Eutectic bonding of Au-backed MMIC (Monolithic Microwave Integrated Circuits) power amplifiers presents a unique challenge due to the thin, brittle nature of the device and the need for a virtually void free attachment of the backside of the device to the heat sink.Both scrub assisted processes and vacuum solder methods have been successfully employed throughout the industry. In the microwave world, the concerns are more than just total voiding under the die which diminishes the ability to draw heat from the component.alter, change, modify - cause to change; make different; cause a transformation; "The advent of the automobile may have altered the growth pattern of the city"; "The discussion has changed my thinking about the issue"It was with rather heavy hearts that the party set off, but Tom's spirits could not long stay clouded, and the scientist was so good-natured about the affair and seemed so eager to do the utmost to render Beecher's trick The solitary and arid blades of grass arose from the passing gusts fearfully perceptible; the bold and rocky mountains were too distinct in their barrenness, and the eye even sought relief, in vain, by attempting to pierce the illimitable unknown with light and order; but to the man who has dethroned God, the spirit-land is, indeed, in the words of the Hebrew poet, "a land of darkness and the shadow of death," without any order, where the light is as darkness.
There are some additional voiding requirements based on maximum voiding in any one of four quadrants, but as with X-ray this criteria is not sufficient and needs to be tailored to the specific device type.Reliability Concerns The voiding criteria in the two test methods discussed above needs to be expanded and tailored to address MMIC devices.The real concern is location of voids relative to the device features.A relatively large void located directly under the channel region in an FET structure on the die where the heat is generated is much more problematic than a void under a non-active area.Mil-Std-883 J TM 2012.9 specifies voiding criteria underneath the die.
This test method is under review in JEDEC to incorporate the newer, real-time X-ray equipment that is now available in the marketplace.It makes sense to overlay the device features onto the X-ray image and highlight the critical areas where no voiding is allowed.The image above displays an X-ray which shows total voiding at 13%; however in this case, the voids happen to be situated in the RF pathway through the MMIC, and therefore presents a potential problem.Another area of concern is voids underneath the bond pads.A void under a 2 or 4 mil thick Ga As die is a problem for two reasons: Voids underground vias are also cause for concern.A small void in the wrong location could be just as critical; therefore, total voiding and the location of the voids should be addressed by the process engineer to assure a reliable die attach. Clean surfaces, good metallization and a flat heat spreader are all critical process engineering concerns.