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The Formation of Whiskers on Electroplated Tin Containing Copper
November 21, 2005 |Estimated reading time: 1 minute
K.-W. Moon, M. E. Williams, C. E. Johnson, G. R. Stafford,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
<?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" />C.A. Handwerker, and W. J. Boettinger
Metallurgy Division, MSEL, NIST, Gaithersburg, MD20899-8555, USA
The probability of whisker growth on as-grown tin(Sn) electrodeposits has been measured as a function of copper(Cu) addition to a commercial bright methanesulfonate electrolyte. To provide reproducible plating conditions and to approximate flow conditions in commercial strip plating, a rotating disk electrode assembly was used. A fixed plating current at 25 *C was used to produce deposits 3 m and 10 m thick.
Two substrates were used: free standing 250 m thick pyrophosphate Cu deposits and 40 nm thick fine grain Cu evaporation deposited on silicon(Si) (100) wafer. Electrolyte Cu2+ concentrations from 0 to 25 x 10-3 mol/L produced deposits with average Cu compositions between 0 % and 3.3 % mass fraction, respectively. In the absence of Cu additions, no whiskers were observed on either substrate after 60 days of room temperature aging. With Cu additions, no whiskers were observed on the Cu-coated Si substrates, but whiskers were observed within two days on the pyrophosphate Cu substrates increasing to a density of 102/mm2 for the highest Cu contents. The deposits on the pyrophosphate Cu and Cu evaporated Si(100) substrates had different preferred orientations: (103) and (101) respectively, but the effect of Cu on the deposit microstructure was the same. The increase in Cu content reduced the Sn grain size from 0.65 m to 0.2 m in the deposits independent of substrate. Much of the Cu in the deposits occurred as Cu-rich particles on the Sn grain boundaries. The primary correlation observed was that Sn-Cu deposits with (103) preferred orientation were most likely to form whiskers.