Stability of channels at a scalloplike Cu 6 Sn 5 layer in solder interconnections
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Dong Hyuk Shin Department of Metallurgy and Materials Science, Hanyang University, Ansan 425-791, Kyungki-Do, Korea (Received 2 November 2000; accepted 2 February 2001)
The thermodynamic stability of the solder channels at a scalloplike Cu6Sn5 layer formed between Sn-containing solders and Cu substrate was evaluated by studying the penetration behavior of the liquid solders into the grain boundaries of a Cu6Sn5 substrate. The orientational relationship between the grains of the Cu6Sn5 layer formed during reflow soldering was also analyzed using the electron backscattered diffraction technique. The results showed that liquid solders penetrate into the grain boundaries at an order of faster speed than the growth rate of the layer, which provided a direct evidence of thermodynamic stability of the channel.
The formation of intermetallic layers consisting of Cu6Sn5 and Cu3Sn phases at an Sn-containing solder/Cu substrate interface is an integral part of interconnections in electronic packages. Formation of a thin intermetallic compound (IMC) layer was reported to improve the joint strength by promoting the metallic bonding, but a thicker layer degrades the strength and wettability of the joint.1–5 Therefore, significant research efforts have been directed to understand and control the kinetics of the layer formation reaction.6 –17 The formation of the IMC layer takes place either between liquid solder and Cu substrate during the reflow soldering and/or between solid solder and Cu substrate during service of the packaged device. The kinetics of the later reaction studied via isothermal aging treatments were found to follow the conventional parabolic time dependence.6–9,14 In the former reaction, however, a layer with a large local variation in thickness, i.e., scalloplike Cu6Sn5 grains,10 –13,15–20 was found to follow a power law type soldering time dependence, of which exponent is in a range from 0.21 to 0.37.10 –12,15,18,22,23 In addition, channels filled with the solder were observed between the scalloplike grains. The deviation of the growth kinetics of the layer from the parabolic time dependence has been attributed to the presence of the channel.11 Bader10 and Tu et al.11 reported that deep channels down to Cu and Cu3Sn layers remain during the entire course of the reflow soldering. Other studies,19,21 however, indicated that the channels became clogged by the lateral growth of the grains, affecting the growth J. Mater. Res., Vol. 16, No. 5, May 2001
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kinetics of the layer. For the channel formation mechanism, Schaefer et al.15,16 suggested grain boundary wetting or grooving by liquid-phase solder. It is obvious from those previous studies that the stability of the solder channel during the reflow soldering is crucial for the growth of the IMC layer. In this study, the thermodynamic stability of the channel was investigated through a simulated experiment: melting of Sncontaining solders on the surface of a Cu6Sn5 intermetallic substrate. If the channel conf
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