1. Introduction

Due to limitations of the electrical line on a printed circuit board (PCB), such as bandwidth, crosstalk, and line density, etc, next-generation computer systems require new alternatives [1]. Thus, many researchers have suggested chip- and board-level optical interconnections based on an optical PCB (OPCB) as potential alternatives [2]–[4]. Most of the studies have focused on the optical interconnections using a rigid OPCB. However, we propose a flexibly interconnected system, as shown in Fig. 1(a). The system is linked using a rigid flexible optical electrical PCB (RFOE-PCB) for chip-to-chip as well as board-to-board interconnections. This configuration has the advantage of being optically and electrically linked simply without constructional restrictions on one or more of the boards. The low-cost of a smaller sized RFOE-PCB limited to only a special section of a large-sized PCB is a further advantage compared to a high-cost OPCB with optical layers covering the entire area of a large-sized PCB.

 

Fig. 1. Conceptual drawings of (a) the proposed optical interconnected system and (b) the multi chip module platform based on the RFOE-PCB.

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Figure 1(b) illustrates in detail a multi-chip module (MCM) platform based on our proposed RFOE-PCB. The MCM is divided into a rigid section and a flexible section. The rigid section can be composed of optical sources and detectors such as a vertical cavity surface emitting laser (VCSEL) and photodiode (PD), very large scale integration (VLSI) chips, embedded active and passive components, electrical connectors, etc. The flexible section is broken into optical waveguides and electrical lines in an embedded manner. Owing to the flexible section, the MCM platform can be easily installed anywhere such as in a board-to board or chip-to-chip interconnection, as shown in Fig. 1(a).

This letter details the RFOE-PCB fabrication process using a flexible optical waveguide that is endurable during the conventional PCB process under both high temperature and pressure. The optical characteristics of the RFOE-PCB are also described and analyzed.

2. Fabrication process of the RFOE-PCB

Figure 2 depicts the fabrication process of the RFOE-PCB in more detail. The steps in this process can be outlined as follows.

 

Fig. 2. Fabrication process of the RFOE-PCB.

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(1) Fabrication of film waveguide sheets with 45° mirror and guide holes for passive alignment: We first prepared a film-type optical waveguide with a 50 µm× 50 µm core size and 25 µm clad thickness. Because the optical waveguide is thin enough to be 100 µm in total thickness, and because both the core and the clad, which have refractive indices of 1.546 and 1.5227, respectively, were fabricated using acrylate polymer materials, it is not brittle but very flexible. The flexible optical waveguides used in the RFOE-PCBs were 5 cm in length and made up of 4-channel arrays. Next, to make film optical waveguide sheets with 45° mirrors, a diamond blading technique was applied [5]. After blading, the surface roughness of the 45°-ended waveguide facet was 40 nm (rms). Due to the fine surface roughness, another additional surface-fining process of the waveguide facet was not necessary. In the following, using an e-beam evaporation process, a Ni layer and Au layer were deposited sequentially as a thin metal mirror on the 45°-ended facets. Next, we fabricated guide holes by laser drilling for passive alignment between the optical waveguide and flexible printed circuit board (FPCB).

(2) Passive alignment between the waveguide sheet and the FPCB: Copper (Cu) patterns for alignment were formed on the flexible copper clad laminate (FCCL). A bonding sheet was prepared to strongly attach the film waveguide and the FCCL. The waveguide sheet and the FCCL sheet were aligned in reference to the Cu patterns and guide holes. This alignment technique using guide holes and Cu patterns is useful when inserting and adding a variety of waveguides at specified regions in a large-area PCB. Also, this technique offers another advantage of preventing the unwanted waste of waveguides.

(3) Fabrication of open area to reduce the optical coupling loss: A cover-lay film, which is a sheet material applied to the outside layers of a flexible circuit to insulate the copper conductor, was covered on the optical waveguide in order to protect it from physical damage. The cover-lay film has approximately 0.5-dB transmission loss. To reduce the optical coupling loss, we opened a partial region of the cover-lay using laser-drilling, where the beam emitted from a VCSEL is coupled to the waveguide.

(4) Fabrication of the rigid PCB: After inserting the completed flexible OE-PCB in the third step between the top and bottom layers, we formed the RFOE-PCB through a 180°C and 35 kg-f/cm² hot press process using prepreg, which is a sheet material such as glass fabric impregnated with a resin cured to an intermediate stage. The prepreg was also opened at the optical coupling region to improve the optical transmission property using laser-drilling.

(5) The completion of the RFOE-PCB: The RFOE-PCB was completed through via and solder-resist process as the final step. Figure 3 shows a photograph of the completed RFOE-PCB, and the right inset of the figure is a demonstration of the beam propagation through the optical layer. For the optical transmission of the RFOE-PCB, an input beam through a bare fiber was installed on the XYZ stage and the fiber was perpendicularly aligned to the 45°-ended optical waveguide. On the opposite side, the output beam reflected by the 45° mirror was observed using a microscope. The flexible region of the RFOE-PCB was highly flexible using the flexible optical waveguide, as shown in the left inset of Fig. 3. Finally, the RFOE-PCB was composed of three electrical layers and one embedded optical layer.

