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Improving Density in Microwave Multilayer PCBs
Improving Density in Microwave Multilayer PCBs
Paper presents work performed to achieve LCP-based high density multilayer structures, describing the breadboards manufactured and tested.
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Authored By:
David Nevo, Olivier Vendier, Jean-Louis Cazaux, Jean-Luc Lortal
Thales Alenia Space
France

Summary
The need in complexity for microwave space products such as active BFNs (Beam Forming Networks) is increasing, with a significantly growing number of amplitude / phase control points (number of beams * numbers of radiating elements). As a consequence, the RF component's package topology is evolving (larger number of I/Os, interconnections densification ...) which directly affect the routing and architecture of the multilayer boards they are mounted on.

It then becomes necessary to improve the density of these boards. It has already been demonstrated the benefits of non-PTFE (Teflon) materials for the manufacturing of microwave multilayer PCBs. The Liquid Crystal Polymer (LCP) is a very interesting candidate allowing, among others, to achieve RF and LF flexible interconnections. It has many advantages for packaging applications or manufacturing multilayer structures (low dielectric constant and losses, low water absorption, low CTE in X and Y axis.)

Mostly, this material is available in very thin layers, allowing to considerably reduce the total thickness of the board and favoring densification (decrease of via diameter, pads, track width...). However, the use of LCP for printed circuit board is fairly recent and few manufacturers have experience with this material. This paper will present the work performed to achieve LCP-based high density multilayer structures, describing the different electrical and technological breadboards manufactured and tested and presenting the results obtained.

Conclusions
The liquid crystal polymer is a very interesting candidate as a printed circuit board substrate and the work presented in this paper confirms this. Its excellent dielectric properties have been first measured up to 30GHz with an accurate characterization of cavity. Very good copper adhesion has been then observed even with high thermal stresses (soldering / desoldering).

A double sided breadboard has allowed to know the etching accuracy that can be achieved for high density circuits (small diameters, very fine track widths / isolations). Finally, the possibility to achieve etched resistors on LCP has been demonstrated both in outer layer and buried in the circuit, provided that you comply with a sufficient track width, in order to reduce their sensitivity to etching tolerances and protect them in case of high pressing temperature.


Initially Published in the IPC Proceedings

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