Cross interactions on interfacial compound formation of solder bumps and metallization layers during reflow

Abstract

While the dimension of solder bumps keeps shrinking to meet higher performance requirements, the formation of interfacial compounds may be affected more profoundly by the other side of metallization layer due to a smaller bump height. In this study, cross interactions on the formation of intermetallic compounds (IMCs) were investigated in eutectic SnPb, SnAg3.5, SnAg3.8Cu0.7, and SnSb5 solders jointed to Cu/Cr–Cu/Ti on the chip side and Au/Ni metallization on the substrate side. It is found that the Cu atoms on the chip side diffused to the substrate side to form (Cux,Ni1−x)6Sn5 or (Niy,Cu1−y)3Sn4 for the four solders during the reflow for joining flip chip packages. For the SnPb solder, Au atoms were observed on the chip side after the reflow, yet few Ni atoms were detected on the chip side. In addition, for SnAg3.5 and SnSn5 solders, the Ni atoms on the substrate side migrated to the chip side during the reflow to change binary Cu6Sn5 into ternary (Cux,Ni1−x)6Sn5 IMCs, in which the Ni weighed approximately 21%. Furthermore, it is intriguing that no Ni atoms were detected on the chip side of the SnAg3.8Cu0.7 joint. The possible driving forces responsible for the diffusion of Au, Ni, and Cu atoms are discussed in this paper.

The Influence of Current Direction on the Cu-Ni Cross-Interaction in Cu/Sn/Ni Diffusion Couples

“…[1][2][3][4][5][6][7][8][9][10][11] It was recently reported that both Cu and Ni atoms can diffuse across the entire solder (hundreds of microns) to opposite sides of the sandwich structure in a very short time (a few seconds) during soldering. [1][2][3][4][5][6][7][8] The diffusion of Cu to the Ni-side can form a single layer of (Cu,Ni) 6 Sn 5 or a bilayer of (Cu,Ni) 6 Sn 5 and (Ni,Cu) 3 Sn 4 over the Ni pad, 1-8 resulting in a brittle solder/pad interface. 6,12 Another potential reliability concern regarding this interaction is the rapid consumption of the Cu pad.…”

Cross-Interaction Between Au/Sn and Cu/Sn Interfacial Reactions

“…[4][5][6][7][8] For example, in a typical Cu/Sn/Ni joint system, a cross-interaction between interfacial Sn/Cu and Sn/Ni reactions occurs. The Cu atoms diffuse across the molten solder ball to form a ternary (Cu,Ni) 6 Sn 5 compound layer on the Ni interface.…”

Effect of Interfacial Reactions on the Reliability of Lead-Free Assemblies after Board Level Drop Tests

“…The small amount of Ni was apparently from the upper component/solder interface through a coupling effect, which was extensively discussed in our previous investigations 20, 21 and those of others. 22,23 In the case of ENIG, a needle-like (Ni,Cu) 3 Sn 4 layer with about 9 at.% Cu content is observed at the solder/pad interface, and a dark, thin, P-rich layer is formed between the Ni layer and IMC. The two cases differ from each other in many aspects (IMC types, interfacial microstructure and solder microstructure); however, different types of IMCs seem to be the major reason leading to the different performances under drop tests.…”