Experimental verification of models for underfill flow driven by capillary forces in flip-chip packaging

Underfill process is a very important step in the flip-chip packaging because of its great impact on the reliability of electronic devices. In the control of the underfill dispensing in flip-chip packaging, an analytical model for the underfill flow behavior is required to perform the control action. Traditionally, the Washburn model is used for predicting the viscous flow behavior in the flip-chip underfill process driven by capillary forces. Unfortunately, some studies in the literature have shown that the model does not match the measured results well due to the neglect of the characteristics such as solder bump resistance and non-Newtonian behavior of underfills. Although some underfill flow models have been developed for considering these characteristics, there is no sufficient account for such a mismatch from the literature. In this article, we present an experimental investigation aimed to understand the possible causes responsible for the observed mismatch with the Washburn model. The experimental investigation confirmed that the underfill fluid used in flip-chip packaging shows a complex non-Newtonian behavior and that the Washburn model is, indeed, only applicable to the Newtonian fluid in this setting. Another contribution of the work reported in this article is the provision of measured data on a test bed which was built upon using the off-the-shelf components; as such the data can be used by other researchers to validate their theoretical findings.