Low cost T/R module packaging is a unnerstone of economical active phased array technology[ll. "his paper describes a MMiC-based T/R module which includes composite metal matrix baseplate, multi-layer thick film substrate, soldered metal seal wall, and unique RF transitions, which provided a low cost solution for a C-Band active array application. Emphasii is placed on the RF feedthrough design and the packaging material choices.
Module DescriptionThe fmal module cross Section is shown in Figure 1. A baseplate of copper tungsten provides secure attachment to the next higher assembly, and supports the substrate and MMICs of the module itself. Hermetically soldered to the baseplate is a 96% alumina substrate. A single layer of gold thick film f o m the microwave interconnect on top, and three layers of thick film gold and dielectric are used to implement the power, ground, and logic circuits on the bottom.MMICs are attached to individual copper-molybdenumcopper carriers which are epoxied to the baseplate through cutouts in the substrate, A metallized alumina seal ring, topped with a seam sealed Kovar lid, completes the hennetic enclosure. 'Ihe use of a metallized seal ring, soldered around its entire perimeter, drove the design of a new RF transition, as well as the use of the multi-layer approach to DC and logic YO.
Design BackgroundThe original approach for this package called for a single layer thick film circuit. The metallized ceramic seal ring was then attached with a non-conductive glass sed, rather than solder, and both RF and DC lines were led on the top of the substrate beneath the wall. Apertures in the wall metalization prevented shorts, and the wall metalization was connected to ground pads on the substrate. The high U 0 count of this module, coupled with high gain, resulted in leakage problems in the first iteration. Mechanical issues related to base flatness and stress reduction also needed resolution. Tbe solution to these problems included multilayer techniques used in other successful T/R module packages[2], as well as design elements unique to this package. 0-7803-2411-0/94/$4.O0@ 1994 IEEE 'Ransition Design A completely enclosed package, with no gaps in the walls, would signifkantly reduce the RF leakage. This would require a new approach to the RF feedthroughs. A possible solution would use a multi-layer thick film feedthrough on the top of the substrate. For the RF lines, this rep resents an asymmetrical striplie section under the wall,with an upper ground plane spacing of about five mils. The limited ground plane spacing leads to line widths which are impractical for thick film implementation, and so this technique was rejected. In its place, a conductor backed coplanar waveguide transition was chosen. However, instead of bringing the microstrip ground plane up to the top of the substrate, the microstrip trace was brougbt down to the ground plane layer through laser drilled vias. A pocket in the base plate shielded the transition region from external influences. The metahtion pattern used is ...
