Dissolution and Interface Reactions between Palladium and Tin (Sn)-Based Solders: Part I. 95.5Sn-3.9Ag-0.6Cu Alloy
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THE development of a lead (Pb)-free soldering technology must consider processing and reliability issues associated with interconnections made to alternative surface finishes.[1,2] Such finishes included electroplated nickel (Ni)–electroplated gold (Au) coatings for component leads and terminations as well as electroless Ni–immersion Au, or ENIG, for printed circuit board finishes.[3,4] Immersion silver (Ag) coating has received increased interest for printed circuit boards.[5–7] A candidate Pb-free surface finish is nickel-palladium (Ni-Pd). The Ni-Pd coating was first introduced on leaded packages.[8,9] Nickel is the solderable finish; Pd is the protective layer. The commercial finish is built up from the following layers: 0.1 lm Ni, 0.01 lm Ni-Pd, 1.2 lm Ni, and last 0.1 lm Pd. An electroless Ni-Pd finish has been developed for printed circuit board applications with the following layers: 0.01 lm Ni-Pd/ 1.3 to 2.0 lm Ni/0.1 to 0.5 lm Pd.[10] When first introduced on component leads, the Pd layer often experienced a slight loss of solderability, particularly after aging under elevated temperature and PAUL T. VIANCO, Distinguished Member, Technical Staff, JEROME A. REJENT and GARY L. ZENDER, Technologists, and PAUL F. HLAVA, Contractor, are with Sandia National Laboratories, Albuquerque, NM 87185. Contact e-mail: ptvianc@ sandia.gov Manuscript submitted June 12, 2009. Article published online September 17, 2010 3042—VOLUME 41A, DECEMBER 2010
humidity conditions.[11] Takao and Hasegawa investigated this solderability loss through laboratory tests by measuring the contact angle, wetting time, and wetting rate parameters.[12] Those authors confirmed that the Pd surface was responsible for the effect. Laboratory wetting angle tests by Wang et al. determined that, intrinsically, a pristine Pd surface exhibits excellent solderability with molten Sn-Pb solder.[13] Therefore, the solderability loss was caused by the formation of a thin palladium-oxide layer on the Pd surface.[14] In order to mitigate the degradation of solderability caused by the palladium-oxide layer, a thin Au layer of 0.05 to 0.2 lm was added on top of Pd as a ‘‘second’’ protective coating. The Au layer is sufficiently thin so that it rapidly dissolves into Sn-based solders and does not pose an immediate embrittlement concern. The excellent solderability of a pristine Pd surface does not imply a rapid dissolution rate. Bader measured the dissolution rate of Pd in molten 60Sn-40Pb solder to be approximately one-half that of Cu.[15] Palladium has also been investigated with respect to its effect on the mechanical properties of soldered interconnections. Vaynman et al. investigated the effects of 0.5 wt pct Pd additions on the stress-strain, creep, and isothermal fatigue of eutectic Sn-Pb solder.[16] Tests performed at 298 K and 353 K (25 °C and 80 °C) showed no significant change to yield stress, creep exponent, and apparent activation energy, or fatigue resistance. There are some data that indicate that Sn-Pb solder joints made to Pd protective finish can experience a s
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