Dielectric losses laminates

The increasing quantity of power dissipation per square unit as a result of the rising density of devices on PCBs leads to higher operating temperatures. Therefore, the thermal resistance of the applied adhesives has to correspond with the operating temperatures. Furthermore, a low dielectrical constant is essential for the adhesives and/or binders used for laminates because the heat dissipation is a result of dielectrical losses. 

THE DIELECTRIC LOSS OF POLYPROPYLENE FILMS AND POLYPROPYLENE-POLYURETHANE LAMINATES AT CRYOGENIC TEtlPERATURES... 

A summary is presented of measurements of the dielectric loss characteristics of six commercial polypropylene films, as well as laminates consisting of two or three polypropylene films bound together with a polyurethane. 

To examine these factors, some additional experiments were carried out using the PP-U-PP(C). One of the polypropylene layers was stripped from this laminate, leaving all the polyurethane attached to the other layer. The stripped layer of polypropylene was then folded on itself and the dielectric loss characteristics of the resulting doubled-up layer (henceforth referred to as OU see Fig. 6), were measured. The remaining polypropylene film with the adhering polyurethane layer was doubled up to form a sandwich consisting of two outer polypropylene layers and two inner polyurethane layers, thus effectively producing a laminate in which the polyurethane is twice as thick as in the PP-U-PP(C). The dielectric characteristics of this new laminate (henceforth referred to as 2U) were also measured. 

Fig. 5. The dielectric loss at 100 Hz of polypropylene-polyurethane laminates, as well as the PP-F from which they are made, as a function of temperature. The low temperature (4.2 to 10 K) data are shown on an expanded scale in the inset. See text for nomenclature.

It is important to note that values of tan 6 at 4 to 10 K for the OU sample coincide closely to those of the annealed PP-F. In both cases, the dielectric loss is lower than that for the unannealed PP-F. This may be attributed to a suppression of the 30 K peak in the former two samples. In short, this illustrates that the polypropylene layers in the laminates undergo changes during the lamination process that must be taken into account in an analysis of the relative contributions of the polypropylene and polyurethane to tan 6. 

The Dielectric Loss of Polypropylene Films and Polypropylene-Polyurethane Laminates at Cryogenic Temperatures... 

Polyethylene can be made into a high-strength, high-modulus fiber (tradenamed Spectra by Allied Signal, Inc.) and then into a fabric. The polyethylene is an ultrahigh-molecular-weight polymer. The density of Spectra is the lowest of all fibers, which makes its use in aerospace laminates especially attractive. However, laminates must not be exposed to temperatures over 250°F. Spectra has very attractive electrical properties with low dielectric constant and loss factor. These properties are useful in radomes. Spectra is also used in ballistic applications like helmets and aircraft panels. 

Related to the dielectric constant is the dissipation factor (Df) or loss tangent. This is a measure of the percentage of the total transmitted power that will be lost as electrons dissipate into the laminate material. See Fig. 12.9 for a schematic representation of the test. 

Test Methods for a-c Loss Characteristics and Permittivity (Dielectric Constant) of Solid Electrical Insulating Materials Test Method for Dissipation Factor and Permittivity Parallel with Laminations of Laminated Sheet and Plate Insulating Materials Test Methods for Relative Permittivity (Dielectric Constant) and Dissipation Factor of Polyethylene hy Liquid Displacement Procedure... 

---