Updated October 4,
here to go to our main page on microwave packaging
here to learn about hermeticity
to go to our new page on microwave circuit card assemblies
to go to our new page on microwave printed circuit boards
Microwave modules are often referred
to as "hybrids". A hybrid circuit in this sense combines
a few technologies into one module, such as discrete FETs attached
to alumina substrates containing circuit traces. No one seems to
remember how or when the microwave module was named a "hybrid"
but the name has stuck for good.
Modules consist of at least the
following parts: a housing or enclosure, a circuit board, and one
or more RF connectors. Typically the circuit board will be populated
with passive and active devices such as inductors, resistors, MMICs,
The process of assembling a microwave
module might follow the following steps:
|1. Clean parts
||Just a fingerprint
on a gold trace could cause some expensive rework later.
|2. Attach substrates
||Could use epoxy
or solder to hold down the substrates, but it must be conductive
to join the ground planes.
|3. Attach component
||Could use solder
or epoxy, but process temperature may be limited by what happened
in the substrate attachment step.
wires or ribbons are used, as opposed to aluminum wires which
are used in assembling silicon stuff.
||Some one has
to check out your work! Inspection can use microscope, X-ray
or sonic scan.
test and alignment
||Here is where
you get to set up the bias networks, and maybe tune up some
of the microwave stuff.
||Laser or resistance
welding is used to attach a lid. This is followed by hermeticity
check, usually fine and gross leak tests.
temperature cycling or shock, or vibration or a combination
|9. Final electrical
your module does what it is supposed to...
|10. Final inspection
||Then ship it!
In production, the assembly process
would be different from the prototype process, and might use some
expensive equipment. For example, epoxy can be dispensed using automated
equipment, and automated pick-and-place equipment can be used to
install the components
The attachment of substrates
can be accomplished using conductive epoxy or solder. The solder
can procured in a preform, which means your friendly solder vendor
will cut out a sheet of solder in the shape of what you are trying
to attach, just as you might cut cookie dough with a cookie cutter.
Otherwise it some in a wire, a sheet, or a paste.
Solder selection depends on many
parameters: what metals you are joining, what will be your operating
temperature range, required thermal conductivity, electrical conductivity,
and tensile strength, corrosion resistance, price considerations,
flux or no flux, etc. You can check out the table
of solders here at Microwaves101.com (which we borrowed for
Housing materials and plating
The choice of housing material
has many considerations, and quite often the choice boils down to
two materials: kovar or aluminum. Let's look at some of the reasons
The thermal expansion coefficient
is an important consideration when materials are to be joined and
then operated at wide temperature range.
Alumina is the "classic"
substrate material for microwave modules. Its main attractions are
that the thermal expansion coefficient is low (an OK match to Kovar), it has low dielectric loss tangent, and when plated properly it is easy to wirebond to. Other substrate materials you might encounter in a hybrid module include co-fired ceramics (contain alumina mixed with glass) and quartz (known for ultra-low loss but poor thermal conductivity).
Contributed by Nameless Insider
from New Jersey: Sometimes called waveguide modes, sometimes "box"
modes, either way this spells trouble. At the end of the day, you
close the cover and the thing misbehaves. Sometimes the cause is
box modes, other times it is in indication to cut back on the drinking
at work. Basically, the shiny shielded metal box to enclose the
PCB is the same as putting your circuit into a cavity resonator.
Two solutions are often employed: 1) construct the box with different
dimensions so that the many resonant modes are at frequencies that
aren't excited (or important). This is a really fun exercise, and
another trip to the shop. Note that software such as Agilent ADS
will try to calculate and model the box modes. 2) Add some absorber
material. So, unless you get paid overtime at work, think twice,
Military and aerospace microwave
modules need to be extremely reliable. To make a module almost indestructible
and give it a long shelf life involves hermetically sealing it.
