Prototyping 1

Antennas and Microwaves

PCB Prototyping

Prototyping of Microwave PCBs

INTRODUCTION

The majority of literature dealing with rapid or home production of PCBs centres around standard FR4 material and the use of rather noxious chemicals such as Ferric Chloride. There are other non-chemical options, such as CNC micro-milling machines, but you probably need to be on good terms with your bank manager to contemplate one of these.

However, microwave PCBs generally have different characteristics and requirements to standard circuit-boards and these can be used to our advantage. The following list identifies some of the typical attributes :

1) The circuits often have a low component count, reducing the need for super fine tracks and multi-layer boards. Microwave components are expensive and most designers will try to get signals down to a convenient intermediate frequency or baseband as soon as possible.

2) At microwave frequencies the actual circuit board material is important, dielectric constant, loss tangent and board thickness must be known and guaranteed throughout the board. In standard FR4 material these parameters are poorly controlled, and it is therefore generally unsuitable by default. The proposed alternative to FR4 has some other benefits which make the whole process possible.

3) Many passive microwave components consist of geometric shapes, the dimensions of which are important in defining their performance e.g. Transmission lines, inter digital filters and couplers etc. Under or over etching in the chemical process (especially homebrewed varieties) can be problematic.

MATERIAL SELECTION

The process can be summed up by the phrase “score and peel”, very simple but surprisingly effective, the key is finding the right material. As I have already mentioned FR4 is unsuitable because of it’s electrical characteristics, or lack of them. Also there is a problem with it’s peel strength, basically the copper is stuck very well to the dielectric substrate. This is normally good, unless you want to remove it, in which case it’s bad.

Enter “Rogers Corporation”, purveyors of fine microwave substrates since the 1960s and traditionally associated with materials such as Reinforced-Teflon-Duroid for use in space and military applications. Of course with the words “microwave”, “space” and “military” in the same sentence the materials are not cheap. However, with the commercial microwave revolution (primarily satellite TV and mobile phones networks) came a demand for low cost alternatives to RT-Duroid, but with FR4-like handling and processing characteristics. PTFE based boards are notoriously difficult to work with because of their soft pliable nature; try peeling the copper off and the whole board will bend.

The new found demand resulted in the “RO4000 series of materials, a glass reinforced hydrocarbon/ceramic laminate designed for performance sensitive, high volume commercial applications” - as stated in the Rogers promotional literature.

RO4350 is the closest to FR4 in that it has a flammability rating, the RO4003 material does not. For reference here are some of the more important parameters for RO4350

@ 10Ghz / 23 Deg C (See the links on page 5 for Rogers Corporation to find out more).

Dielectric constant Er 3.48 +/-0.05

Dissipation factor tan(delta) 0.004

Copper Thickness 35 um

Substrate Thickness 0.25 / 0.51 / 0.76 / 1.52 (mm)

Apart from having some guaranteed electrical characteristics, it is rigid, and best of all for our purposes, has a peel strength somewhat lower than FR4, especially when heated. This means that once you have scored through the copper layer, tracing out your design, you can simply peel off the bits you don’t want.

The following pages describe the various options for accurately scoring the copper and

give a few examples of what can be done.

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PCB Prototyping

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