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An Investigation of Diffusion Barrier Characteristics of Electroless Co(W,P) Layer to Lead-Free SnBi Solder
Keywords: diffusion barrier, lead-free, electroless Co(W, P)
This study investigated the interfacial reactions between SnBi solder and electroless Co(W,P) layer with various crystallinities by utilizing the liquid-state aging test (250aC for 1 hr) and solid-state aging test (120aC up to 1000 hrs) so as to explore the feasibility of electroless Co(W,P) to UBM for low-temperature, lead-free flip-chip bonding. In the sample containing SnBi/amorphous Co(W,P) subjected to liquid-state aging test, a P-rich layer containing Co, Sn, P elements formed in between solder and electroless Co(W,P) whereas the lath-like CoSn3 intermetallic compound (IMC) spalled into solder. The P-rich layer became thicker when the aging time was increased. For the samples subjected to solid-state aging test, a thick IMC layer formed in between solder and Co(W,P) without spallation and the P-rich layer was similar observed. In the samples containing SnBi/polycrystalline Co(W,P) subjected to liquid- and solid-state aging tests, an about 1 m-thick interfacial layer containing various elements including Co, Sn, P, W and Bi formed in between solder and Co(W,P). The needle-like CoSn3 IMCs interlarded with Bi element formed on Co(W,P) without spallation. In conjunction with the analytical results obtained in the case of SnBi/amorphous Co(W,P), it is concluded that, regardless of its crystallinity, electroless Co(W,P) layer is in fact a sacrificial-type barrier to SnBi solder owing to the formation of P-rich layer.
Hung-Chun Pan, Ph.D Candidate
National Chiao Tung University
Hsinchu, Taiwan 30049,

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