Residual stress and interfacial reaction of the electroplated Ni-Cu alloy under bump metallurgy in the flip-chip solder joint

Abstract

Pure Ni, the Ni-Cu alloy, and pure Cu layers as the under bump metallurgy (UBM) for a flip-chip solder joint were deposited by electrolytic plating. For the pure Ni layer, residual stress can be controlled by adding a wetting agent and decreasing current density, and it is always under tensile stress. The Ni-Cu alloys of different Cu compositions from ϳ20wt.%Cu to 100wt.%Cu were deposited with varying current density in a single bath. The residual stress was a strong function of current density and Cu composition. Decreasing current density and increasing Cu content simultaneously causes the residual stress of the metal layers to sharply decrease. For the pure Cu layer, the stress is compressive. The Cu layer acts as a cushion layer for the UBM. The residual stress of the UBM strongly depends on the fraction of the Cu cushion layer. Interfacial reaction of the UBM with Sn-3.5 wt.% Ag was studied. As the Cu contents of Ni-Cu alloys increased, the dissolution rate increased. Several different intermetallic compounds (IMCs) were found. The lattice constants of alloys and the IMC increase with increasing Cu contents because the larger Cu atoms substitute for the smaller Ni atoms in the crystallites. The Cu content of the IMC are strongly dependent on the composition of the alloys. Ball shear tests were done with different metal-layer schemes. The failure occurs through the IMC and solder.

Sensitivity analysis of plating conditions on mechanical properties of thin film for MEMS applications

“…5 presents the dependence of the average residual stress in nickel films on the applied current density at a temperature of 40°C along the plating thickness. The residual stress in the given plating thickness increased as increasing current density, which is consistent with the result of other experiments [17,18]. It has been reported that the residual stress increased 160 to 220MPa when the current density increased 2 to 5mA/cm 2 in the nickel sulfamate plating condition at 50°C [17].…”

“…The residual stress in the given plating thickness increased as increasing current density, which is consistent with the result of other experiments [17,18]. It has been reported that the residual stress increased 160 to 220MPa when the current density increased 2 to 5mA/cm 2 in the nickel sulfamate plating condition at 50°C [17]. However, those reports did not investigate the effect of the plating thickness on the residual stress.…”

Kinetics of intermetallic compound formation in thermally evaporated Ag-In bilayers

“…15 Development of IMCs upon aging of solders at RT and at moderate operational temperatures can greatly deteriorate the electrical and mechanical properties of the solder joints. [16][17][18][19] It was also shown that IMC formation strongly affects the bond strength of Ag-In joints. 20,21 Consequently, fundamental knowledge of the kinetics of Ag-In IMC formation is of utmost importance in order to predict and optimize their long-term stability.…”

To inhibit the consumption of Cu during multiple reflows of Pb-free on Cu

“…It means that it will be very beneficial if we can inhibit the Cu-Sn reaction after chip-join in the first reflow. Indeed, several approaches have been proposed to inhibit the growth of the IMCs, including altering solder composition, and incorporating Ni into the Cu UBM [11][12]. Yet, none of them is effective.…”