Transmission Electron Microscopy Characterization of Ni(V) Metallization Stressed Under High Current Density in Flip Chip Solder Joints

Abstract

The Ni(V) under bump metallization (UBM) in flip chip solder joints is known to be consumed in a two-stage process during current stressing. The Ni(V) UBM transforms first to the ''consumed Ni(V)'' state. Then, this consumed Ni(V) transforms to a so-called porous structure. In this study, the details of the consumed Ni(V) and the porous structure were analyzed by transmission electron microscopy. Bright-field images showed that the consumed Ni(V) was a continuous layer without columnar structures and that the porous structure had many voids in the matrix. The compositional analyses showed that V atoms were immobile and trapped in the original Ni(V) layer. Cu and Sn atoms diffused into the original Ni(V) layer, and Ni atoms diffused outward. Selected-area diffraction patterns and high-resolution transmission electron microscopy of the porous structure showed that it was composed of an amorphous matrix with fine crystalline Cu 6 Sn 5 and VSn 2 phases.

Experimental Evidence for Formation of Ni-Al Compound in Flip-Chip Joints Under Current Stressing

“…Figure 3b shows the line scan results for each main element in this range (Al, Ni, V, Cu, Sn, and O, respectively). Owing to the consumption of Ni(V), as discussed in previous studies, [5][6][7][8] almost no Ni signal was detected within the porous structure. However, an abrupt Ni signal was detected at the location of the continuous layer described in the previous paragraph.…”

“…The original Ni(V) UBM first transforms into so-called consumed Ni(V), then this consumed Ni(V) transforms into a so-called porous structure. [5][6][7][8] The consumed Ni(V) has been termed ''white patch'' in some studies. 2 The porous structure was composed of an amorphous matrix and crystalline phases comprising very fine grains.…”

“…2 The porous structure was composed of an amorphous matrix and crystalline phases comprising very fine grains. 8 Figure 1c, d presents SEM images that show the microstructure of Ni(V) stressed for 150 h and 300 h, respectively. These images illustrate the morphologies of the consumed Ni(V) and the porous structure under SEM observation.…”

“…1,2 The Al/Ni(V)/Cu trilayer structure is one of the most popular under bump metallurgy (UBM) designs in modern flip-chip packages. [1][2][3][4][5][6][7][8][9] During assembly processes, solder joints undergo several reflow cycles and the entire Cu layer in Al/Ni(V)/Cu dissolves into the solder. A layer of Cu 6 Sn 5 intermetallic compound (IMC) is simultaneously formed between the exposed Ni(V) and the solder.…”

“…A layer of Cu 6 Sn 5 intermetallic compound (IMC) is simultaneously formed between the exposed Ni(V) and the solder. [1][2][3][4][8][9][10] Subsequently, the solder begins to react with Ni(V) and the consumption of Ni(V) commences. Several studies on the interfacial reactions between Al/Ni(V)/Cu and solders have been reported in the literature.…”