Multiple reflow study of ball grid array (BGA) solder joints on Au/Ni metallization |
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Authors: | W H Zhong Y C Chan B Y Wu M O Alam J F Guan |
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Affiliation: | (1) Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, P. R. China;(2) Department of Mineral Processing, Wuhan University of Science and Technology, Wuhan, P. R. China |
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Abstract: | This paper evaluates the shearing behavior of ball grid array (BGA) solder joints on Au/Ni/Cu pads of FR4 substrates after
multiple reflow soldering. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with Sn–3Ag–0.5Cu (SAC) and Sn–37Pb
(SP) solders, in terms of fracture surfaces, shearing forces and microstructures. Three failure modes, ball cut, a combination
of solder shear and solder/pad bond separation, and pad lift, are assessed for the different solders and reflow cycles. It
is found that the shearing forces of the SP and SAC solder joints tend to increase slightly with an increase in the number
of reflow cycles due to diffusion-induced solid solution strengthening of the bulk solder and augmentation of the shearing
area. However, the shearing forces of the SACI solder joints decrease slightly after four cycles of reflow, which is ascribed
to the thermal degradation of both the solder/intermetallic compound (IMC) and IMC/Ni interfaces. The SACI solder joints yield
the highest strengths, whereas the SP solder joints give the smallest values, irrespective of the number of reflow cycles.
Thickening of the interfacial IMC layer and coarsening of the dispersing IMC particles within the bulk solders were also observed.
Nevertheless, the variation of shearing forces and IMC thickness with different numbers of reflow cycles was not so significant
since the Ni under layer acted as an effective diffusion barrier. In addition, the initially-formed IMC layer retarded the
further extensive dissolution of the pad material and its interaction with the solder. |
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