Analysis of a Thermosetting Compound During Mold Filling
Thermosetting molding compounds such as epoxy are widely used to encapsulate semiconductor devices and electronic modules. In recent years, the number of electronic parts encapsulated in an electronic module has increased dramatically in response to the demands for higher performance. As a result, the configuration of inserted parts during molding of the electronic modules has become very complicated. Due to the demands for miniaturization, package thickness has also been reduced. These trends lead to complications in the flow of molten thermosetting compounds in the mold cavity, thus making the prediction of void formation or gold-wire deformation much more difficult.
Three-dimensional flow analyses of thermosetting compounds have been conducted by a FLOW-3D user in Japan who developed customized routines for the code with the objective of minimizing the design phase before mass production and enhancing the product quality. Special models for predicting viscosity changes as a function of time and temperature and for calculating the amount of wire deformation as a function of viscosity, wire configuration, and other parameters, were developed by the user and integrated into FLOW-3D.
During mold filling, the temperature of the material rises. To predict viscosity changes under these non-isothermal conditions, a model for calculating viscosity changes associated with a chemical reaction was derived from tests using an epoxy molding compound normally used for encapsulating semiconductor devices. This empirical model incorporates a non-dimensional relationship for time and viscosity.
In semiconductor devices and electronic modules, very thin gold wires are used to connect large-scale integration chips to the terminals of substrates and lead frames.
During mold filling, these wires can be deformed due to the flow of molten materials. Predicting the
amount of gold wire deformation is very important for preventing the occurrence of circuit defects. A model for deformation was derived from a combination of empirical data and theoretical viscosity values for several semiconductor devices. The horizontal displacement of each gold wire is similar to that of an elastic beam under three-point bending with a center load.
The mold-filling dynamics of an epoxy compound were analyzed using the modified FLOW-3D during transfer molding of an in-production high performance electronic module (Fig. 1 & 2). The changes in the three-dimensional distributions of parameters such as temperature, viscosity, velocity, and pressure were compared with the observed flow front patterns.
The predicted results of cavity filling behavior corresponded very well with short shot data (Figs. 3-5). As well, the predicted amount of gold-wire deformation also corresponded well with observed data obtained by X-ray inspection (Fig. 6).
Appreciation to Junichi Saeki and Tsutomu Kono, Production Engineering Research Laboratory, Hitachi Ltd.