Effect of Copper Grain Size on the Interfacial Microstructure of a Sn/Cu Joint

Abstract

For pursuing a more comprehensive mechanism of void formation at the Sn/Cu interface, different grain size Cu samples with and without twinned boundaries are electrodeposited for the Sn/Cu joint and the IMC analysis. The formation of voids in the Sn/Cu joint is divided into three parts for discussion: Cu diffusion, Sn diffusion, and impurity diffusion. For the void formation at the Cu/Cu3Sn interface, the concept of the mechanism is from the Kirkendall effect, which is specially focused on the speed difference of Cu diffusion between at the Cu grain boundary and in the copper lattice in this study. The speed difference caused the vacancy formation. Impurities also play an important role in promoting the vacancy aggregation from a cluster to a void. Whether it is a diffusion effect or an impurity effect, it occurs at a Cu grain boundary. The voids would be formed when the density of the Cu grain boundary is high. The grain size of Cu is inversely proportional to the density of the Cu grain boundary; as a result, Cu grain size strongly affects the interfacial microstructure of a Sn/Cu joint, that is, the larger the Cu grain size is, the less the void at the Sn/Cu interface will be formed.

Effects of impurities on void formation at the interface between Sn-3.0Ag-0.5Cu and Cu electroplated films

“…Therefore, the microstructure of Cu B was similar to that of Cu C, but they were dissimilar to that of Cu A without the columnar grains. Some articles emphasized that the microstructures of the Cu lms signi cantly induced or suppressed the Kirkendall effect [7,8,12]. Among them, the smaller grain size in a Cu lm provided numerous grain boundaries for Cu fast diffusion into the Sn-rich solder [7,13], and the grain boundaries between columnar grains were the paths for vacancy aggregation on the Cu surface [8].…”

“…Some articles emphasized that the microstructures of the Cu lms signi cantly induced or suppressed the Kirkendall effect [7,8,12]. Among them, the smaller grain size in a Cu lm provided numerous grain boundaries for Cu fast diffusion into the Sn-rich solder [7,13], and the grain boundaries between columnar grains were the paths for vacancy aggregation on the Cu surface [8]. They enhanced the Kirkendall effect at the Sn-rich solder/Cu interface unless Cu nanotwin existed in the Cu lms [8,12].…”

“…Some studies have been dedicated to decreasing the level of the impurity incorporation in electroplated Cu to suppress the void formation through the optimization of additive formulation [1,4,5]. On the other hand, plenty of studies presented the void formation at the interfaces of solder joints could be suppressed by constructing the Cu microstructures with coherent twin boundary or larger grain size [6][7][8]. These speci c microstructures enabled to block the paths for vacancy accumulation, making voids hardly to nucleate.…”

Effects of Impurity on Void Formation at Interface between Sn-3.0Ag-0.5Cu and Cu Electroplated Film

“…3b and c. Therefore, the microstructure of Cu B was similar to that of Cu C, but they were dissimilar to that of Cu A without the columnar grains. Some articles emphasized that the microstructures of the Cu lms signi cantly induced or suppressed the Kirkendall effect [7,8,12]. Among them, the smaller grain size in a Cu lm provided numerous grain boundaries for Cu fast diffusion into the Sn-rich solder [7,13], and the grain boundaries between columnar grains were the paths for vacancy aggregation on the Cu surface [8].…”

Significant Hall–Petch effect in micro-nanocrystalline electroplated copper controlled by SPS concentration

“…In contrast, organic additives added to the electroplating solutions are vital in controlling the deposition rate of reduced Cu atoms and microstructures of the electroplated Cu. For instance, some additives in the plating solutions can be used to fabricate Cu films with nanotwin structures to enhance their electricity, strength, and void suppression 5 , 8 , 9 . One of the additives is the chloride ion (Cl − ) from NaCl or HCl, which increases the reduction rate of Cu ions 10 .…”