A measure of hermeticity is the leak rate, which is expressed in
atmosphere-cc/second. For a pressure differential of one atmosphere,
the following table shows how long one cubic centimeter or gas will
leak from a module:
||One cubic centimeter
Hermetic cover seals are usually accomplished in two ways: either
laser welding or resistance-welding. Resistance
welding is more often done on metallic housings that contain some
type of steel, such as kovar. Resistance welding is also known as
seam-sealing, or roller-welding. It is accomplished by driving a
DC electrical current through the housing/lid interface using a
pair of conductive wheels, or rollers, which heat the module/lid
joint enough to melt it due to I2R losses. As the metal
heats up and joins, the rollers are passed around the housing until
the lid is completely joined to the housing body.
are often used on aluminum housings. In this process a laser is
used to melt the lid and form a weld joint which is steered around
the housing until the lid is completely sealed.
Here is a link to a vendor that
would like to sell you some cool sealing equipment. Unitek
Miyachi has a bunch of different offerings, so check out all
of their areas before you decide which you want.
Hermetic modules are typically
sealed in an inert nitrogen atmosphere, so that it is drier than
a Tucson winter inside. Both types of welds (resistance and laser)
are typically performed in an inert atmosphere of nitrogen using
a "glovebox" that allows the operator to handle the hardware
using rubber gloves while maintaining a positive pressure within
the chamber. The guys at Terra Universal would be happy to sell
you your very own seam-sealing glove box, pictured below:
In case of an electrical failure,
if you are lucky you can de-lid a hermetic module maybe two times
to rework the insides, for both laser sealed and seam-sealed units.
In either case you have to be really careful about grinding off
the lid and getting a lot of particles inside the guts of the module,
which will cause all manner of short-circuits later. The best way
to accomplish this delicate task is to stop grinding when the lid
is a few mils thick at the edges, then cut off the lid with a sharp
blade, like you would open up a can of dog food.
Speaking of particles, there
is a test you can do to a sealed module to determine how much crap
is floating around in it. This test is called particle-impact
noise detection, or PIND for short. The PIND test set-up has three
components, the controller, the shaker, and the oscilloscope. The
controller commands the shaker to shake the unit, and the oscilloscope
displays the noise generated by the loose particles. Typical systems
can detect particles with masses of 1 micro-grams and diameters
of less than 1 mil (0.001 inch) at accelerations as low as 2 Gs
and 40 hertz.
The image of the PIND setup is
courtesy of NASA's Goddard Space
One downside to hermetic sealing
is that whatever you sealed up in the housing is stuck in there
forever, and reactions from different components may ensue. One
phenomenon that is widely discussed in microwave modules is the
issue of hydrogen poisoning of GaAs
devices employing Schottky gates with platinum or palladium in the
gate metal (that includes most MMICs). Where does the hydrogen come
from inside the module? Perhaps from outgassing from materials such
as epoxies. Without "getting" into the physics of the
problem here, one solution that is offered is the use of a hydrogen
"getter", which could be attached to the lid prior to
sealing. One way to minimize the effects of hydrogen poisoning is
to bake-out the module in a vacuum prior to sealing, in a process
step known as vacuum bakeout. What else would it be called?
There are two types of leak tests
to verify the hermeticity of a module. These are known as fine and
Fine leaks are defined as less
than 10-5 atm-cc/sec. Fine leak testing is done by placing
the module into a pressurized helium atmosphere (known as "bombing")
for two hours at up to 90 PSI. Helium is used for two reasons. First,
it is a really small molecule, so it fits between small crevices
and will find its way into a failed module. Second, as students
of the Hindenburg disaster know, it is inert, and therefore not
flammable. Plus, it helps you make that cool "Bee Gees"
voice when you inhale it. After bombing the module with helium,
a sniffer is used to determine whether helium is leaking out of
the module. The technical term for the sniffer is a helium mass
Guess what? A module that passes
fine leak might still have a big leak in it! In this case, all of
the helium would escape between the bombing and sniffing operations,
and you wouldn't know it. So a gross leak test is done to check
for big holes. A gross leak is defined as greater than 10-5
atm-cc/sec. The test for gross leak is similar to the dunk tank
that your local Goodyear tire guy would use to find a leak in a
tire, except a more inert liquid is used instead of water. The procedure
goes like this: the module is immersed in liquid fluorocarbon (the
same stuff that is wrecking the ozone layer) in a pressure chamber
which is cranked up to 90 PSI for an hour. The part is then placed
in a hot bath of liquid fluorocarbon (unpressurized), and an operator
watches it for 60 seconds to see if any bubbles come out which would
indicate a failure.