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Method to measure the effects on surface roughness on the high frequency transmission line
Keywords: surface roughness, microwave measurement, pseudo microstrip line
In high end server of recent years, the transmission loss by the effects on surface roughness in printed circuit boards (PCB) has become important from signal integrity point of view, so that many theses concerning the effects on surface roughness have been reported. However, for the existing measurement methods using microstrip line (MSL) or stripline, it is necessary to prepare the separate samples that measure the effects on surface roughness, because the area of high current density is between conductor layer and insulator layer in PCB. In this case, it is difficult to eliminate the effect on the margin of error by manufacturing dispersion from the measurement results. Therefore, the purpose of this study is to develop the method to measure the effects on surface roughness, using the sample that has different roughness but treated on the same surface. In this study, the transmission line and the ground (GND) pads are routed on the one side of PCB and their terminals for the measurement are routed on the other side. At the measurement, the transmission line and the GND pads are attached to the low dielectric loss material which covers the Cu plate. As a result, GND pads and Cu plate are capacitively coupled then Cu plate works as GND, which resulting in the formation of pseudo-MSL structure. Since the area of high current density in this structure is between the transmission line and the low dielectric loss material at GND side, the effects on surface roughness on the same transmission line can be measured repeatedly by detaching it from the low dielectric loss material. Therefore, the measurement of the transmission characteristics becomes possible using the same substrate that has a different roughness. First, this structure was investigated by 3D electromagnetic simulations. It was verified that the Cu plate functions as the GND at the frequency range between 2 GHz to 30GHz which means that this structure is equivalent to that of MSL. Then, the sample was evaluated with a vector network analyzer. As a result of the conductor loss from the experimental data, when the line length was 20mm and when the surface roughness was Ra=0.5um, the losses were measured 0.35 dB and 0.76 dB at 10GHz and 20GHz, respectively. As described above, it was shown that the effects on surface roughness can be measured quantitatively using our original sample structure. Therefore, it was succeeded to measure frequency dependency characteristics of the effects on surface roughness without the margin of error for first time. It will be possible to adopt the effects on surface roughness on the transmission line simulations to develop the high speed transmission PCB. At the presentation, we will discuss the relation between the surface roughness and the electronics characterization, and the results of the electromagnetic simulation for the samples, which roughness were changed by the various surface treatments such as chemical treatment, mirror polishing, and FIB process.
Toshiki Iwai,
Fujitsu Laboratories Ltd.
Atsugi, Kanagawa

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