Objective   The traditional electric interconnection method has become the bottleneck to limit the rapid development of high-speed communication electronic products for its inherent physical characteristics in the case of high frequency. Optical interconnection technology can be used instead of electric interconnection technology to realize high-speed, large-capacity, high-density and flexible information transmission in electronic products and eliminate the technical bottleneck. As a new development direction of board-level optoelectronic interconnection technology, FEOPCB can realize flexible interconnection among different subsystems and meet the development trend of lightweight, miniaturization and high performance of high-speed electronic systems. However, during the high-temperature lamination manufacturing process of FEOPCB, the embedded fibers will generate thermal stress, which will cause damage to the embedded fibers, affecting high-speed signal transmission performance and reliability of FEOPCB. Therefore, it is necessary to establish the finite element model with bare optical fibers embedded for simulating and analyzing the thermal force to guide the design and manufacturing of FEOPCB. For this purpose, the research work on simulation and test of fiber characteristics in FEOPCB process was carried out in this paper.

  Methods   In order to reduce the bending radius and improve its reliability, high-precision rectangular positioning grooves for the fibers were designed and fabricated on polyimide substrate with double-sided copper-clad. Firstly, finite element simulation models of fibers with and without coating layer embedded in PI substrate were established and the manufacturing process of FEOPCB was simulated and analyzed with the influence factors of stress for embedded fibers (Fig.1). The results indicate that the stress of the coated fibers is much smaller than that of the uncoated fiber (Fig.5-7). Then, the laser-etching technology was used to fabricate the high-precision rectangular positioning grooves on the double-sided copper-clad PI substrate for the coated fibers (Fig.8). FEOPCB was fabricated by the high-temperature lamination process (Fig.9).

  Results and Discussions   FEOPCB has completed the reliability tests of temperature shock, low temperature, high temperature, wet heat and bending fatigue for 100 000 times (Fig.11). Through the observation and analysis with optical microscopy, the result shows that the embedded fibers have no high temperature degradation and cracking defects under high temperature lamination process (Fig.12). The minimum bending radius of FEOPCB is as small as 2 mm, and the bending loss is 0.57 dB and 1.12 dB respectively under 90° and 180° bending conditions (Fig.14). The measurement results show that there is no crosstalk between adjacent fibers. Finally, the high-speed signal transmission performance was measured which indicated that a 10 Gbps communication rate with bit error rate of 10-16 could be reached at the wavelength of 850 nm (Fig.17).

  Conclusions   In this study, the finite element analysis method is used to establish the model with coated or uncoated optical fiber embedded in rectangular groove of PI substrate, and the stress and influence factors of embedded optical fiber are analyzed. The results show that the stress of uncoated fiber decreases from 129.72 MPa to 116.80 MPa, and the stress of coated fiber decreases from 89.47 MPa to 52.02 MPa with the increase of intermediate PI layer thickness. The stress value tends to be stable, when the thickness value is greater than 140 μm. With the increase of the thickness of the filling adhesive at the bottom of the optical fiber, the stress on the uncoated optical fiber decreases from 125.04 MPa to 86.82 MPa, and then increases to 93.53 MPa, and the stress on the coated optical fiber increases from 81.30 MPa to 84.52 MPa. The transverse and longitudinal offsets of the embedded optical fibers at both ends of FEOPCB were measured, and the maximum values were 3.87 μm and 7.15 μm, respectively. It can ensure high coupling efficiency between bare optical fiber and photoelectric chip. FEOPCB has completed the reliability experiments and performance tests perfectly. The research results show that the coated optical fiber can be compatible with the printed circuit board lamination process. FEOPCB has high reliability and can meet the requirements of high-speed signal transmission.