Kunihiko Nishi

Analysis of package cracking during reflow soldering process

Package cracking that occurs in surface-mount devices that have absorbed moisture has been studied by means of a moisture-diffusion analysis of the plastic, deformation and stress analysis of the package, and measurement of some high-temperature properties of the plastic. The validity of the analysis has been confirmed by a measurement of the deformation of packages heated by infrared radiation. Several packages with different level of moisture saturation and hysteresis of moisture absorption have been heated by soldering and the occurrence of package cracking investigated. The vapor pressure and distribution of moisture content of these packages have been calculated by the present analysis. It was found that generated vapor pressure is lower than saturated vapor pressure and depends on the moisture content at the part of the plastic facing the space in which vapor pressure is generated. This example shows that it is possible to evaluate package cracking by the present analysis method quantitatively. >

Moisture-Induced Aluminum Corrosion and Stress on the Chip in Plastic-Encapsulated LSIs

Stress of encapsulant resin which acts on LSI chip was measured to evaluate passivation defects by a new method. It was cleared that the resin thermal stress gave damage to the LSI passivation film and aluminum corrosion took place by moisture penetrating through the film damage up to the metal surface in a humid atmosphere. The stress values obtained by the new method for various resins showed good correlation to the passivation defect densities. When a resin with half stress was applied, the defect density was halved. To improve the moisture resistance of plastic-encapsulated LSIs, the application of the low stress resin is supposed to be a good method.

A Study of Parasitic MOS Formation Mechanism in Plastic Encapsulated MOS Devices

With a view to preventing the formation of parasitic MOS in plastic encapsulated MOS devices, we studied the mechanism of the formation and found that parasitic MOS is due not only to surface mobile charge on oxidized silicon but also to bulk charge in plastic materials. Concerning an epoxy resin for semiconductor use, which is the most widely used material for plastic encapsulation, the behavior of bulk charge under pressure was investigated by electrical measurement and that under application of high voltage dc was also investigated electrically and by EPMA analysis. As a result, it was established that the majority of the mobile charge is actually certain ions with a negative charge and containing chlorine, rather than electrons or dipoles. It was further confirmed that parasitic MOS formation is extremely scant in MOS devices encapsulated with the same type of epoxy resins with little hydrolyzable chloride ion, and hence that minimizing the ionizable component in plastic materials is highly effective in preventing parasitic MOS formation. Motion of charge along the interface between silicon oxide layer and plastic could not be clarified. However, it is thought that parasitic MOS formation due to this motion can be prevented by providing an appropriate guard ring.

Development of Coated-Wire Bonding Technology.

Wire bonding technology is being applied to narrower pad piches and longer spans in LSI packages with the increase in number of input-output pins and shrinkage in chip size. This makes adjoining wires liable to touch during the molding process. To solve fundamentally this problem, the authors have been developing bonding technologies, using coated gold wires which do not suffer from short circuit failures even if the wire touch occurs.In this study, various coated wires were evaluated to select the optimum coating film with improved resistance to the wire touch at high tempratures and improved bonding continuity. Packages were then assembled with the selected wires, and various reliability tests were conducted.The best properties were obtained with the coating film of 0.4 μm thick heat-resistant formal, which showed high insulation reliability in a high temperature wire-crossing test and enabled continual bonding for more than one hundred thousand wire connections. It was confirmed that the assembled packages have sufficient insulation reliability between touching wires and that no failures occurred in various reliability tests including temperature cycling and high temperature high humidity bias tests.