Reliability in microwave modules, MICs (microwave integrated circuits), hybrids, RF MMIC (monolithic microwave integrated circuits) modules, can be achieved using a unique set of processes and materials. Microwave packaging technology in this article refers to relatively small packages (smaller than the outline of your cell phone) hermetically sealed or "nearly" hermetically sealed chip and wire modules. Proper spacing of MICs interconnected with other passive components inside a ceramic or metal can to achieve a specific RF function is an art form that takes years to master. These are some key issues to consider when assembling your microwave module.

In the beginning phases of microwave design, design for manufacturability (DFM) is an essential concept to consider. A poor design results in yield loss,

production delays, reliability problems and unhappy customers. Microwave hybrid circuit designers must "design with the process in mind". Any circuit that can’t be assembled within reasonable cost and schedule constraints is a bust. DFM requires designers to perform detailed computer stress and thermal modeling analysis up front before ever committing to production. Finite element analysis (FEA) identifies weakness in the mechanical design and guides the selection of materials based on the package geometry and coefficient of thermal expansion for each candidate material. Thermal modeling is another critical DFM requirement. Heat build up inside a microwave power module is the number one cause of early field failures. Based on the thermal analysis model designers can select the proper heat spreaders to keep the junction temperatures below specification and assure reliable operation.

Unique materials and processes are required for the successful creation of microwave modules. Substrates made from copper clad Teflon are often used because of their desirable dielectric constant, low loss tangent and other RF properties. However, the teflon "soft board" material creates problems in die attach and wirebond. When the ultrasonic energy from a wirebonder hits the soft substrate the energy is dissipated and this effectively shrinks the bond window and leads to a lot of frustration during assembly. Gold tin (AuSn) eutectic die attach of gold back MMICs is a very common process in power modules. MMICs are made from very thin (.004 " and less) gallium arsenide material, which has about one half the fracture toughness of silicon. These fragile chips have to be carefully handled and soldered down to obtain a complete void free interface to the substrate, or risk hot spots and stress build up on the die surface. Deep access wire and ribbon bonding often follow after die attach. This is another critical process step far beyond the scope of this article.

To protect the unit from moisture you will need to hermetically seal the device. Laser welding aluminum alloys or seam sealing iron-nickel alloys is the preferred method to create a hermetic seal. What is hermeticity? The dictionary definition of the term "hermetic" means a seal that is gas tight or impervious to gas flow. In the context of a hermetically sealed microwave module it implies an airtight seal that will keep moisture and other harmful gases from penetrating the sealed package. Of course it also means that it keeps hydrogen and other gases from escaping, which can lead to a problem known as "hydrogen poisoning".

MIL-STD-883 Test Method 1014 is the universally accepted test designed to determine the effectiveness or hermeticity of the seal. Lately, microwave companies are turning to plastics and other materials such as Liquid Crystal Polymers (LCP) to make the package.

When the term "near -hermetic" or "non-hermetic" packaging is used it implies the package is made from polymeric materials as opposed to glasses, metals and ceramics. Non hermetic parts made from plastics reduce cost, weight, size and if designed, manufactured and tested properly hold the promise of a reliable substitute for a hermetic can. Military specs do not recognize a "non" or "near-hermetic" package as a hermetic device.

Designing, assembling and sealing a microwave module require a plethora of additional specs to be achieved. It begins with a careful design and focus on manufacturability. Assembly processes such as substrate fabrication, die attach and wirebond present their own set of problems unique to devices that operate at microwave frequencies and above. Deciding on whether or not to hermetically seal the device should be based on the susceptibility of the device to moisture, the expected lifetime of the product and the predicted end use environment.