Using MITS FP-21T Precision milling machine at the University of Belgrade

Introduction process
The FP-21T Precision milling machine was purchased from a grant of the Serbian Ministry of Science and Technological Development. The grant was given to the Group for Electromagnetics, Antennas, and Microwaves at the School of Electrical Engineering. For about half a century, the Group has been one of the world leaders in the theoretical research, computational techniques, and engineering development. The group has 10 full-time faculty members, who cover teaching at the bachelor, master, and Ph.D. levels. At all three levels, there are curricula dedicated to microwave engineering and the total number of enrolled students is about 120 (out of about 3000 at the whole School of Electrical Engineering).According to the rules set by the Ministry, all research and academic institutions in Serbia have access to the machine. In particular, we expect the machine to be heavily utilized by the staff of the Microwave Institute in Belgrade, which is involved in the design and production of microwave and millimeter wave devices.Getting started with the machine was facilitated by the training obtained from MITS in October 2008. It was followed by a period of a few weeks of self-training. As the result, we now feel confident to be able to produce even tricky printed patterns.We have to emphasize the good support obtained from MITS, whose staff is always ready to help, answer any question we may have, and promptly improve software and manuals wherever appropriate.

FP-21T Precision enables resolution below 0.1 mm

The FP-21T Precision machine has an important role both in the teaching and research activities of our group. The machine enables rapid experimental verification of the design of microwave devices and, in particular, millimeter-wave devices.

The target substrate is in most cases a Teflon-based laminate, Rogers 4003 or a similar substrate. Some high-frequency circuits (e.g., filters) require very narrow gaps between microstrip traces.

Before purchasing the FP-21T Precision machine, our PCBs were produced by etching. The standard resolution of the available etching technology is 0.2 mm.
With particular care and manual processing, the resolution could be pushed down to nearly 0.1 mm, but with low yield.

However, the FP-21T Precision machine has enabled easy manufacturing with resolution below 0.1 mm.
We have successfully milled patterns with 0.05 mm resolution and we have been investigating possibilities even to push this limit further.

The new technology, which we have mastered owing to the FP-21T Precision machine, enabled production of filters with wider bandwidth than we could produce by etching and facilitated production of devices for the millimeter-wave range. Although we have been using the machine for only 3 months, we have already made several wideband coupled-resonator filters.

A significant step-up in PCB quality and fast turn-around time

Besides achieving a significant step-up in the quality of manufacturing of microwave printed-circuit boards, we have attained fast turn-around time, which is necessary for the development of laboratory prototypes. Instead of outsourcing the PCB manufacturing, we can now quickly get prototypes, even in the evening and on weekends, which is important in the research activities of both the faculty members and Ph.D. students.

Sharp edges obtained with suitable tools

The milling process should be carefully implemented. Microwave milling tools are of great help not only to obtain vertical metallization walls, but also to accurately control the width of conductors (strips, traces) and gaps.
Incremental milling helps to obtain clean conductor edges. Conical tools leave sloped conductors walls, with sharp and clean edges. However, good control of the tool depth is necessary to maintain the necessary precision of patterns.

Capable of designing microwave circuit with large copper leftover surfaces

Besides using the milling machine for efficient prototyping, the milling process has motivated us to undertake research activities aimed at obtaining accurate models of milled microwave circuits and also at speeding up the milling process by leaving large copper areas intact (rather than rubbing them out).

These large areas are coupled to the microwave circuit and they have a twofold influence. The first influence is modification of the characteristic impedances of traces in a broad frequency range. The second influence is caused by parasitic resonances of the copper areas. In our research, we have performed computer simulations, followed by experimental verification, which have given us sufficient information to be able to keep both influences under full control.
Hence, we are capable of designing microwave circuits with large copper leftover surfaces, keeping intact the circuit performance.

In conclusion, the FP-21T Precision milling machine has given us a significant technology boost which is of great help in our teaching, research, and design activities.

Professor Dr. Antonije Djordjevic
University of Belgrade
School of Electrical Engineering