Microelectronics, humidity

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. 

Relative to microelectronic applications, the out-of-plane dielectric constant for BPDA-PFMB films measmed after aging at 50% relative humidity for 48 h at 23°C was between 2.8 and 2.9 (0.1 kHz to 1 MHz) (ASTM D-150-81These values are considerably lower than that of commercial polyimides such as PMDA-ODA (pyromellitic dianhydride, PMDA) (s = 3.5 at 1 kHz and 3.3 at 10 MHz). The dielectric constant and tan 8 (dissipation factor) were temperature- and frequency-dependent. The dielectric constant, which was independent of temperature until near 210°C increased above this point until a frequency-dependent maximum was reached at about 290°C. The dissipation factor, which was also independent of temperatme below 200°C, underwent a rapid increase with no maximum between 200 and 400°C owing to ion conductivity. The temperatme at which this increase occurred increased as the frequency increased. The films also... 

Other advantages of microfabricated devices include faster response times, and the fabrication of multiple test sites for simultaneous replicate assays in one microfabricated device. This analytical redundancy provides a safeguard that is not easily attained in a conventional macroscale analyzer, where duplicate assays represent the usual extent of repetitive assay of a sample. Encapsulation technology used in the microelectronics industry may also be applicable to microscale devices and could be extended to operations over a wide range of environmental conditions of humidity, and temperature. 

Microelectronics and computer programming have allowed the printing processes to be analysed, and sensed remotely so that many of the variables in the printing are monitored, adjusted, and controlled automatically from a console. Such fundamentals as flow of ink from the duct, colour strength, ink viscosity, temperature, and humidity all are monitored and checked against pre-determined limits, so that they can be adjusted to maintain consistency of print quality. 

Polymer/metal multilayer devices are used in the microelectronics industry. These devices are composed of alternating layers of polymer and metal, the metal is etched into lines and, except where via holes permit the contact of different metal layers, the polymer serves as an insulator. Because the polymer must withstand rather hostile environments during fabrication, the choice is narrowed to those which are stable to chemical treatment, high temperature (for short periods of time) and humidity. The polymers of choice here are the polyimides, although others are certainly used. 

While it is true that, with the exception of the proton, the trace ions found in polymers do not migrate in the absence of water(11,56 58), it is also true that microelectronic devices and components are not hermetically sealed they are always exposed to atmospheric humidity. It is, therefore, necessary to know, both qualitatively and quantitatively, which trace ions are present. This writer has had experience(591 with four techniques which are capable of identifying contaminants in the ppm-ppb (i.e. ug/ml-ng/ml) concentration region. Each has its own advantages and disadvantages, necessitating the simultaneous use of several of the techniques found in Table I. A particular advantage of ion... 

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. 

It is well known that the combination of surface water and surface impurities leads to high failure rates for microelectronic devices (1-6). Under high humidity conditions, solid surface contaminants take up water from the environment and dissolve to form liquid droplets, or vacuoles. These vacuoles exhibit high conductivity, relative to dry surface impurities, and facilitate electrochemical attack on metallization, which, in turn, leads to device failure. 

The original intent of this project was to identify and/or perfect vendor-supplied, state-of-the-art hydro-phobic polymer sealants, such that the end product could meet military criteria for hermeticity in hybrid microelectronic devices. Solubility and diffusivity constants and permeabilities for several types of hydrophobic polymers were determined. Data from the laboratory and theory from literature both indicate that moisture impermeable polymers are not attainable, at least to meet military standards. The project objective was then redirected towards finding a means of passivating the surface of a microelectronic device against moisture. It was found that interface modification, i.e., passivation of the chemically reactive surface species, provides significant protection in high temperature and humidity environments. Plasma polymerization of hexamethyIdi-silazane (HMDS) afforded phenomenal protection to moisture sensitive thin film nichrome resistors even under the most severe test conditions. 

Ahn H, Park J-H, Kim S-B, Jee SH, Yoon YS, Kim DJ (2010) Vertically aligned ZnO nanorod sensor on flexible substrate for ethanol gas monitoring. Electrochem Solid-State Lett 13(11) J125-J128 Arena A, Donato B, Saitta G (2009) Capacitive humidity sensors based on MWCNTs/polyelectrolyte interfaces deposited on flexible substrates. Microelectron J 40 887-890... 

Islam T, Pramanik C, Saha H (2005) Modeling, simulation and temperature compensation of porous polysfiicon capacitive humidity sensor using ANN technique. Microelectron Reliab 45 697-703... 

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... 

Polpmides have excellent thermal stability, solvent resistance, radiation resistance, wear resistance, hydrolytic stability, low dielectric constant, high mechanical properties, good chemical resistance and a low dielectric constant. Due to these superior properties, the application field of polyimide has generally been enlarged from printed circuit boards and electrical insulation layers in microelectronics to functional layers of humidity sensors, shielding layers for sensor surfaces and novel platforms for thermal sensor devices, temperature sensor arrays, micro-hotplates integrated into gas sensors and biosensors. 

Ammonium sulfate particles have been implicated in the corrosion of microelectronics in humid air. Smyrl and Butler placed a copper micro-mirror on the end of a fiber into an aerated solution of ammo-... 

For comparison, one of the most common potyimides used as a dielectric in electronics applications is PMDA-ODA (9), made Ity reaction of pyromellitic dianhydride (PMDA) and 4,4 -diaminodiphenylether (ODA). For PMDA-ODA, the dielectric constant at 0% RH is 3.10 and at 58% RH the dielectric constant increases to 3.71. The smaller change in dielectric constant as a function of relative humidity for 5a, 5b, and 5c compared to PMDA-ODA may offer a performance advantage in microelectronic applications where low dielectric constant at high relative humidities is required. 

As will be discussed in the next subsection, to be truly effective, predictive capabilities must be based on fundamental physical understanding. The models presented in Table 19.2 are simply empirical correlations. Limited progress has been made to date to improve this situation. Comizzoli [46] has provided a mechanistic explanation and associated mathematical expression for the exponential dependence of surface conductance (and MTTF) on relative humidity that is contained in some of the models listed above. Pecht and Ko [83] developed a comprehensive model for predicting absolute time to failure when microelectronic die metallization is corroded by an electrolytic process. Their phenomenological... 

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