Dielectric loss moisture

The moisture content of a plastic affects such conditions as electrical insulation resistance, dielectric losses, mechanical properties, dimensions, and appearances. The effect on the properties due to moisture content depends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a product test under extreme conditions of usage in which critical dimensions and needed properties are verified. Plastics with very low water-moisture absorption rates tend to have better dimensional stability. 

Moisture has, in itself, usually not much effect on polymer properties, though the amount of moisture which can be absorbed by polymers varies within wide limits (between zero and a few %). Logically, the electric properties such as resistivity and dielectric losses are the most sensitive to water. As to mechanical properties, nylons show the strongest dependence on water absorption. PA-6 is able to take up a... 

The anhydride hardened epoxies generally have better dielectric loss properties above the Tg than do the novolac epoxies. Consequently, a higher device Junction temperature can be tolerated because of less leakage at the plastic/chip interface and because the required heat dissipation can be obtained via the filler. This is, unfortunately, obtained at the expense of moisture resistance because the moisture resistance of the anhydride-hardened epoxies is not as good as the novolac epoxies. 

Most applications of polymers in electronic packaging have a number of common requirements. Most pervasive is the moisture issue, but other important requirements are low dielectric constant, low dielectric losses (dissipation factor), and good adhesion to various substrates and metallizations. With the... 

The nature of the interaction between water and the polymers is important because absorbed water can adversely affect thermal, electrical and mechanical properties of the polymer. Moisture absorption increases the dielectric constant, (5.6) and dielectric loss, (7) and has been related to device reliability problems. (8) Water-induced plasticization causes hygroscopic expansion, lowering of Tg, and degradation of mechanical properties. (9)... 

Food is a complex mixture of different components and its dielectric properties are highly dependent on its compositions. Moisture content and salt concentration usually play a major role in determining the dielectric properties of foods. Figure 3.1 shows the relationship between the dielectric loss factor components and temperature. 

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

The dielectric constant at 20°C increased from 3.39 to 3.84 due to the 1.7 %wt. moisture (2.0 %v.). The calculated increase of the dielectric constant from 3.39 to 3.60 is only about 50 % of the total effect. The Maxwell-Wagner theory thus seems to describe roughly the frequency/temperature location of the dielectric loss maximum due to absorbed moisture. However, it does not adequately describe the increase of the dielectric constant due to the moisture uptake from the air. A possible reason for this discrepancy might be that one of the assumptions does not hold, viz. that the conductivity of the resin matrix is negligibly small. 

The influence of moisture on the dielectric properties The relative high dielectric constant of water makes the dielectric constant of a polymer very sensitive for small amounts of absorbted moisture, see 5.2.1. PK terpolymer absorbs under room temperature conditions 0.5 - 0.7 %wt. of moisture. The influence of such a moisture concentration on the dielectric constant and the dielectric losses were determined. 

The dielectric losses at temperatures below Tg are shifted to a higher level due to the moisture absorption. Remarkable, however, is that the intensity of the dielectrically measured y-relaxation as such is practically not influenced by the samples moisture content. Dynamic mechanically, a strong influence of the moisture content on the intensity of the y-relaxation was measured, see Figure 9.9. 

Two basic contributions are expected to the variation of dielectric properties of a hydrated material with respect to those of a dry one that of the polar water molecules themselves and the second one due to the modification of the various polarization and relaxation mechanisms of the matrix material itself by water [37]. In the low frequency region of measurements, there is a third contribution, often ignored in works dealing with high frequency measurements, which arises from the influence of moisture on conductivity and conductivity effects. The increase of electrical conductivity of the sample is the major effect present in wet samples dielectric response is often masked by conductivity, and it superposes the dielectric processes in the loss spectra and demands a conductivity correction of the dielectric loss spectra [9]. This dc conductivity strongly affects the modifled loss factor, e". In this case, it can be expressed as shown in the following equation ... 

The dielectric loss usually increases with increasing moisture content but levels off at values in the range of 20%-30% and may decrease at still higher moisture. 

FIGURE 13.3 The critical moisture content m. The dielectric loss factor is e". The region helow is indicative of bound water, whereas above free water is more easily removed. 

The temperature dependence of a dielectric constant is quite complex, and it may inaease or deaease with temperature depending upon the material (see Section 13.2.4). In general, however, a material below its freezing point exhibits lowered dielectric constant and dielectric loss. Above freezing, the situation is not clear-cut, and since moisture and tanpera-ture are important to both drying and dielectric properties, it is important to understand the functional relationships in materials to be dried. Wood, for example, has a positive temperature coefficient at low moisture content [5] that is, its dielectric loss increases with tanperature. This may lead to runaway heating, which in turn will cause the wood to bum internally if heating continues once the wood is dried. 

The commercially important properties of Et>-Nb copolymers include low density, high transparency and low color, high moisture barrier and low moisture absorption, low optical distortion, excellent feature replication, resistance to polar solvents, high purity, shatter resistance, good biocompatibiUty, extremely low dielectric loss, high temperature capability, and compatibility with polyethylenes. The resins also have the low shrinkage and warpage typical of amorphous polymers. 

The fact that plastics are good insulators does not mean that plastics are inert in an electrical field. They can in fact, be made to conduct electricity by the addition of fillers such as carbon black and metallic flake. The type and degree of interaction depends on the polarity of the basic resin material and the ability of an electrical field to produce ions that will cause current flows. In most applications for plastics, the intrinsic properties of the polymer are related to the performance under specific test conditions. The properties of interest are the dielectric strength, the dielectric constant at a range of frequencies, the dielectric loss factor at a range of frequencies, the volume resistivity, the surface resistivity, and the arc resistance. The last three are sensitive to moisture content in many materials. These properties are determined by the use of standardized tests described by ASTM (Table 16-1). These properties of the plastics are temperature dependent as are many of their other properties. Temperature dependence must be recognized to avoid problems in electrical products made of plastics. 

Interpenetrating polymer networks (IPNs) have been the subject of extensive study since their advent in the 1960s. The IPN is a combination of two polymers in network form, at least one of which is synthesized and/or cross-linked in the immediale presence of the other. In this section, we will refer to the research in recent years on the IPN or semi-IPN based on BMI. A special class that is part of the IPN network is made up of the bismaleimide-triazine (BT) resins based on BMI and dicyanate ester (CE), which has many attractive properties such as very good thermal stability, moisture resistance, low dielectric constant, and low dielectric loss. The cure mechanism of their blends was highly controversial and sometimes conflicting, and all research done on this subject until the year 2006 led to the conclusion that CE... 

In addition to providing circuit interconnection, a multilayer printed wiring board (ML-PWB) provides the electrical and mechanical platform for the system. This means that the electrical and thermal properties of the ML-PWB material are very important for the proper functioning of the system. Among the properties of importance are dielectric constant, Du (also known as Er) dielectric loss, Df (or tan 8) glass transition temperature, Tg time to delamination,Txxx thermal decomposition temperature.Ta coefficient of thermal expansion, CTE and moisture absorption. The following sections discuss the importance of these properties to an ML-PWB snbstrate. 

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