Dielectric losses epoxies

Grade G-10, glass fabric with epoxy resin binder, has extremely high mechanical strength (flexural, impact, and bonding) at room temperature and good dielectric loss and electric strength properties under both dry and humid conditions. 

Additions of BN powder to epoxies, urethanes, silicones, and other polymers are ideal for potting compounds. BN increases the thermal conductivity and reduces thermal expansion and makes the composites electrically insulating while not abrading delicate electronic parts and interconnections. BN additions reduce surface and dynamic friction of rubber parts. In epoxy resins, or generally resins, it is used to adjust the electrical conductivity, dielectric loss behavior, and thermal conductivity, to create ideal thermal and electrical behavior of the materials [146]. 

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. 

Figure 8 shows the temperature dependencies of e" at four frequencies for Epikote 1001 (M w=1396) in comparison with that of a/E0 calculated from the data by the DC conduction measurements [10]. A broad peak is observed for each of the four frequencies at low temperatures on the plot of e", which is due to the rotational diffusion of the dipole moments. A good agreement is observed between e" (plots) and o/E0 (a solid curve) at higher temperatures and at lower frequencies in Fig. 8. The dielectric loss e" can be used as an indicator of the ionic conduction in the DGEBA oligomer at a fixed frequency at the temperatures where the dipole component is negligible. The ionic conduction from the dielectric loss can be measured in a short period of time and is widely used for the cure analysis of epoxy resin systems [62,79-82].

Although some competitor resins (e.g., polyimides and cyanate esters) are replacing epoxy resins in some more demanding applications, in which the superior glass transition temperatures or lower dielectric permittivity/low dielectric loss are preferred, brominated epoxies are still widely used. 

Major results. Figure 14.7 shows that the resistivity of aluminum-filled PMMA changes abruptly. Smaller volumes of filler contribute a little to resistivity but, after certain threshold value of filler concentration, further additions have little contribution. A similar relationship was obtained for nickel powder the only difference is in the final value of resistivity, which was lower for nickel due to its higher conductivity. The same conclusions can be obtained from conductivity deteiminations of epoxy resins filled with copper and nickel. Figure 14.8 shows the effect of temperature on the electric conductivity of butyl rubber filled with different grades of carbon black. In both cases, conductivity decreases with temperature, but lamp black is substantially more sensitive to temperature changes. Even more pronounced changes with temperature were detected for the dielectric loss factor and dissipation factor for mineral filled epoxy." ... 

The dielectric loss/frequency relation of a liquid epoxy/polyamide resin system... 

At very low temperatures, most degrees of freedom are frozen. The detailed chemical structure of the polymer chains does not remarkably influence most of the elastic and thermal properties at these temperatures. (Properties, such as mechanical strength or dielectric loss, may be influenced by the chemical structure because of factors such as steric hindrance and dielectric polarization.) Cross-linking is one structural feature of epoxy resins which might influence low-temperature properties. 

Fig. 10. Mechanical and dielectric loss vs. temperature for various epoxy resins and polyethylene.

Amorphous epoxy resins are polar polymers and have comparably higher losses. Some loss measurements on epoxy resins are plotted versus temperature in Fig. 11. It is interesting to note that the mechanical and dielectric losses are only different by a factor of roughly 2. Also, the dependence on temperature is rather similar. The dielectric and the mechanical parameters of Fig. 11 were determined at different frequencies. At low temperatures, this is a minor error. Mechanical measurements performed at a higher frequency, namely 50 Hz, would, at most yield lower values, more similar to the dielectric ones. Thus, for epoxy resins, the electrical and mechanical dipole forces are similar. 

Fig. 55. Dynamic cure of a brominated epoxy-phenolic system at a heating rate of 7 C/min (125) superimposed on the dielectric loss factor plot are djmamic viscosity data taken with a rheometrics rheometer. 1 Pa s = 10 P.

Fig. 19. Dielectric loss factor as a function of time for isothermal curing of an epoxy resin. From Ref. 82.

The volume resistivity, permittivity, and dielectric loss factor of nanostructured interpenetrating polymer networks based on natural rubber/polystyrene have been found to increase as a function of blend composition, reaching a maximum of 10 -10 Hz dielectric loss factor [27]. Measurements of volume resistivity have also been reported on epoxy resin-polyaniline blends resulting in the establishment of a correlation between a shoulder on the 1583 cm band with the degree of volume resistivity [31]. 

BMI resins were modified with a wide range of other polymers in order to achieve improved properties. Toughened BMIs may be obtained by using polyetherketones (Han et al. 2009), while enhanced processing characteristics and low dielectric losses may be achieved for BMI by modification with allyl phenyl compounds, allyl epoxy resins, and epoxy acrylate resins (Liang et al. 2007). Flame retardancy was accomplished by modifying BMIs with fully end-capped hyperbranched polysilox-ane (Zhuo et al. 2011a). 

Dielectric Loss (Dfor Tan 5). The energy absorbed by the dielectric media is called dielectric loss. Attenuation is proportional to tan 5 and signal frequency. For standard FR-4 ML-PWB materials, tan 5 is 0.02, which translates into serious losses at frequencies above 1 GHz. For circuits operating above 1 GHz, a lower loss material is required.There are a number of material choices for lower dielectric attenuation, including blended epoxies, hydrocarbon ceramic, polytetrafluoroethylene (PTFE), and PTFE with ceramic. The tan d values of these materials are approximately. 01,. 004,. 002, and. 001, respectively. 

Both BT and PPO-epoxy boards provide small improvements in electrical properties. If the real need is to reduce Dg so that the board thickness can be reduced, these materials may be useful. However, if dielectric loss is a serious problem, the only real solution is to use PI PE. Unfortunately, PTFE is very expensive and difficult to process, so it should be specified only where absolutely necessary. Eor example, RF circuits will nearly always require PTFE. 

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