We have used scanning electron microscopy to study the interfacial morphology of the Cu-Sn compounds formed between a eutectic SnPb alloy and Cu at 200 °C. A selective etching reveals the three-dimensional morphology of the Cu-Sn compounds. On the solder side, the compounds grow rapidly as big scallops and the interface becomes extremely rough as compared to the Cu side of the interface. In order to understand this rapid and extremely irregular growth of the Cu-Sn compounds, we propose that it is caused by the dissolution of Cu into the liquid solder and the coarsening of the scallop-type compounds by Ostwald ripening. The growth of the Cu-Sn compounds has a serious impact on solder joint rework in electronic packaging.
The growth and morphology of intermetallic compounds between the solder and substrate play an important role in the solderability and reliability of electronic solder joints. Solder on thin films, as in chip joint, acts as an electrical and mechanical/physical interconnection between the chip and the substrate. We have studied the interfacial reactions between eutectic SnPb (63Sn37Pb, wt%) and Cr/Cu/Au thin films. Our results found here have been compared to the solder reaction on bulk Cu. The eutectic solder has 7° of wetting angle on Cr/Cu/Au thin films rather than 11° on Cu substrate. Sideband around the solder cap was found in both the thin film case and the Cu case. Spalling of Cu6Sn5 compound grains occurred in the thin-film case when the Cu film was consumed but not in the case of bulk Cu. We observed a shape change from hemispherical ‘‘scallops’’ to spheroids before spalling took place. The shape change is assisted by ripening a reaction among the scallops. We have calculated a critical size of the scallop, depending on the Cu film thickness, when the shape change or spalling starts.
In reacting eutectic SnPb solder with Ti/Cu and Cr/Cu/Au thin film metallization on Si wafers, we have observed spalling of Cu6Sn5 spheroids when the solder consumes the Cu. The formation of the spheroids is assisted by the ripening reaction among the compound grains. In addition we have observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were soldered face-to-face. The spalling occurs predominantly at the interface at the bottom of the solder joint. It suggests that gravity plays a role.
On Au/Cu/Cr thin film surface, a drop of molten Sn first spreads out to wet the surface, but it then pulls back to dewet. The latter is due to the spalling of Cu–Sn compounds and exposing the Cr surface to the molten Sn when all of the Cu film has been consumed by the wetting reaction. Dewetting is clearly undesirable for solder joints in electronic packaging; the phenomenon is presented here.
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