During soldering and solder reflow process, the surface finish layer on the PCB laminate
reacts and dissolves into the molten solder and the formation of intermetallic compounds
(IMC, hereafter). This interfacial reaction would significantly influence the mechanical
properties of the solder joint. Moreover, in the extreme case, when the solder joint is
subjected to multiple reflow cycles, it would exhibit a drastic change in its microstructure
and mechanical properties due to interfacial reactions. Hence the choice of a suitable
surface finish is crucial to control the interfacial reactions.
In this study, the effects of surface finish layer on the BGA (Ball Grid Array) solder joints
have been investigated as a function of the reflow cycle. A Pb-free solder alloy, Sn-
3.8Ag-0.7Cu (in wt%) has been employed as the solder ball material. Five types of
surface finish combinations including Cu-Cu, Cu-Au/Ni(P), Au/Ni-Au/Ni(P),
Au/Pd/Ni(P)-Cu, and Au/Pd/Ni(P)-AuPd/Ni(P) on opposite ends of the BGA balls, have
been investigated. IMC growth was measured as a function of reflow cycles for each
surface finish combination. It was found that when Cu was used at least on one side, the
IMC grew much faster than those having just the Ni layer. The hardness in the BGA
solder ball, was affected mainly by the choice of surface finish, not so much by the
reflow cycles. The BGA solder joints having a Ni layer on both sides of the BGA balls
showed a higher hardness value than that without a Ni layer. The microstructure of the
solder joints varied with the number of reflow cycle and a variance of surface finish. The
b-Sn dendrite density and its size appear to affect the hardness value. It appears that the
characteristics of the BGA solder joints depend to a great deal on the choice of a surface
finish layer due to its interfacial reactions with the solder ball.
By: W. K. Choi, S. K. Kang, D.-Y. Shih, P. Lauro, D. Henderson, T. Gosselin, D. N. Leonard
Published in: RC22160 in 2001
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