Encapsulation arrangement

An encapsulation arrangement for encapsulating an electric component is arranged on a carrying structure. The encapsulation arrangement includes a plurality of expandable plastic particles. Each particle encapsulates a gaseous medium.

DETAILED DESCRIPTION OF THE EMBODIMENTS According to the invention, the encapsulation arrangement consists of gas field plastic (micro) beads or spheres instead of glass. Surprisingly, it has been observed that plastic microspheres used in the encapsulant have low dielectric constant, higher coefficient of fullness, low viscosity and lower elasticity coefficient. For better understanding of the features of the present invention, it is described in conjunction with illustrative examples shown in FIGS. 1 - 5 . FIGS. 1 to 4 illustrate one example of the steps of applying and curing the encapsulation arrangement on a carrier structure. FIG. 1 is a cross section through the carrier structure 10 , such as a printed circuit board, a substrate or the like. A cavity 11 is arranged on one surface of the carrier 10 for receiving an electrical component 12 , such as a circuit and particularly a naked circuit for microwave applications. The electric component is connected to circuit conductors (not shown) by means of wire bonds 13 . The component is placed in the cavity and fixed, e.g. by means of an adhesive agent. In FIG. 2 , the glob-top material consisting of plastic, gas field microspheres 14 are dispensed pensed by means of a dispenser 15 . Examples of microspheres that can be used are: EXPANCEL® (by Akzo Nobel), which comprises spherical plastic particles. The microspheres consist of a polymer shell encapsulating a gas. When the gas inside the shell is heated, it increases its pressure and the thermoplastic shell softens, resulting in a dramatic increase in the volume of the microspheres. Dualite® and MICROPEARL™ (by Pierce & Stevens), which are heat expandable polymeric microspheres and expand to form a low-density sphere. Of coarse, above products are given as examples and other plastic microspheres may also be used. Moreover, the invention is not limited to the spherical particles and particles having other shapes can also be used. Microspheres can have different sizes for different applications; a typical diameter for non-expanded spheres is 1-50 &mgr;m, which are expanded to a diameter of 30-300 &mgr;m. Epoxies, silicones, phenols and similar material can be used as die material. In FIG. 3 , the dispensed microspheres are exposed to heat 16 or other radiation that generates heat resulting in expansion of spheres. The expansion and curing temperature may range from 100° C. to 210° C., preferably from 110° C. to 190° C. Most of microspheres have curing temperatures within the expansion temperature range. FIGS. 6 a and 6 b show, in a schematic way, the non-expanded and expanded micro spheres 14 and 14 ′, respectively, in the same section of the encapsulation. FIG. 4 shows the final stage, where the encapsulation arrangement 17 has cured and seals the component. In FIG. 5 , another example is shown, in which a circuit 12 ′ is arranged directly on a substrate 10 ′, and provided with the encapsulation arrangement 17 ′ according to the invention. The advantages of the encapsulation material according to the invention compared to glass spheres can be summarized by: The expanded (and cured) material has a low dielectric coefficient, which results in low losses at high frequencies. The small size of non-expanded particles allows a higher filling ratio, which also allows lower dielectric coefficient. The small size of a particle also results in lower viscosity and reliable dispensing. The plastic spheres have the advantage of not being crushed in the dispenser, e.g. a feed gear, because they are softer, smaller and more flexible. The expanded spheres have lower elasticity module, which results in lower mechanical stress. The stresses from the thermal mismatch are absorbed easier by the material with lower elasticity module; thus, fractures in the substrate and wire bonds can be avoided. The invention is not limited to the shown embodiments but can be varied in a number of ways without departing from the scope of the appended claims and the arrangement and the method can be implemented in various ways depending on application, functional units, needs and requirements etc.