Microelectronic encapsulation, epoxy

The effects of temperature and relative humidity on the kinetics of moisture sorption in epoxy materials for microelectronics encapsulation are not generally known. In a previous paper QJ we examined moisture sorption as a function of temperature under conditions of 100 percent relative humidity. Conjugate sorption measurements were combined with mechanical, dielectric and thermal methods of analysis to examine moisture related micro-structural alterations. 

Ea, above and below Tg. Three case studies illustrate the range of applicability of the bending beam setup and factors contributing to the stress state. The first is a comparison of two polymers for interlayer dielectrics PMDA-ODA (pyromellitic acid dianhydride - oxydiamine) and a bis-benzocyclobutene. The second is of a neat epoxy resin commonly used for microelectronics encapsulation (epoxidized ortho-cresol novolac cured with a phenolic novolac). The third is a screen-printable polyimide coating used for protection of the integrated-circuit chip. An outline of our stress model is sketched, and example results are presented. 

Three case studies are examined which illustrate the use of the bending beam stress experiment. The first is a comparison of two polymers for interlayer dielectrics. The second is of a neat epoxy resin commonly used for microelectronics encapsulation. The third is a polyimide coating used for protection of an integrated-circuit chip. 

Epoxy molding compounds, used to encapsulate microelectronic devices, contain bromine to provide flame retardancy to the package. This bromine, typically added as tetrabromo bisphenol-A or its epoxy derivative, has been found to contain many hydrolyzable bromides. These bromides, along with the presence of chloride impurities, are detrimental to the life of the electronic component. Bromine especially has been suspected (proven) to cause wire bond failure when subjected to moisture and/or high temperatures. With the addition of a more thermally and hydrolytic stable bromine compound, flame retardancy does not have to be compromised to increase the device reliability. Stable brominated cresol epoxy novolac, when formulated into a microelectronic encapsulant, increases the reliability of the device without sacrificing any of the beneficial properties of present-day molding compounds. 

Table I shows the typical analytical properties of stable brominated CEN and the standard high purity resin blend of CEN (QUATREX 3430) and the epoxy of TBBA (QUATREX 6410). This resin blend of CEN and the epoxy of TBBA was mixed in a ratio that corresponds with what is typically used in microelectronic encapsulants. The stable bromine CEN was synthesized to match the bromine content of the resin blend. The total bromine content of the resin blend and the stable bromine CEN is 7% however, the hydrolyzable bromide impurities of the stable bromine CEN is much lower than that of the standard resin blend. This low content of hydrolyzable bromide, along with the low chloride content, contributes to an increase in device reliability...

The electronics industry desires improved flame suppressant additives for microelectronic encapsulants due to bromine induced failure. Epoxy derivatives of novolacs containing meta-bromo phenol have exhibited exceptional hydrolytic and thermal stability in contrast to standard CEN resins with conventional TBBA epoxy resins. When formulated into a microelectronic encapsulant, this stable bromine epoxy novolac contributes to significant enhancements in device reliability over standard resins. The stable bromine CEN encapsulant took about 30% more time to reach 50% failure than the bias pressure cooker device test. In the high temperature storage device test, the stable bromine CEN encapsulant took about 400% more time to reach 50% failure than the standard compound. Finally, the replacement of the standard resins with stable bromine CEN does not adversely affect the desirable reactivity, mechanical, flame retardance or thermal properties of standard molding compounds. 

Catalysts for Epoxy Molding Compounds in Microelectronic Encapsulation... 

The key to the development of the proper epoxy molding compounds for microelectronic encapsulation is the catalyst in the formulation. In spite of serious limitations in epoxy molding compound performance in sensitive microelectronic devices about ten years ago, the many new developments in catalysts during the last few years have enabled tremendous improvements. However, the exact curing mechanisms of various catalysts in epoxy molding compounds are still not fully understood today. 

Epoxy molding compounds in microelectronics encapsulation are usually composed of more than ten kinds of raw materials each of which has its own qtedal fimc-... 

Fundamental characteristics of the molding compound in microelectronics encapsulation to consider are moldability, mechanical and electrical properties, and humidity and heat resistance. These depend significantly on the corresponding charactoistks of the base epoxy resins used. Epoxy resins can be classified rou y into two types structoterminal and structopendant types. Each type includes a variety of epoxy r ns having different chemical structures In these epoxy resins, crosslinking takes place... 

Table 4. Formulation of base epoxy resin for recent molding compounds in microelectronics encapsulation...

Kinjo, N., Ogata, M., Nishi, K. and Kaneda, A. Epoxy Molding Compounds as Encapsulation Materials for Microelectronic Devices. Vol. 88, pp. 1 —48. 

The protection of microelectronics from the effects of humidity and corrosive environments presents especially demanding requirements on protective coatings and encapsulants. Silicone polymers, epoxies, and imide resins are among the materials that have been used for the encapsulation of microelectronics. The physiological environment to which implanted medical electronic devices are exposed poses an especially challenging protection problem. In this volume, Troyk et al. outline the demands placed on such systems in medical applications, and discuss the properties of a variety of silicone-based encapsulants. 

Due to their small feature sizes, microelectronic circuits need protection from environmental hazards such as mechanical damage and adverse chemical influences from moisture and contaminants. Several approaches are currently in use, for example, hermetic encapsulation of the device in sealed metal or ceramic enclosures, application of soft silicone gels as a cover over integrated circuitry, and encapsulation by transfer molding, which is the topic of this report. Both silicone resins and epoxy resins are used for this purpose. As the quality and performance of the epoxy encapsulants improved, the need for the generally more expensive silicone resins diminished. The present work is exclusively devoted to epoxy transfer molding compounds. 

Epoxy resins are widely used for semiconductor device encapsulation in the microelectronic industry. The major criteria for measuring quality of epoxy resin in microelectronic applications are the level of total leachable halide and the epoxide content. Both factors have a great influence on the reliability of an encapsulated semiconductor device. 

Performance of Stable Brominated Epoxies in Encapsulants for Microelectronic Devices... 

The mechanism for organometallics and Lewis acids in phenolic or anhydride cured epoxy molding compounds are still not fully understood. Lewis bases such as imidazoles can be reacted with organic acids to form salts in order to improve latency. Imidazoles are, so far, the most widely accepted as a compatible catalyst family for encapsulating microelectronics. 

The major use of CE resins is in the electronic industry, including printed wiring circuit boards, thin cards, multichip module laminates and ship encapsulants. CE resins have replaced epoxy and bismaleimide resins to a great extent for such microelectronic application due to their comparatively lower moisture absorption and dielectric dissipation factor. 

A. Christou, Reliability Aspects of Moisture and Ionic Contamination Diffusion Through Hybrid Encapsulants, in Proceedings of the Technical Program—International Microelectronics Conference (1978) p. 237, Industrial Scientific Conference Management, Inc., New York. Electric insulators and dielectrics Silicone/epoxy-polyurethane interpenetrating networks, moisture, and ion diffusion through potting compounds. 

Cure Kinetics of Epoxy Cresol Novolac Encapsulant for Microelectronics... 

Epoxy Moiding Compounds as Encapsulation Materials for Microelectronic Devices 3... 

---