Underflow process for direct-chip-attachment packaging

In flip-chip packaging, an underfill mixture is placed into the chip-to-substrate standoff created by the array of solder bumps, using a capillary flow process. The underfill mixture is densely filled with solid silica particles to achieve the desired effective coefficient of thermal expansion. Thus, during the flow process the underfill mixture is a dense suspension of solid particles in a liquid carrier. The flow behavior is a complex function of the mixture properties, the wetting properties, and the flow geometries. This paper reports on the use of a plane channel capillary flow to characterize underfill materials. We define and explore a metric termed the flow parameter which scales as σ cos(θ)/μ_app. The measured flow behavior provides evidence that both the contact angle (θ) and the suspension viscosity (μ_app) vary with time under the influence of changing flow conditions. The flow parameter is useful in detecting both of these phenomena. The contact angle variation is consistent with the literature on wetting dynamics, where θ is observed to be a function of the contact line speed. Nonlinear fluid behavior is evident for both model suspensions and commercial underfill materials.