Effects of Electroplating Additives on the Interfacial Reactions between Sn and Cu Electroplated Layers

Abstract

Three combinations of additives based on polyethylene glycol (PEG), Cl − ions, bis(3-sulfopropyl)disulfide (SPS), and Alcian Blue (ABPV) were used to fabricate the Cu plated layer. The additive combination was found to play an important role in the microstructural evolution of the Cu plated layer. The PC (PEG + Cl − ) case produced the Cu plated layer with grains of a smaller size and random morphology, while the PCS (PEG + Cl − + SPS) and PCSA (PEG + Cl − + SPS + ABPV) cases produced the Cu plated layers with grains of a larger size and faceted morphology. Experimental results based on metallurgical examination and trace element analysis indicated that the interfacial reactions between Sn and Cu plated layers strongly depended upon the additive combination used in the Cu plating solution, and the dependence was reasonably correlated with the grain morphology of the Cu plated layer combined with the incorporation of organic impurities.Cu is commonly used for conductors and metallizations in printed circuit board (PCB) and Si integrated-circuit (IC) chip due to its superior electrical conductivity and good wettability with solder materials. Sn is extensively used in microelectronic solders due to its suitable melting point, efficient mechanical property, and low cost. So, the Sn/Cu joints massively exist in the microelectronic products. To form a Sn/Cu joint, reflow is conducted during which the Sn melts and wets the Cu substrate. The intermetallic compounds (IMC) are usually formed as a result of the interfacial reaction between Sn and Cu. Related investigations have been extensively performed to provide helpful information for reliability evaluation of solder joints. [1][2][3][4][5][6][7] The Cu conductor and metallization on PCB and Si chip can be fabricated using various methods, among which electroplating is widely used due to economic consideration. Electroplating is an electrochemical process involving complicated interaction between various additives used in the Cu plating bath, such as suppressor, accelerator, and so on. These additives interacted with each other on the Cu surface and inevitably were partially incorporated into the Cu plated layer during electroplating. The incorporation of organic impurities in the Cu plated layers during plating was found to have significant effects on the interfacial reactions between Sn-based solder and Cu plated substrate. [8][9][10][11][12][13] The S residue of about 17 ppm was detected in the Cu plated layer when a high level (3 × 10 −5 M) of accelerator, SPS (bissodium sulfopropyldisulfide, C 6 H 12 O 6 S 4 Na 2 ), was added in the Cu plating bath. 8 Large amounts of void were formed at the Cu/Cu 3 Sn interface along with the solder/Cu interfacial reaction during 150 • C aging, which was attributed to the reduction of free energy barrier for void formation due to interfacial segregation of S. Voids were continuously formed during aging and merged into several gaps within the Cu-Sn IMCs, forming an alternating layered structure composed of gaps and IMCs at the Sn-3...

Impurity Effects in Electroplated-Copper Solder Joints

“…Wu et al [30] found that the addition of only suppressor (PEG + Cl − ) in the plating bath originally containing basic electrolyte (H 2 SO 4 and CuSO 4 ) resulted in a high level of impurity incorporation in the electroplated Cu layer. Addition of accelerator (SPS), together with the suppressor, remarkably reduced the impurity concentration by two orders of magnitude.…”

“…Recently, our group published a series of papers exploring the effects of additive formulas on the impurity level in the Cu-plated layer and the interfacial reactions in the Sn/Cu joints [18,30,31]. First, three additive formulas were tested including PC, PCS, and PCSA (A is an abbreviation of leveler "ABPV") with a plating current density of 32 A/ft 2 (34.44 mA/cm 2 ) [30].…”

“…First, three additive formulas were tested including PC, PCS, and PCSA (A is an abbreviation of leveler "ABPV") with a plating current density of 32 A/ft 2 (34.44 mA/cm 2 ) [30]. As shown in Figure 5, the PC-based plating solution produced a Cu layer whose interior grain morphology was irregular.…”

“…This also implies that the use of a low level of SPS (2 ppm) causes a negligible impurity incorporation in the Cu-plated layer (Figure 5d). Our group also investigated the interfacial reactions between solder and three plated Cu substrates subjected to thermal aging at 200 • C [30]. As shown in Figure 6, a common Cu 6 Sn 5 /Cu 3 Sn dual-layer structure without noticeable voids was formed at the interfaces where the Cu substrates were electroplated using the formulas of PCS and PCSA (Figure 6b,c).…”

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

“…The impurities incorporated in the Cu lms played the roles to produce plenty of vacancies nucleating at the SAC305/Cu C interface. Segregation of the impure species or their derivatives in the grain boundaries might also annihilate the vacancy sinking sites which accelerated the accumulation of vacancies to form voids [14][15][16]. Severe void propagation gave rise to the formation of continuous crevices or cracks which was a kind of volumetric defects in crystal caused by void nucleation during annealing [17,18].…”