& wire construction
Updated June 15,
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This material was provided by
State College Pennsylvania. The Unknown Editor couldn't resist
adding a few comments. The pictures are excellent, and no worries,
they are non-ITAR. Many thanks!
Check out this dimes-worth of
MMICs. The three-port circuits at 2, 7 and 9 o'clock are all mixers.
The "giant" circuit at 11 o'clock is a wideband, two-stage
dual amplifier, the smaller circuit at 1 o'clock is a single stage
amplifier, also of very wide band. The circuit at 4 o'clock appears
to be a use of "digital" capabilities on a GaAs circuit
(perhaps a phase shifter integrated with a shift register), which
provides bragging rights for the foundry that created it but could
probably be replaced with a cheaper silicon alternative.
Chip and wire refers to a method
of construction where size and performance matters. Yes dear, smaller
and discrete FETs are the order
of the day for chip & wire construction. These are designed
with your microstrip and coplanar layouts in mind. Most of your
favorite large packaged components can be purchased in unpackaged
die form. With non-microwave companies (boo!) like Analog Devices,
Linear Technologies and Maxim you may have to say please more than
once. Obtaining unpackaged dice (dice is the accepted plural of
the word die) is a very real problem for military contractors, who
need MMICs in die form but in low quantities; commercial MMIC houses
often don't seem to care about this potentially lucrative end of
the business. Our advice: hire some quality people, buy some inspection
scopes, and show your patriotism!
Chips used in chip-and-wire are
not only GaAs MMICs, but include silicon circuits as well. Silicon
supports RF, but also provides cheap logic and driver circuitry.
Yes, you can wirebond using gold wire to aluminum pads on silicon
Connections between components
are made using ball or wedge bonding. We have a web page for wire
bonding, well at least we're working on it. You'll see plenty
of examples of ball and wedge bonds below. Clicking on most of the
images on this page will reveal higher-resolution photos.
The vast majority of circuit
boards for chip and wire construction are ceramic (alumina).
Everything you see on this page uses thin film alumina boards. Rogers
Duroid is very good for its low dielectric constant but that soft
crayon board material
doesn't take well to the heat of ball bonding or the pressure of
thermosonic compression. When they say "softboard" they
Parallel gap welding is typically
used to interconnect boards using gold ribbon. This welding can
even be used to connect Rogers Duroid boards.
Using chip and wire construction,
packaging parasitics are reduced and you no longer have those nasty
connectors taking up
valuable space between each part. You can even countersink that
MMIC to minimize lead length (parasitic
inductance). MMICs can be placed on tabs to bring the top surface
level with that of the interconnect substrate, and thereby reduce
theseries inductance. But added complexity means you'd better think
carefully about how much better the part is going to work and if
it is worth it. In the end your goiing to have potentially greater
gaps because of the tolerance stackup.
Chip & wire is smaller, lighter
and can support higher frequency compared to surface mount and flip
chip. What is the down side to chip and wire construction? cost:
- Gold is the preferred medium
for construction (wires, plating, traces) and those custom thin-film
alumina boards are not cheap.
- MMICs and other active components
come in passivated and unpassivated. Unpassivated means that part
must be stored in a dry environment (dry nitrogen). The package
your design is in must be hermetically sealed. Passivated and
unpassivated, it would be embarrassing to have your design fail
in a 2 or 3 years due to corrosion. Hermetic housings don't come
cheap, you can figure a minimum of $100 for the simplest housing,
- Special ball, wedge and welding
equipment cost much more than a Radio Shack solder station. Your
technicians are either working under a microscope or have the
eyes of the Six Million Dollar man.
The images below will combine
some of the gifts that the Gods of electronics have given us.
Below are two MMIC amplifier
pairs in balanced configuration with two-section Wilkinsons
to split/combine the signal. Wilkinson are in-phase divider/combiners,
two sections implies that this module is intended for wide bandwidth.
If you want the benefit of improved 2nd order cancellation, the
split phase needs to be rotated by 90 degrees (in quadrature). Lange
couplers shorted at two ports are used here to rotate transmission
phase by 90 degrees with respect to a "through line" of
the same length, a clever use of this ubiquitous coupler. In the
lower left and upper right of the ceramic boards, ribbon was welded
down to reduce the path length. This type of tuning is often performed
on chip-and-wire modules, both before and during test. The "DC"
feedthroughs through the floor and the RF connector (which is 2.4mm
judging by the threads) pointed northward give a hint that this
is a double-sided module. The lid must fit the profile of the module
cavity, because it is non-rectangular, we can assume that the lid
will be laser-sealed as opposed to seam-welded.
Chip & wire construction
can be used to make all types of circuits. Below is a dielectric
resonator oscillator (DRO) feeding a broadside coupler and a MMIC
amplifier. Above and below the amplifier die are multi-tap resistors
used for adjusting the bias voltages of the MMIC.
Shown below is a balanced
amplifier that takes up less than 0.3 square inches. FETs are
used here instead of MMICs. The coils are gold plated, the resistor
and capacitor contacts have gold contacts and the Kovar carrier
that the alumina thin film boards are epoxied to is gold plated.
In this example, ball bonds are used to connect the FETs. Thermosonic
"wedge" bonding is used on the coils. You can see the
parallel gap ribbon weld at the bottom right (connecting the Lange
board to a mitered bend board.)
Carrier plates allow sub-components
to be separately assembled and tested before integration into the
housing. It takes a steady hand and good tweezers to place and tighten
the hardware (screw and washer) without dropping something on top
of a gold wire and messing up an entire days work! The tradeoff
between carriers and no carriers in a housing is complex, you have
to weigh the advantages (ability to pretest, easy replacement) against
disadvantages (additional parasitic inductance between carriers,
potential ground plane problems), and more parts on the bill of
materials. Screws by the nature of their manufacture are filthy
things, they come with machine oil residue and chunks of metal left
over from the threading operation. You should always clean hardware
using ultrasonic bath using a benign solvent like isopropyl alcohol
prior to installing inside microwave modules.