New technology for electrical/optical systems on module and board level: the EOCB approach

Abstract

Besides the optical wide and local area networks (WAN & LAN) there is a growing request for short-distance solutions like in backplanes, boards, and modules. Optical interconnections within systems and packages transmit data rates up to some Gbit/s with low loss, crosstalk, EMV- sensitivity or reflections. But here the fibers provide no satisfying assembling strategies. To find a more efficient solution, the Fraunhofer IZM has developed a packaging concept, which is based on a hybrid carrier containing both, electrical and optical interconnects: the EOCB (Electrical Optical Circuit Board). The key element is an additional optical layer with waveguide structures. This layer is handled by standard PCB technology. The result is, that waveguides are incorporated into the circuit board. For first tests we used the hot embossing process for foil structuring. After core filling and sealing it with an over- cladding, the optical layer is given into the PCB process. But other solutions for the waveguide structuring are possible too. They are currently under development. The EOCB uses multimode waveguides to relax assembly tolerances. It has an interface to common SMD technology. Therefore the EOCB concept merges with current methods of manufacturing equipment. Special optoelectronic components have been designed and fabricated to demonstrate the concept. First results will be presented on the integration of the future information medium "light" into the board. Optical Interconnects in Systems and Packages The need for optical interconnects is driven by the demand of growing data rates and higher performance for new equipment generations. Today light has already been established as a transport medium for information transfer to overcome the disadvantages of electrical links for the Gbit/s- transmissions in systems and packages, e.g. in data- communication and computer sector. Glass and polymer optical fibers combined with optoelectronic components are the state of the art for these links with highest data rates. However, these assemblies have disadvantages, too. One disadvantage in particular is that no passive optical structure can be implemented such as splitters or combiners but only by splicing. Furthermore, these systems with electrical and optical functions can not be assembled by standard SMD- processing. Additional steps which increase the costs are necessary. Approaches from Hewlett Packard and Motorola with MT-connectors as a part of the optoelectronical components have demonstrated that it is possible to overcome the problem of pigtails in the assembling process (1). The evolution of optoelectronical device packages is shown in Figure 1. After the introduction of SMD- compatible devices with optical MT connectors a hybrid carrier concept will provide a complete electrical and optical SMD compatibility. The new EOCB approach combines multimode waveguides with PCB technology. Innovative packaging solutions for optoelectronical devices will be supported by the availability of VCSELs and assembly techniques like flip chip, chip-scaled package and ball-grid array technology.