Spalling of intermetallic compounds during the reaction between lead-free solders and electroless Ni-P metallization
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S.K. Kang and D.Y. Shih IBM T.J. Watson Research Center, P.O. Box 218/Route 134, Yorktown Heights, New York 10598
T.Y. Lee Department of Materials Science and Engineering, Hanbat National University, San 16-1, DukMyoung-Dong, Yuseong-Gu, Deajeon 305-764, Korea (Received 10 November 2003; accepted 6 May 2004)
Intermetallic compound (IMC) spalling from electroless Ni-P film was investigated with lead-free solders in terms of solder-deposition methods (electroplating, solder paste, and thin foil), P content in the Ni-P film (4.6, 9, and 13 wt% P), and solder thickness (120 versus .200 m). The reaction of Ni-P with Sn3.5Ag paste easily led to IMC spalling after 2-min reflow at 250 °C while IMCs adhered to the Ni-P layer after 10-min reflow with electroplated Sn or Sn3.5Ag. It has been shown that not only the solder composition but also the deposition method is important for IMC spalling from the Ni-P layer. The spalling increased with P content as well as with solder volume. Ni3Sn4 intermetallics formed as a needle-shaped morphology at an early stage and changed into a chunk-shape. Needle-shaped compounds exhibited a higher propensity for spalling than chunk-shaped compounds because many channels among the needle-shaped IMCs facilitated Sn penetration. A reaction between the penetrated Sn and the Ni3P layer formed a Ni3SnP layer and Ni3Sn4 IMCs spalled off the Ni3SnP surface. Dewetting of solder from the Ni3SnP layer, however, did not occur even after spalling of most IMCs.
I. INTRODUCTION
Solder bumps in flip-chip technologies of microelectronic devices is an interconnection for electrical data and thermal energy dissipation from an IC chip to a substrate.1 The substrate in microelectronics packaging is usually organic PCB (Print Circuit Board), or LTCC (Low Temperature Cofired Ceramic).2,3 The solder joint between an IC chip and a substrate is a key interconnection for the electrical performance and reliability of an electronic system. A reliable solder joint can be formed by a metallurgical reaction between a molten solder and under-bump metallization (UBM) on a chip or metallization on a substrate, which produces stable IMCs at the joint interfaces.4,5 Pb-containing solders have been extensively used in microelectronic interconnections. Recently, the industry is searching for Pb-free solders to replace Pb-containing solders for environmental reasons as well as consumers’ desire for green products. Most of
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0297 2428
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 8, Aug 2004 Downloaded: 14 Mar 2015
Pb-free solders are Sn-based alloys such as Sn-3.5Ag, Sn-3.8Ag-0.7Cu, Sn-0.7Cu, and others (all in wt% unless specified otherwise). The interfacial reactions between Pb-free solders and UBM have become a critical issue, because both the Sn content in the solders and their reflow temperatures are higher than the conventional PbSn eutectic solder.6–12 The previous studies on the interfacial reactions between hi
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