Design of printed circuit board with separate heat metal block for high-speed optical transceiver module

Abstract

Recently, the size of high-speed optical transceiver module is reduced gradually, but the channel number is increased. Because the densities of optic-electronic devices and driver chips are increased in the optical transceiver modules, the module temperature will raise. In electronic elements, the thermal noise is increased due to the higher temperature. The thermal noise will lead to the degeneration of signal transmission performance. Therefore, we use a separate heat metal block embedded in the high-speed optical transceiver module. In addition, the design of printed circuit board (PCB) in high-speed optical transceiver module will be also investigated. The coplanar waveguide is used and the impedance match of signal trace is also designed in our PCB. The coupling capacitors are added near the high-frequency signal input or output ends to block DC level. The coupling capacitor pad size is different with the width of signal trace, so the anti-pad is designed under the coupling capacitor. The anti-pad is used to improve the impedance mismatch of signal path through the coupling capacitor. The ground vias are also designed in our PCB. The ground of coplanar waveguide is connected with the other ground layer through many ground vias to achieve equal potential for all ground. While the impedance match and the equal potential for all ground can be carefully designed, the quality of high-speed signal transmission will become better in PCB. In high-speed optical modules, the heat sources are usually from the high-power electronic devices, such as laser diode driver chips, receiver amplifier chips, and clock and data recovery chips. The temperature of the high-power chips will influence the quality of signal transmission, and even lead the module failure. Therefore, the metal block is embedded at the location of high-power chips in the high-speed PCB, and the high-power chips are mounted on the separate heat metal block. The metal block has to be isolated with PCB to avoid the short circuit with signal traces. We used the six-layer PCB to design a 4×25-Gb/s optical transceiver module. The 1.2-cmx1-cm separate heat metal block was embedded in the 0.43-cm ×0.53-cm PCB, and the laser diode driver chip and receiver amplifier chip were mount on the metal block. The 25-Gb/s eye diagrams of transmitter and receiver were measured. For transmitter, the rise time, fall time, and jitter are 14.53-ps, 17.61-ps, and 10.854-ps, respectively. For receiver, the rise time, fall time, and jitter are 18.12-ps, 18.12-ps, and 12.051-ps, respectively. Using the PCB with separate heat metal block, the 4×25-Gb/s optical transceiver module has been designed and realized.