Spherical filler-induced damage in IC plastic packages

The stress concentration mechanism in a molding compound composed of spherical fillers, which is used for IC packages, was analyzed. A stacked structure consisting of a small filler under a large filler causes the stress to increase in an encapsulated silicon chip. Not only thermal stress but also deformation that results from mounting the packages on printed circuit boards causes compressive stress normal to the chip surface. The stress increase ratio sometimes exceeds 20, depending on the dimensions of the small filler and the large filler. The maximum stress reaches 200 MPa. Such a high stress causes not only mechanical failures such as cracking of thin films for interconnection of LSI chips, but also decreases the reliability of the devices by increasing leakage current in the diode. The leakage current of a Schottky-type diode strongly depends on the stress normal to the Schottky junction. The leakage current abruptly increases when the normal stress exceeds 130 MPa. Thus, when above-mentioned stacked structure is created during encapsulation of packages, it can cause an abnormal leakage current in some products. Passivation film coatings thicker than 2 μm can reduce the stress. Controlling of the maximum diameter of the fillers is also an effective way to decrease the stress.