A Division of General Electronic Devices
Manufacturer of Quartz Crystals, Oscillators and Filters
(760) 979-1441 • 320 South Pacific St., San Marcos, CA 92078
Hermetic Seam SealingMicroelectronic circuits typically have a metal (Kovar) container (Package) with an open top and upstanding sides (Figure 1a). A Kovar lid .010-.015" thick with a .005" thick perimeter etched" "race track" area is placed over the open top to form a hermetically sealed package by seam welding. Alternately, a ceramic package with a Kovar "picture frame" seal ring is also used (See Figure 1b below). The package base material is typically Kovar, gold or nickel-plated, with matched glass to metal seals or a cofired ceramic package.
Figure 1a: "Bath-tub" plug-in packages enclosure
Figure 1b: Cofired packed with Kovar seal ring and etched lid
Resistance welding is typical used for hermetic seam sealing of microelectronic packages. The package and lid are passed under a pair of tapered conical copper electrode wheels (See Figure 2). The welding power supply produces a series of current pulses, which are conducted from one electrode through the lid to the other electrode to form a series of overlapping spot welds.
The localized heat at the lid/package interface forms either a weld or braze type seal. The nickel or gold plating acts as the brazing medium.
Two sides of the package are welded in parallel simultaneously and then the package is rotated 900 to weld the other two sides. (See Figure 3a & b). During the welding process, the lid may be held down manually by hand with pressure, magnetically, with vacuum through the work holder, or with a robotic piston. Alternately, the lid can be tack welded prior to vacuum bake.
Figure 3a: 1st weld sequence Figure 3b: Electrodes rotates 900
The localized heating is generated by the I2R losses at the relatively high resistance of the small contact area between the electrodes and the lid edges. This localized temperature can be controlled from 2000C for soldering to 15000C for Kovar welding while keeping the package base temperature and components to under 1250C.
The weld cross-section is shown in Figure 4.
Figure 4: Micro cross section of a Au-plated stepped lid package showing melted plating forming braze at lid/sidewall
Typically, hybrid packages are vacuum baked out at a high temperature and vacuum for 1500C for 8-24 hours and immediately transferred into a glove box, which contains a dry nitrogen environment of less than a 100ppm of water (Figure 5).
Figure 5: Hermetic seam welder glove box