The effect of filler gradation on the flow length of Underfill

With the popularization of the flip-chip packaging technology, the underfill as an important material designed for the durability promotion has attracted extensive attention in the electronic packaging. Due to the large difference in coefficient of thermal expansion (CTE) between the chip and substrate, the generated thermal shearing stress at the working conditions of the chips could lead to serious solder joint fracture, which could be effectively eliminated by the applied underfill between the solders. The underfill is mainly composed of epoxy resin and the silica fillers. After curing, the underfill has low CTE and high modulus that provides the good protection for the solders. In the application, the fluidity of the underfill is important for achieving the fully protection without drawbacks. As reported, the fluidity of the underfill is greatly dependent on the size distribution of the silica fillers, where the micro-size fillers always result in better fluidity, while the nano-size fillers will result in the dramatic decrease of the fluidity, and it is hard to achieve the high filling ratio and the good fluidity simultaneously for the single-sized silica fillers. To solve the trade-off effect of the size distribution of the fillers, the optimization of the gradation between the micro- and nano-sized silica fillers is essential for the performance enhancement of the underfill.In this paper, four kinds of silica filler with different particle sizes ranging from 300 nm to 5 μm are used for the gradation optimization investigation. The DLS particle size analyzer is used to test the gradation of the fillers. A dynamic random filling theory is applied for the gradation optimizing calculations. The nano-sized silica fillers have large specific surface areas, which are easy to be agglomeration, and result in the dramatic increase of viscosity and the smaller flow length of the underfill. The micro-sized particles can enhance the fluidity of the underfill, but can also block the encapsulation inlet, and lead to the smaller filling ratio. The underfill flow length tests show that under the optimal filler gradation, the high fluidity and the large filling ratio can be attained simultaneously, and the performance of the underfill can also be largely improved.