Characterization of a no-flow underfill encapsulant during the solder reflow process

A challenge in flip-chip technology development is to improve the thermo-mechanical reliability of the flip-chip assembly. To increase reliability, an underfill encapsulant is applied to the gap between IC chip and substrate to provide thermal-mechanical protection as well as environmental protection to the assembly. Two processes for applying the underfill encapsulant to the gap between IC chip and substrate can be described as the fast-flow method and the no-flow (reflowable underfill) method. The fast-flow method is currently the most widely used method. The no-flow method is a new innovative method that provides cost savings. In order to develop novel underfill encapsulants for the no-flow process, a better understanding of the underfill properties during the solder reflow is needed. This paper studies two aspects of the No-Flow underfill: fluxing activity and viscosity during reflow. These two aspects are important for proper interconnect formation. Solder wetting studies were conducted by applying the no-flow underfill on top of solder beads on substrates of different metallizations. The samples were then placed in a 7-zone reflow oven on different eutectic type heating cycles. Cross sections of the samples were taken and the angle the solder makes with the substrate was determined. The viscosity of the underfill during reflow is important to allow proper solder interconnects. To acquire the viscosity of the underfill just before, during, and shortly after the solder reflow temperature, a no-flow underfill encapsulant developed at the Georgia Institute of Technology was studied. Samples of this underfill were placed in a 5-zone reflow oven on a standard eutectic cycle and taken out at different points. The samples were then analyzed by differential scanning calorimetry (DSC) to find the % conversion (amount of cure) of the underfill material. These % conversions were then used to find the complex viscosity at different points in the reflow process. In this paper, we present the experimental procedures and results of the No-Flow underfill's fluxing abilities and viscosity during reflow heating conditions.