Due to their small size and low cost, millimeterwave monolithic microwave integrated circuits (MMICs) are being more widely used in the commercial market. Yet these MMICs are fragile because of their size and fine features. Usually, they require specialized equipment for die attachment and bonding. In a four-page application note titled, "Bonding, Handling, and Mounting Procedures for Millimeterwave PHEMT MMICs," M/A-COM Technology Solutions provides tips for gallium-arsenide (GaAs) MMIC low-noise amplifiers (LNAs), power amplifiers (PAs), gain blocks, and driver amplifiers.
The note breaks handling and assembly into two areas: putting the chip into the circuit (die down) and making top contact to the chip (top bonding). Among the precautions provided are handling the MMICs in a cleanroom-type environment and not touching the die surface. To improve chip bonding, the carrier should be plasma cleaned before any eutectic is used. Among the other common problems that arise during bonding are poor reliability due to the entrapment of contaminates under the bond and mechanical failure of the bond under thermal shock or temperature cycling. These conditions result from improper wetting of the die to the ground plane.
The note covers chip die-down bonding techniques beginning with the eutectic bonding of chips and PAs. It points out that the eutectic bonder is one of the most convenient ways of bonding chips onto a metal ground plane or circuit. For LNAs, die bonding with conductive epoxies may lead to good die-down bonds. Yet PAs may not perform to specification using this approach. In contrast, LNAs work well when epoxies are used.
To obtain consistently strong bonds, electrically conductive epoxy is required and must be within the warranty shelf and/or pot life. It is advisable to use one-half the value of the recommended pot life to ensure good quality. In addition, silver conductive epoxies should not be used where they will come into contact with lead tin solders or high tin solder.
In terms of top bonding, techniques are usually chosen according to the size of the chip's top contact, type of chip, the chip's sensitivity to temperature and pressure, type of circuit board, and equipment available. Because GaAs is very brittle, extra care should be taken during wire bonding. Bonding parameters should be adjusted to maximize reproducibility at a high bond pull strength. In acceptable bonds, wire will not separate when tested. In addition, there will be no fractures in the bond, no separation of metallization, or wire breaks before the bond.
m/a-Com technology solutions, 100 Chelmsford st., lowell, ma 01851; (978) 656-2500, internet: www.macomtech.com.