Factors Controlling the Defect Formation in Cu-Sn Micro Joints in 2.5D and 3D Interconnects

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The Cu-Sn reaction upon soldering has been thoroughly studied and the sporadic formation of Kirkendall voids within the intermetallic bonds between ball grid array (BGA) or surface mount technology (SMT) solder joints and Cu pads has been demonstrated as well-known concern. However, in the realm of ever developing device miniaturization, there is very limited information on the impact of space confinement on the dynamics of the Cu/solder joint interface. It is known that no voiding has been found in solder joints with high-purity Cu but this phenomenon presents itself, albeit to a different extent, when the joint is realized with electroplated Cu. Also, it is believed, that the voiding propensity in the Cu3Sn intermetallic compounds (IMC) is more likely to be higher in micro joints than in BGA/CSP scale (macro) joints for a given quality of electroplated Cu. Along with that it has been shown that the IMCs on Cu surfaces seem to grow much faster when the solder becomes very thin, and this is accompanied with a greatly enhanced risk of Kirkendall void nucleation.The emphasis of this work is on understanding the voiding behavior within the intermetallic bonds of Cu-Sn micro joints (5 – 30 µm) with the eventual goal of learning how it can be controlled or prevented. Our systematic studies have corroborated the association of voiding with the incorporation of small concentrations of impurities resulting from organic additives degradation during the Cu electroplating as already illustrated for macro joints. Our results also show lack of any voiding not only in macro- but also in micro joints when the interconnection is realized with high purity Cu, likely because no organic impurities are present in sufficient amount to prompt defect nucleation and growth. Our research further demonstrates that the IMC growth rate varies with factors and parameters such as solder volume to pad area ratio, solder composition, pad finish, initial assembly process parameters, subsequent heat treatments, and specifics of the further thermomechanical history, thus impacting differently the overall voiding level. Following this lead, the analysis of micro joints subjected to different synthetic and processing treatments clearly demonstrates the propensity for voiding to be substantially higher, for a given quality of electroplated Cu, in very small solder joints (5 – 30 µm), such as those found in 3D assemblies, unlike in macro joint counterparts. The risk and severity of voiding in these thinner joints is confirmed to depend on the type and amount of incorporated organic impurities in the electroplated Cu. Therefore, in the present report we show how the bath additive chemistry and various Cu electroplating parameters play a systematic role in the type and quantity of impurity incorporation, and thus the propensity for voiding. Approaches to control the electroplating Cu quality in a way that leads eventually to void-free micro joints are also discussed.