpendicular to the film. The flow path of tin atoms appears to originate from the stressed Sn/Ni interface moving along the interface radially toward the root of the hillock. Key Words: tin whiskers, tin hillocks, growth mechanism, flow path, FIB Introduction In microelectronic industry, tin whiskers growing from tin coatings has been an important problem for many years. The literature was reviewed recently by Galyon and Palmer [1]. The problem was mitigated by electroplating tin-lead (Sn with 3-10% Pb by weight) alloys instead of pure tin by Arnold [2] in1959 even though to this day we still do not know clearly why it worked. However in recent years, some environmental related regulations such as Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) listed Pb as a toxic element. In fact Europe has not allowed lead-containing solders since July 1, 2006. So this old problem of tin whiskers surfaced again. Some landmark discoveries were as follows: Koonce and Arnold in 1954 [3] showed that the growth was from the base and not from the top by observing that the tip morphology was unchanged while the whisker grew longer. Fisher et al. [4] in 1954 found that a compressive stress of 52 MPa could accelerate the whisker growth to 1 micron per sec at room temperature and the rate was too fast for lattice diffusion. Baker [5] in 1957 showed that the base of the whisker was usually a grain boundary or an interface boundary inconsistent with a dislocation mechanism. Lindborg [6] in 1975 used x-ray to determine stresses in electroplated Zn films and showed that a minimum stress level was required for the initiation of whisker growth. Not much activity appeared in the 80’s and 90’s. Tu [7] in 1994 suggested that the oxide layer surrounding the whisker must be broken to allow the whisker to grow. Lee and Lee [8] in 1998 measured the residual stresses in Sn film on Cu substrates and found 11MPa tensile as deposited which changed to -8 MPa compressive after storage about 5 days. They also determined the thickness effect, 60 whiskers per mm 2 for a thickness of 1 micron decreased continuously to 20 whiskers per mm 2 for a thickness of 4.5 microns. In 2001 Zhang et al [9.10] used FIB and found that the root of the whisker was close to intermetallic particles projecting upwards from the tin/substrate interface. Later [11,12] they studied the effect of a Sn/Ni interface suggesting that Sn diffuses into Ni faster than Ni into Sn. Choi et al [13,14] used micro-focus x-ray from a synchrotron facility and found that the stress surrounding the whisker area was more compressive than the whisker area and the whisker grain had a different orientation (210) than the surrounding grains (321). Boettinger et al. [15] measured and analyzed the residual stress development in Sn, Sn-Cu and Sn-Pb layers electrodeposited on phosphor bronze cantilever beams and observed whisker and hillock formation. Tu and Li [16] introduced the idea that grain boundary fluid flow may be faster than grain boundary diffusion. This idea comes from a comparison between self diffusion and viscous flow in liquids [17]. More recently Woodrow [18] did a delicate experiment by plating one layer of isotope tin 118 on brass coupon, then covered by another layer of isotope tin 120 . The area of tin 120 was larger than that of tin 118 , so in the middle there were two layers surrounded by a single layer of tin 120 . The two layers share the common columnar grains so there were no grain boundaries between the two layers. After some time, in the whiskers grown in the single layer area, tin 118 was also found there. Since almost all the grain boundaries were perpendicular to the surface, the diffusion of tin atoms must have taken place along the interface of copper and tin unless the atoms could move above or under the oxide layer which appeared unlikely. Williams et al. [19] electrolytically deposited 15 micron Sn and, in another experiment, Sn (1.4-3.7w/oCu) Cu alloy onto a W substrate and found hillocks on Sn layers and whiskers on Sn-Cu layers. The purpose was to show that it is not necessary to form intermetallic compounds with the substrate for whisker formation. But they did not rule out the possibility of residual stresses caused by electroplating. Since the driving force is generally accepted as a stress gradient around the root of a whisker, we designed a device in which a known compressive stress can be applied mechanically by bending a silicon wafer coated with tin. To show whether the interface could be a path for tin atoms we deposited only one grain layer of tin to eliminate all the horizontal grain boundaries. 978-1-4244-2231-9/08/$25.00 ©2008 IEEE 472 2008 Electronic Components and Technology Conference