Fluid Structure Interaction Modeling for Dynamic Wire Sweep

Abstract

In this paper, dynamic wire sweep in a package during molding is analyzed with Fluid Structure Interaction (FSI) modeling, as well as an experimental validation. Wire bonding (WB) is widely used in integrated circuit (IC) packaging, connecting between chips and substrate. Subsequent molding fluid flow with a certain viscosity and with a certain speed perpendicular to wire curve easily sweeps wires, leading to potential electrical failure. Wires are as thin as tens of micrometers to enable more input-output on a limited chip area. Compared to tens of millimeters in package scale, hundreds of wires are not feasible to model in a package model due to meshing limit. This paper uses an overall flow model considering non-Newton fluid characteristics, from which fluid velocity field is taken as boundary of wire submodel. Overall model contains compound curing kinetic property to capture epoxy reaction during flow because epoxy gelation time is not long. The submodel considers solid-fluid coupling with FSI, as well as thermoset material property, therefore, narrow gap filling around wires and chips could be evaluated. Experimental wire sweep shows consistency with FSI submodel, while non-FSI method could not capture wire sweep in narrow tunnel of compression molding. Curing thermoset material also prevents wires from recovering back elastically. Factors such as wire size, speed, and wire distance are simulated for package design. Results show wire-to-wire distance couples with wire size and impacts on sweep, which could be optimized at design stage with FSI simulation. Small distance may introduce filling issue as molding fluid contains fillers. Front wire may not be able to protect its back wires if their distance is too long. Thicker wire, lower wire loop, and lower inlet speed would help to minimize wire sweep.