Rheological analysis of an underfill material

In microelectronics packaging, flip-chip interconnection is gaining momentum as it accommodates thinner packages, higher I/O and shorter electrical connections, hence reducing heat generation and increasing clock speed. The former should improve package reliability. Overall, it reduces manufacturing time/cost by eliminating wire bonding. It is necessary to use underfill material to improve flip-chip reliability by reducing stress on the solder bumps and the device due to thermal mismatch between substrate, solder bumps and silicon die. The low dielectric constant of the underfill also reduces noise capacitance, and minimizes crosstalk in high frequency devices. These experiments are focused on the effect of the filler level on the underfill material properties. The rheological properties of underfill determines the material's ability to fill narrow gaps and to spread uniformly. The nature of the resin matrix, the filler shape, size and volume fraction and the flow process temperature affect the rheology. The underfill viscosity has been investigated as a function of time. It was found that viscosity increases with increasing underfill thaw time and with increasing filler percentage. The relationship between underfill flow rates and filler percentage was examined at various temperatures. From the results, the underfill flow rate was found to increase with temperature and decrease with increasing filler level. The underfill dielectric constant for different chemistries and filler types is discussed. A model for the rheological properties of the material and its relationship to the filler level is also discussed.