Hybrid modules

Updated January 11, 2007

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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.

Module assembly

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, etc.

The process of assembling a microwave module might follow the following steps:

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

Solder

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 the experts!)

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 for this.

The thermal expansion coefficient is an important consideration when materials are to be joined and then operated at wide temperature range.

Substrate materials

Alumina is the "classic" substrate material for microwave modules. Its main attraction is the

Moding in packages

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, machine once.

Hermeticity

Military and aerospace microwave modules need to be extremely reliable. To make a module almost indestructable 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:

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.

Lasers 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:

http://www.terrauniversal.com/products/gloveboxes/gallery/glovebox1a.htm

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 Flight Center.

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?

Leak testing

There are two types of leak tests to verify the hermeticity of a module. These are known as fine and gross leak.

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 spectrometer.

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.

Attention, leak testing equipment suppliers: contact us about putting your ad here!