Residual Stress Mapping of Epoxy Molding Compound in a Ball Grid Array Microelectronic Package Using a Fluorescent Sensor

Abstract

Thermal residual stresses developed at the time of semiconductor molding may cause serious problems both in their structural and functional performance; therefore, residual stress assessment in microelectronic devices is a mandatory evaluation step. Fluorescence piezo-spectroscopy was applied to evaluate residual stresses with a microscopic resolution inside a semiconductor encapsulant. In order to obtain reliable stress information, a low fraction of alumina powder, as a fluorescent sensor, was embedded into the silica/epoxy molding compound. Residual stress was transferred from the molding compound to the alumina phase and could be monitored by recording the shift of the sharp and intense fluorescence spectrum of Cr3+ in alumina. Two-dimensional residual-stress maps, recorded near the edge of the silicon chip, revealed a strong stress concentration in the molding compound. Experimental results were compared with calculations obtained by the linear finite element method. Such a comparison showed that the experimental stress values were systematically larger than the corresponding calculated values due to local delamination at the chip edge.

Raman imaging of residual stress distribution in epoxy resin and metal interface

“…To evaluate the local stress distribution in a bulk sample, precise accuracy is paramount; hence, a fundamental investigation of measurement precision was performed as described in this report. In previous reports aimed at measuring the stress distribution of bulk samples, a PbO thin film as a Raman active material was coated on an epoxy resin, or an alumina particle as a tracer for detecting the stress was mixed with the epoxy resin, and the peak shift derived from the alumina tracer was measured . However, this method may be problematic in that the physical properties of the resin may be changed by mixing in an inorganic filler with a different hardness .…”