Electronics - Microwave Package Weld
Microwave circuitry is different from many other electronic devices in that the package around the active elements is also part of the microwave circuit. Since most microwave circuits involved expensive radar or communication systems, they require a very high reliability hermetic seal and a high yield. Therefore, their packaging must be robust, hermetically sealed, and capable of being opened and reworked when needed.
Laser welding meets these requirements. A laser weld produces a hermetic seal so that no contaminants can reach the circuits and the weld also traps a very high quality inert atmosphere in the package. Laser welding has very low heat input and can be placed very close to polymer seals, glass-to-metal seals, soldered components, and electronic circuits.
For aerospace applications the alloy of choice for these packages is aluminum. It has the electrical and thermal properties, is easy to machine, and very light weight. 6061-T6 alloy is used for the base of the package and then a 4047 alloy lid is laser welded onto the base with a butt, lap, or fillet joint. The 12% silicon-aluminum alloy 4047 can be fusion welded to the 6061 without cracking creating a strong hermetic seal. For rework, an end mill will machine through the laser weld back to the same level as the original base. Once repaired, another lid can be located on the package and laser welded again. The number of repairs is virtually unlimited. In some instances a laser weld is required through the lid into dividing labyrinth walls in the base to isolate certain parts of the circuit from others. Some microwave packages can use stainless steel or kovar components.
Most systems that weld microwave packages are called glovebox systems. These systems contain a high quality atmosphere of 90% nitrogen or argon and 10% helium. Special scrubbers remove oxygen and water from the atmosphere so that components welded in the system will have an atmosphere of less than 10ppm water and oxygen in them. Helium in the atmosphere allows for a mass spectrometer leak check of the package to better than 10-7 cc of He per second without bombing the component in a high-pressure helium atmosphere first. Inside the glovebox is a two three or four axis CNC motion system for welding. Attached to the main glovebox atmosphere where the welding occurs are a pass-through chamber, a vacuum bake-out oven, and the atmosphere purification system. The pass-through chamber is used to move parts and tools into and out of the glovebox. The chamber is pulled to vacuum before being opened to the inert atmosphere to keep the atmosphere free from contaminants. A vacuum bake-out oven removes contaminants from the parts before welding by accelerating the out-gassing of any volatile materials. Atmosphere purification is performed by activated getters that continuously remove oxygen and air from the atmosphere and are periodically generated by baking in a hydrogen-nitrogen atmosphere.
Pulsed YAG lasers are the best choice for microwave package sealing. Their high peak powers and low heat input can handle all the different alloys and create a very tolerant process while maintaining deep penetrations. Fiber optic beam delivery is most common for these parts because of the consistent focus spot size and top-hat energy distribution which helps with fillet and butt joints to fill any gaps and get good metallurgical mixing of the alloys. Welding speeds are generally 150-300mm/min for aluminum alloys and up to 5 times those rates for iron alloys. Weld penetrations of over 1.5mm are possible if needed.
Typical lasers that have been used for this application
JK125P, JK300P & JK300HP
For Fiber-Delivered Cutting, Welding, Heat Treating
JK450HP & JK600HP
Lamp-Pumped Nd:YAG, 450 & 600W
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