RFOE-PCB manufacturing has led to the need for positional accuracy between the reference metal pads on its top and the optical waveguide layer embedded within it. In particular, it is very important to accurately align the window aperture of the optical source/detector and the 45° mirror plane formed at the end of the waveguide. As previously mentioned, the guide pin used on the fabrication process helps their positions to be set accurately. Thus, we cut off the RFOE-PCB and polished the cross-section of it. The positional accuracy was measured to within about ±10 µm at the lateral axis. A ±10 µm misalignment in the RFOE-PCB was calculated with 2 dB additional coupling loss by ray optic simulation. Nevertheless, such a positioning error has a sufficient power margin as calculated 7.5 dB in an unbent optical link.

 

Fig. 3. Photographs of the completed RFOE-PCB, the used flexible optical waveguide, and an emitted beam at the output port.

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3. Experimental results and discussions

As optical characteristics, we measured the propagation loss of a bare optical waveguide as a reference, as well as the optical propagation loss and bending loss of the RFOE-PCB after PCB lamination process. The propagation loss of the optical waveguide was measured to be 0.24 dB/cm at an 850 nm wavelength using a cut-back method. The optical propagation loss of RFOE-PCB after the hot-pressed lamination process was increased by an average of 0.21 dB/cm for all channels. In addition, we measured the variation of the optical losses with the bending radii to investigate the excess loss quantitatively when the flexible region of the RFOE-PCB was arbitrarily bent. The bending losses of the 360°-turn optical waveguide, as shown in an inset of Fig. 4, were calculated for different numerical aperture (NA) and bending radii by ray-tracing optic simulation. We then made an experimental set-up in the same manner as the simulation and measured the losses. The NA of the waveguide used in this work was 0.27 and the measured bending loss was analogous to the simulated data of NA=0.3. We successfully accomplished bending experiments up to a sharp bending radius of 1 mm without any physical damage to the RFOE-PCB. The optical loss of the RFOE-PCB at a bending radius of 5 mm was measured to be less than 1 dB. As far as the type of RFOE-PCB is concerned, these results have been little reported thus far.

 

Fig. 4. Comparison of the bending losses between the measured data and the simulated data.

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We carried out a reflow test of flexible OE-PCB in order to check up the compatibility with lead free soldering process. First, we checked if there is any abnormality by a microscope such as bulging or separation after one to three reflow test. And then, insertion losses were measured to observe the heat-resisting property of the embedded optical waveguide. Figure 5 (a) shows the reflow test condition which is suitable for typical lead free solders. The reflow condition is actually measured data by thermocouple. Figure 5(b) and (c) show micrographs of a cross-sectional facet of the flexible OE-PCB before and after the reflow test, respectively. We cannot observe any abnormality, as shown in Fig. 5(b) and (c). Next, we measured the variation in insertion losses as an optical property before and after one to three reflow test. As the experimental setup, a launching fiber with 50 µm core diameter and a detecting fiber with 100 µm core diameter were used. Optical source was an 850 nm VCSEL multimode source. As the result, the variation in insertion losses was very slight, as shown in Fig. 5(d). Thus, the reflow test was successfully accomplished for all 4 channels as losses within ±0.5 dB which means to be within measuring error range. Conclusively, our flexible OE-PCB is compatible with soldering process.

 

Fig. 5. Results after the reflow test. (a) Condition of the reflow test. Photographs of cross-sectional facet of the flexible OE-PCB (b) before and (c) after the reflow test. (d) Variation in insertion losses before and after the reflow test.

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Finally, an optical transmission test was performed using a commercial VCSEL transmitter and PD receiver, i.e., a Newfocus 1780 model and Teradian 2.5 Gb/s receiver, respectively. The 850-nm VCSEL/PD modules and the fabricated RFOE-PCB were optically coupled using 50 µm and 62.5 µm multimode fibers, respectively. The 2.5 Gb/s source data used was a pseudo random bit sequence with 2²³-1 signal length. Figure 6 shows a clear eye diagram of the 2.5 Gb/s data transmission rate with 20% eye mask margin. Because total link loss was somewhat large as about 13 dB but the power budget of modules using in this experiment was above 17 dB, the 2.5 Gb/s data transmission was successfully achieved. Meanwhile, we cannot achieve the 10 Gb/s data transmission because the link loss exceeded the power budget which resulted from the poor sensitivity of a 10 Gb/s receiver module. As a future work, improvement in optical coupling loss is required for the successful 10 Gb/s data transmission.

 

Fig. 6. Measured 2.5-Gb/s eye diagram with 20% eye margin through the fabricated RFOE-PCB.

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4. Conclusions

We suggested chip- and board-level optical interconnection schemes using RFOE-PCBs and described the schemes and the RFOE-PCB in detail. The proposed RFOE-PCB was successfully fabricated and demonstrated passing by various tests such as the optical bending test, the thermal reflow test, and the data transmission test. We accomplished bending experiments up to a sharp bending radius of 1 mm without any physical damage, the reflow test as a slight variation in insertion losses within ±0.5 dB, and the optical transmission test with clean 2.5 Gb/s eye diagrams. The optical interconnection technology based on the RFOE-PCB is expected to make a great contribution to the next generation PCB industry as well as to high performance computer and server systems.