Abstract
In this article, a novel planar helix traveling wave tube structure is proposed for S/X-band amplification. The planar helix structure is developed between two printed circuit boards such that they utilize solder balls to connect each other. This technique which comes originally from microelecronic packaging technology called ball grid array. To evaluate the performance of the proposed structure, one period of its geometry is modeled and the cold test parameters are calculated in CST microwave studioTM. The fundamental space harmonic mode overlaps well with the beam-line in the dispersion diagram for both bands. The in- and out-coupler ports are also designed in the form of coplanar waveguides having good matching. For 150 periods of the planar helix, the 15 dB gain is obtained in particle-in-cell simulations for both bands. Finally, the fabrication process is implemented and the cold test is done.
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Appendix
Appendix
1.1 Fabrication process
A fabricated planar helix based on BGA technology is illustrated in Fig. 17. The top and bottom of the planar helix are structured copper over PCBs which consist of ball pads and oblique connection lines. One of the PCBs is equipped by solder paste through stencil printing which can find a spheric form after the first thermal reflow process. At the last step, both PCBs are aligned and connected to each other after the second thermal reflow process. We have used FR-substrate which is suitable only for measurement of cold test parameters. The used solder alloy is a lead-free solder Sn-3.0 Ag-0.5 Cu (SAC) with a melting point of 217 \(^\circ C\) which is a popular solder in microelectronic packaging Coombs (1987); Gilleo (2001); Puttlitz Karl and Totta Paul (2012); Tong et al. (2013). The scanning electron microscope images show the cross-section view of the interaction region and the space between layers for passing electron beam in Fig. 18. According to Fig. 18 the distance between the two PCB halves is \(440\,\upmu\)m.
Bumping process for developing planar helix based BGA technology
Fabricated the planar helix with solder balls bumping
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Babaeihaselghobi, A., Nadeem Akram, M., Badri Ghavifekr, H. et al. Design and implementation of a planar helix for traveling wave tubes based on package compatible ball grid array technology. Microsyst Technol 26, 1681–1687 (2020). https://doi.org/10.1007/s00542-019-04713-8
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DOI: https://doi.org/10.1007/s00542-019-04713-8