Equi-Axed Grain Formation in Electrodeposited Sn-Bi
E Sandnes, Maureen E. Williams, Mark D. Vaudin, Gery R. Stafford
Sn is widely used as a coating in the electronics industry because it provides excellent solderability, ductility, electrical conductivity and corrosion resistance. However, Sn whiskers have been observed to grow spontaneously from Sn electrodeposits and are known to cause short circuits in fine pitch pre-tinned electrical components. We report here a deposition strategy that produces an equi-axed and size-tunable grain structure in Sn-Bi alloys. An equi-axed grain structure should allow a more uniform creep to relieve compressive stress with no localized surface disturbance. The standard potential for Bi is about 0.45V more positive than Sn. Pulsed deposition can selectively turn on and off the Sn deposition reaction. During the off-cycle, a displacement reaction between metallic Sn on the electrode surface and Bi(superscript 3+) in solution selectively dissolves Sn and deposits Bi, effectively terminating the growth from the previous cycle and forcing the Sn to nucleate a new grain on the Bi-enriched surface. The grain size is tunable by varying the pulsing conditions, and an equi-axed structure can be obtained with as little as 3% atomic fraction Bi. This surface enrichment of Bi by potential modulation is similar to that which occurs naturally in Sn-Pb, and provides an avenue for breaking up the columnar grain structure inherent to pure Sn, thus eliminating a diffusion path for Sn whisker growth.
, Williams, M.
, Vaudin, M.
and Stafford, G.
Equi-Axed Grain Formation in Electrodeposited Sn-Bi, Journal of Electronic Materials, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=853522
(Accessed June 5, 2023)