Techniques for determining the flow properties of underfill materials

It is predicted that the number of ICs flip chipped in the near future will experience exponential growth. In light of this, an area ripe for attention is the development of characterization tools for monitoring the flow properties of the underfill encapsulant. Three tools were developed that emulate the flow characteristics of the flip chip. Two of these tools are based on Washburn-Rideal, or capillary flow. One tool, the capacitance underfill tool, follows changes in the dielectric constant as the encapsulant flows between parallel plates. As a liquid flows between two thinly gapped parallel plates, the flow rate can be measured if we treat the plates as a capacitor and the flow between the plates as a displacement of the dielectric material between them. The advantage of this technique is that the plates can be gapped to mimic the actual flip chip configuration. The second tool, a small “V” groove, is designed as a quick diagnostic for production and process material control. The encapsulant is dispensed into a reservoir that feeds a 90° 1 mm V-shaped groove. The fixture, with built-in reference fiducials, can be placed on a hot plate for elevated temperature operation. The data from both tools is plotted as the square of the distance versus time. The slope of this curve (proportional to viscosity over surface tension) is the important factor in determining the critical material properties for underfilling of the flip chip device. Computational modeling of the underfill process constitutes the third tool. A validated model that runs on a desktop PC provides a quick assessment of the underfill flow pattern. A GUI interface allows easy input of material data and chip configuration parameters such as bump layout, gap height, and fill pattern. The computational based model predicts the flow pattern, possible voiding, and fill time of the underfilling process