Abstract
Broadband Wireless Access systems, such as Local Multipoint Distribution Service (LMDS) and
Multipoint Video Distribution Systems (MVDS) tend to operate at mm-wave frequencies where large
allocations of spectrum are available. These frequencies were previously utilised by low volume
applications such as radio astronomy and military systems. The techniques developed and the prices of
parts reflected this. As the consumer market for Broadband Wireless Access equipment matures, new
techniques are being developed which allow the production of mm-wave equipment in high volumes at
low prices. This paper describes the design and development of upconverter and downconverter Multi-
Chip Modules (MCMs) suitable for manufacture at low cost, in high volumes. The modules utilise printed
filters, GaAs MMICs, discrete SMT components and a PTFE composite soft substrate.
Introduction
Equipment operating at mm-wave frequencies was traditionally the preserve of radio astronomy and the
military. Systems were produced in low volumes and were normally hand-crafted; as such they tended to
be very expensive. Today the use of mm-wave systems for commercial applications is growing rapidly. In
particular, broad band multi-media services, such as LMDS and MVDS, have the potential to become very
high volume markets for mm-wave electronics. This growth in the commercial use of mm-wave
equipment has placed tremendous pressure on suppliers to reduce their costs.
GaAs Monolithic Microwave Integrated Circuits (MMICs) offer a means of fabricating large quantities of
highly reproducible, low-cost mm-wave circuits. However, module assembly techniques, suitable for
circuits operating at mm-wave frequencies, can be complex and costly. They also tend to be incompatible
with low cost assembly methods, which could be adopted for the biasing, control and IF circuitry. This
paper describes the development of a low cost route for producing mm-wave sub-system assemblies. A
PTFE substrate is used, rather than ceramic and the attachment of bare die and SMT components can be
performed in a single process step. Details of the design, fabrication and measurement of a 27.5 to
29.5GHz upconverter and a 27.5 to 29.5GHz downconverter, developed using these techniques, are
presented.
Substrate Manufacture
Careful choice of substrate is vital. As well as being low cost, the substrate should possess the following
properties for optimum use at mm-wave frequencies:
• Thin substrate height (to reduce dispersion and radiation losses)
• Low dielectric constant (helps reduce effects of tolerance variations and avoids dimensions of
distributed structures becoming impractical)
• Well controlled dielectric constant (reduces performance variation)
• Low dissipation factor (low loss)
With these factors in mind, the material chosen was Rogers RT Duroid
®
5880; a glass microfibre
UHLQIRUFHG37)(FRPSRVLWHZLWKDGLVVLSDWLRQIDFWRU WDQ RI DQGD UHODWLYH GLHOHFWULFFRQVWDQW (εr of 2.2 (standard tolerance of ±0.02).
It is also available in thin substrate
heights and 0.005” was selected. The
material is also available with brass
backing. As well as providing
mechanical rigidity, this allows bare
MMIC die to be mounted in to pockets
cut in to the substrate. This results in the
surface of the die being almost level with
the surface of the substrate, so
minimising bondwire lengths.
The first stage of the substrate
processing is the drilling of through
substrate via holes and any fixing holes.
Selective plating of the through substrate
via holes, to contact the front side
metallisation with the back, then takes
place. Next the pockets are cut into the
substrate, revealing the brass backing.
Selective nickel gold plating of the
copper tracking and the brass of the
pocket bases, provides protection from
contamination and a surface suitable for
gold wire bonding.
Assembly
The first stage in the assembly process is
to dispense conducting epoxy in to the
pocket bases for die attachment and onto
the placement pads for the attachment of
any SMT components, which are used.
For development work dispensing is
carried out manually but automated
dispensing methods would be used for
volume manufacture. Both the bare die
and the SMT components are then
placed. Once again this is done by hand
during development but would utilise
automated pick and place machinery in
production. The epoxy is then cured,
fixing all components, bare die and
SMT, in place in one step.
After curing the die are bonded to tracks
or pads on the Duroid using 0.001”
diameter gold wire. Wedge-wedge
thermosonic bonding is used. Figure 1
shows the completed assembly of the 27.5 to 29.5GHz downconverter MCM. Some diagnostic sub-circuits
have also been fabricated on the same tile. No discrete microwave capacitors were used, all de-coupling is
realised using 0402 SMT components. This reduces component cost and assembly complexity. A photograph of an upconverter MCM (which also
includes some diagnostic sub-circuits) is shown in
Figure 2. The 4GHz IF amplifier and the RF power
detection circuit have been realised entirely with
SMT components and can be seen at the bottom left
and bottom right of the photograph, respectively. |
