Northboro, MA--As engineers downsize microwave circuits, high-power-density components create thermal management problems. One way to dissipate the excess heat is to mount high-temperature components directly onto heat sinks. This requires large holes (through-vias) in alumina substrates. Through-vias, however, complicate manufacturing, weaken substrates, cause cracks, and shorten life.

GDE Systems of San Diego, CA, tackled this problem by eliminating through-vias. Taking advantage of diamond's unparalleled thermal conductivity, GDE's engineers surface-mounted all components of a microwave frequency divider to a CVD diamond substrate made by Norton Diamond Film. The synthetic-diamond substrate simplifies manufacturing and lowers temperatures by 20 to 30C.

GDE ran into thermal problems when it cut the size of automated test equipment for the F-16 fighter by an order of magnitude. In particular, downsizing the microwave frequency divider drove heat densities up, which raised temperatures and threatened performance and longevity. To maintain safe temperatures, engineers tried placing the alumina-substrate-mounted gallium arsenide dies in through-vias and directly onto chip carriers/heat sinks.

This technique lowered temperatures to acceptable levels, but complicated manufacturing and raised costs, says principal designer Kent Roberts of GDE's Automated Test Systems Group. Precision through-vias require laser cutting and disturb uniform photoresist spreading. And finally, to make good thermal contact, devices are scrubbed-in-rubbed back and forth-a painstaking process in tight through-vias.

Seeking to simplify the manufacturing process, Roberts decided to explore diamond's potential as a substrate. Depending upon its purity, the thermal conductivity of diamond ranges from 6 to 20 W/cm-C. Diamond's high electrical resistivity also makes it appropriate for electronic applications.

Roberts turned to Fred Borchelt, manager of market development at Norton Diamond Film. Norton can produce 250-micron-thick diamond substrates as much as six inches in diameter. Test-bed circuits made at GDE proved that CVD diamond, by acting as a high-conductivity heat spreader, could lower junction temperatures by as much as 50 degrees C.

Measurements also revealed that the material possessed a suitable dielectric constant and a loss tangent of less than 0.001 when subjected to microwave frequencies. (The loss tangent is the ratio of the irrecoverable to the recoverable part of the electrical energy introduced into an insulator by an electric field.)

"Gallium arsenide power-amplifier temperatures have dropped 20 degrees to 30 degrees C or more, which should improve product life by 8 to 10 times," says Roberts. Diamond substrates require less handwork than alumina substrates. No large cutouts are required to receive dies, less precision handling must be done, and fabrication, especially with microwave devices, becomes simpler, according to Roberts.

Additional details, frequency divider...Contact Kent Roberts, Engineering Staff Specialist, GDE Systems Inc., MailZone 7134H, Box 85310, San Diego, CA 92186-5310, (619)573-7368.

Additional details, CVD diamond films...Contact Fred Borchelt, Manager, Market Development, Norton Diamond Film, Goddard Rd., Northboro, MA 01532-1545, (508)351-7968.