Correlation between Ni 3 Sn 4 intermetallics and Ni 3 P due to solder reaction-assisted crystallization of electroless N
- PDF / 403,750 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 14 Downloads / 195 Views
Ni3Sn4 intermetallic was formed by the depletion of Ni from electroless Ni–P, and a Ni3P layer was formed simultaneously due to solder reaction-assisted crystallization during solder reflow. Both Ni3Sn4 and Ni3P grew rapidly due to the solder reaction-assisted crystallization and their growth was diffusion controlled during the first 15 min of annealing at 220 °C. After that, the growth rate of Ni3Sn4 was greatly reduced and the crystallization of electroless Ni–P to Ni3P was no longer induced. Based on kinetic data and scanning electron microscope morphology observations, underlying mechanisms causing this specific phenomenon are proposed. This finding is indeed very crucial since we may control the growth of Ni–Sn intermetallics by monitoring the solder reaction-assisted crystallization of electroless Ni–P.
I. INTRODUCTION
In the near future, conventional peripheral leaded packages are not expected to be able to handle high pin count integrated circuits (ICs), so the use of area array packaging technology is expected to increase. To accommodate a large number of input/output (I/O) counts, area arrays of solder bumps are being used in the controlled collapsed chip connection (C4) configuration for chip joints, and ball grid arrays (BGA) are being used for lower level joints.1–3 Copper (Cu) is widely used in the under-bump metallization (UBM) and substrate metallization for flip chip and BGA applications. At the liquid lead–tin solder/Cu interface, tin (Sn) reacts rapidly with Cu to form Cu–Sn intermetallic compounds (IMC), which weakens the solder joints due to the brittle nature of the IMC.4–6 Tu et al. have found that a Cu-based UBM is incompatible with eutectic Pb–Sn solder due to the spalling of Cu–Sn intermetallic compounds.7–9 Nickel (Ni) has been proposed to replace Cu or act as a diffusion barrier in the Au/Cu metallization because Ni–Sn compounds show a very slow IMC growth rate10 and because Ni has a relatively low diffusion rate through Au and Cu.11–12 Electroless nickel-phosphorus (Ni–P) has attracted much interest and finds widespread use in printed circuit board (PCB) fabrication and UBM for flip chip technology since it offers a low-cost alternative to more expensive physical Ni deposition methods, good corrosion resistivity, strong adhesion, and good solder wetting.13–15 Normally, the electroless Ni–P deposit is crystalline at lower P contents (9.5 at.%) the deposit is an amorphous phase.16 It has 2534
http://journals.cambridge.org
J. Mater. Res., Vol. 15, No. 11, Nov 2000 Downloaded: 20 Mar 2015
been found that the amorphous Ni–P alloy will undergo a self-crystallization transformation to Ni and Ni3P at temperatures above 300 °C.17 Recently, Jang et al.14 have found that the solder reaction will assist crystallization of electroless Ni–P UBM in flip chip packages even if the reflow temperature is well below the selfcrystallization temperature. The crystallization of electroless Ni–P to Ni3P is induced by the depletion of Ni from electroless Ni–P by the formation of Ni3Sn4. In this work, we stud
Data Loading...
