As one of the earliest mature advanced packaging technologies, flip chip (FC) assembly achieves leadless packaging through bump interconnection. Owing to its excellent electrical performance, high I/O density and strong reliability, it is widely applied in the semiconductor packaging field. In 2.5D/3D packaging, the FC structure is the key to achieving vertical chip integration, such as the interconnection between the interposer and chips and PCB boards, as well as die-to-die (D2D) interconnection. Under thermal loading, a bare FC structure is prone to CTE failure; meanwhile, direct exposure to the atmosphere can also easily cause moisture corrosion. The underfill filling process greatly improves the reliability of the FC structure. Nowadays, the increase in chip size and the reduction in bump pitch pose challenges to the traditional capillary underfill filling (CUF) process. Common failures of CUF include delamination at heterogeneous interfaces, voids and cracks, all of which are related to insufficient underfill flow and wetting. In addition, when the bump pitch falls below a critical gap, the CUF filling time increases sharply. Improving the wettability of the underfill on the substrate can further reduce the critical gap. Plasma cleaning activation can effectively remove residual flux on the substrate, improve surface roughness and enhance surface free energy, thereby improving the wettability of the underfill and its adhesion to the interface.

Effect of Plasma Cleaning Activation on Underfill Packaging Performance

As mentioned above, plasma cleaning activation can improve the wettability of the underfill on the substrate surface beneath the chip, while in the actual FC packaging process, the actual filling effect of the underfill must be given attention. Figure 1 shows the macroscopic packaging morphology of Si/underfill/Si structures with and without plasma cleaning under the same filling time of 10 mins. It can be seen that in the sample without plasma cleaning activation treatment, overflowing underfill is visible at the chip edge, whereas after plasma activation treatment, the wettability of the underfill in the gap is significantly improved. Under the same dispensing volume and dispensing time, the volume of underfill overflowing at the edge of the untreated sample is macroscopically significantly larger than that of the plasma-treated group. This may be because the poor wettability between the underfill and the interface leads to an increase in underfill viscosity during dispensing, thereby reducing its fluidity. For a chip with length L, its capillary flow filling time tfill can be estimated by the following formula:

From Equation (1-1), tfill is proportional to the viscosity η of the underfill and the chip size L, and inversely proportional to the pressure difference Δp, the gap height h and the cosine of the contact angle θ. Since the underfill enters the gap between the chip and the substrate mainly through capillary action, plasma cleaning reduces the contact angle of the underfill, thereby increasing the pressure difference between the chip and the substrate. As a result, within the same time, the underfill can fill the surface beneath the chip more rapidly after plasma cleaning activation treatment.

Figure 1 Underfill filling and curing effects under different treatment processes: (a) untreated; (b) after plasma cleaning activation

Cross-sectional morphology analysis was conducted on Si/underfill/Si structures prepared after different plasma cleaning activation processes, and their SEM cross-sections are shown in Figure 2. It can be seen from the figure that the Si/underfill/Si structure without plasma cleaning activation treatment exhibited interface delamination during filling, with the delamination located mainly at the interface between the underfill and the upper dummy chip; in addition, submicron voids were observed inside the underfill. After plasma cleaning treatment, although a small number of submicron voids still remained in the cross-section of the filled package, under pure O2 plasma cleaning activation conditions, both the interface delamination and the number of submicron voids were significantly reduced. This trend is consistent with the change in water contact angle, indicating that under conditions of a lower polar contact angle, the underfill has better filling and wetting performance, which helps to improve the package interface quality and reliability.

Figure 2 Cross-sections of Si/underfill/Si adhesion specimens under different plasma cleaning processes: (a) untreated; (b) MW 20% O2 plasma cleaning; (c) RF pure O2 plasma cleaning; (d) MW pure O2 plasma cleaning