Effects of Whiskers on Nanocrystalline Copper

David M. Lee, Lesly A. Piñol
Johns Hopkins Applied Physics Laboratory Laurel, MD USA

Spontaneously forming tin whiskers, which emerge unpredictably from pure tin surfaces, have regained prevalence as a topic within the electronics research community. This has resulted from the ROHS-driven conversion to "lead-free" solderable finish processes. Intrinsic stresses (and/or gradients) in plated films are considered to be a primary driving force behind the growth of tin whiskers.

This paper compares the formation of tin whiskers on nanocrystalline and conventional polycrystalline copper deposits. Nanocrystalline copper under-metal deposits were investigated, in terms of their ability to mitigate whisker formation, because of their fine grain size and reduced film stress. Pure tin films were deposited using matte and bright electroplating, electroless plating, and electron beam evaporation. The samples were then subjected to thermal cycling conditions in order to expedite whisker growth. The resultant surface morphologies and whisker formations were evaluated.

Nanocrystalline copper as a whisker mitigating underlayer for pure tin finishes had mixed results after exposure to thermal cycling. The whisker density was reduced when tin was deposited by evaporation or bright electroplating. Whiskers were eliminated entirely when electroplated with bright tin on deposits with an average grain size of 100nm (Nano B) . Matte electroplated films did not whisker on polycrystalline copper, but whiskered moderately on Nano B and profusely on deposits with an average grain size of 50nm (Nano A) and 200nm (Nano C).

Electroless plated films were not found to whisker under any circumstance. This may be a result of the very porous deposit of the electroless tin at a thickness of 1 micron. The lack of stress in the porous deposit may have reduced the potential for whiskering.

Lead-free solder is a reality in today‟s microelectronics industry, and poses many challenges, the principal of which is the ubiquitous nature of tine whiskers. Since lead-free solder has the potential to perform well for ultra fine-pitch substrates, given its superior wettability and decreased susceptibility to "bridging" over eutectic tin-lead solder, it is critically important that a robust whisker mitigation strategy be developed.