Measured Average Absorption Cross-Sections of Printed Circuit Boards from 2 to 20 GHz

Abstract

Absorption by the contents of an equipment enclosure, particularly printed circuit boards (PCBs), affect the enclosure's shielding performance. At high frequencies, this absorption can be quantified using the angle of arrival and polarization averaged absorption cross-section (ACS). However, there is no available data on the high-frequency absorption characteristics of modern PCBs. In this study, we apply a reverberation chamber to the determination of the average ACS of a large number of PCBs taken from contemporary information and communication technology (ICT) equipment to provide a unique and comprehensive dataset. The ACS was found to range from 4 × 10 −4 -10 −2 m 2 from 2-20 GHz and different classes of PCB could be identified according to their surface characteristics. The "shadowing effect" of densely packed PCBs was also quantified for a subset of the PCBs. It was found that the ACS of a PCB in the stack was reduced by 20%-40% compared to its value when isolated. By way of a review of the general power balance analysis of an electrically large populated equipment enclosure in an external environment, we show how the acquired data will be useful for future qualification methodologies for ICT enclosures and PCBs.

Representative Contents Design for Shielding Enclosure Qualification From 2 to 20 GHz

“…We see that the SE of an empty enclosure, , is as much a measure of its wall losses as it is of the transmission through its apertures [3], [4]. Furthermore, considering the cases where the enclosure is either empty or populated we find [3], [10] (2) which shows that knowledge of is in itself insufficient for predicting the SE of a populated enclosure. Indeed, the ATCS of an enclosure is a much more useful metric as it allows the SE of a populated enclosure to be predicted from the AACS of the contents using (2), under the assumption that in practical cases .…”

“…In [10] and [12] it is shown that the SE for a populated enclosure is (1) where is the total average transmission cross-section (ATCS) of the enclosure's apertures, is the total AACS of the inside of the enclosure walls and is the total AACS of the contents, including any antennas. This equation shows that both the total AACS of the contents and the total ATCS of the apertures directly determine the SE of the enclosure at high frequencies.…”

“…The target AACS for a particular ReCo should be statistically determined from a class of PCBs of similar type and size to those intended to be used in the enclosure. In [10], the AACS of a large sample of PCBs was measured. The corresponding AAE ranged from -19 dB up to -4 dB with a mean of about -8 dB.…”

“…The method is described in detail in [8], [9] and a systematic application of the method to enclosure shielding is given in [10]. This analysis shows that the planewave absorption cross-section (ACS) of a printed circuit board (PCB) averaged over angles of arrival and polarizations of the incident plane-wave is the intrinsic property of a PCB that determines its loading effect on an enclosure and hence the perturbation in SE caused by the contents.…”

“…Such an average ACS (AACS) can be measured in a reverberation chamber (RC) [11]. A database of measured AACSs for real PCBs taken from information and communication technology (ICT) equipment was reported in [10]; according to the PWB analysis such measured AACS data can be used to calibrate ReCos for SE measurements.…”