Characterization of PCB Material & Manufacturing Technology for High-Frequency


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In this paper, we investigate how these structuring processes impact the losses of radio frequency (RF) signals up to 110 GHz. Furthermore, as different transmission line modes may exhibit distinct loss behavior, the second point is how these losses depend on the selected transmission line mode, namely microstrip (MS), conductor backed coplanar waveguides (CBCPW), and substrate integrated waveguides (SIW).

Materials and Evaluation Board

In order to investigate the relationship between losses and structuring processes, various PCBs have been designed and manufactured using distinct dielectric materials. Material type A is a very low dielectric constant (Dk) material based on PTFE with ingredients of fused silica, continuous filament, and titanium dioxide (i.e., ceramic filler). According to the manufacturer’s datasheet, the dielectric constant of this material is 3.00 with an LT of 0.001 at 10 GHz. Material type B is a low Dk material with 2 x 2116 glass type and 54% nominal resin content on either side of the fiber. The compound has a Dk of 3.61 and an LT of 0.004 at 10 GHz, according to the datasheet. The basic stack-up for both materials consists of the described RF-substrates with a top and bottom copper layer (Figure 3). To avoid difficulties during the measurements on a waver prober, due to the small overall thickness of less than 200 µm, an additional carrier is attached on the bottom side to minimize the risk of waviness of the evaluation board. The thickness of the conductive layer varies within 28-37 µm and 22-25 µm for panel plating and pattern plating process respectively. Thickness of the dielectric layer ranges within 125–130 µm. The outermost surface, the signal layer, is covered with immersion nickel/gold. The thickness of nickel and gold is 6 µm ±1 µm and 0.5 µm ± 0.1 µm, respectively.

Figure_3.jpg

Figure 3: Stack-up.

Design Description

The losses should be continuously evaluated up to a maximum frequency of 110 GHz, therefore, line elements with different lengths are designed and manufactured for the transmission modes of interest. For best measurement accuracy all lines have been designed for a 50 Ω wave impedance. All measurements have been carried out using a wafer prober, which requires appropriate launching pads for contacting the probe-tips to PCBs. Furthermore, the waver prober limits the maximum probe separation and consequently allows the longest straight line to be around 25 mm.

The first mode to be investigated is MS (Figure 4a). MS lines feature a quasi-TEM propagation mode with a significant amount of the fields in air, depending on the line width to substrate height dimensions. The trace width for an MS transmission mode WMS is determined to be 285 µm and 260 µm, for material type A and B, respectively. The second mode is CBCPW (Figure 4b), a quasi-TEM propagation mode also, whereby in this case most of the field is concentrated in the substrate. The involved field pattern of CBCPW strongly depends on the manufactured dimensions, as for thick substrates the fields tend to propagate in the gap and for thin substrates, as in this case, in the substrate itself. The trace width for CBCPW, WCBCPW, is determined to be 280 µm and 250 µm, and the gap GCBCPW to be 250 µm and 250 µm for material type A and B, respectively. For CBCPW the connection of the top to the bottom ground plane is also important to keep in mind, as via placement and via spacing can degrade the measurements drastically.[1] The last mode is a TE10 in an SIW (Figure 4c) where the entire field is in the substrate. For this design the width of the SIW, WSIW, defines the cutoff frequency fcutoff, and thus the frequency range of operation. WSIW is set for both materials to be 1700 µm as there is expected just a slight shift of fcutoff, however, with the benefit of keeping the manufacturing dimensions entirely equal. Although all samples are manufactured on low loss materials, there should be an apparent impact on the losses if a significant amount of the field is in air as the LT of air is close to 0. Figure 4d provides an overview how the top copper layer of the evaluation boards looks like exemplary for the SIW lines.

Figure_4.jpg

Figure 4: MS, CBCPW, and SIW designs with dimensions.

Measurement Equipment

All measurements have been conducted using an R&S ZVA67 vector network analyzer (VNA) that is enhanced by two external R&S ZVA110E W-band converters which allows continuous measurements up to 110 GHz (Figure 5). The available wafer prober is a Cascade PA300 semi-automatic probe system and Cascade ACP-A-GSG tungsten probes with a pitch of 100 µm have been used. Furthermore, a W-band calibration substrate is used to relocate the reference plane for all S-parameter measurements to the probe tip.

Figure_5.JPG

Figure 5: Measurement equipment.

Analysis and Measurement Results: Fabrication Process

The evaluation boards have been manufactured using two different PCB fabrication processes, pattern platting and panel plating. The two main differences of these fabrication processes are firstly the manufacturing tolerances of trace widths and gaps, and secondly the shape of the traces itself. According to the micro-section analysis carried out on evaluation boards manufactured with the nominal dimensions of Figure 4, traces of the panel plating samples have a trace width of 241 µm on top and 269 µm on the bottom side for material type B, which is a width difference of 13%, with a total copper thickness of 30 µm ± 5 µm. The shape of the trace looks similar to a trapezoid. The results of the pattern plating process are 249 µm on top and 259 µm on the bottom side, which is a difference of only 3% in width at a copper thickness of 30 µm ± 3µm.

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