If the Navy radar and DoD airborne programs are to meet their cost and schedule targets, it is critical that new processes be developed to manufacture the required microwave assemblies. A critical technology in these programs is the direct attach of Monolithic Microwave Integrated Circuits (MMICs). The direct attach or flip-chip approach has been developed but has limited production capability and device availability. The technology used to form the attachment bumps on the flip-chip die is critical, especially for the radio frequency (RF) signals required.

To meet this demand, automated processes must be put in place to allow for increased volume. This can be accomplished by increasing the supplier base of bumped devices, which will reduce risks during production by providing additional sources. The commercialization of this bumping technology is critical to ensure the support and long term affordability of this technology for critical Navy RF applications.

The development of microwave MMIC bumped devices has enabled highly integrated microwave functions, packaged to fit the lattice of high performance, active, electronically-scanned array radars. Typically, components are mounted onto an interconnect medium or substrate using epoxy or gold/tin based solders, and then standard gold wire bonds are used to connect the MMIC inputs and outputs (I/Os) to the substrate. This process has been the preferred approach for many years. More recently, the commercial microwave electronics sector began using surface mount techniques that combine device attach with interconnect in one process. The advantage of using this flip-chip surface mount technique is reduced cost and space usage. This permits higher component density with higher performance than wired devices and improves yields. The silver bump material offers increased thermal management advantages as well as increased reliability for certain applications.

Currently, bumped flip-chip devices are being assembled in MMIC Transmit / Receive (T/R) modules in the F-18 Active Electronically Scanned Array (AESA) radar for a major military aircraft platform (Figure 1-1). MMICs are a key cost driver for the T/R modules, and this is directly impacted by the attachment of these devices using bump technology. Additional capacity and an alternative, second source of devices will provide cost advantages and the needed volume for aggressive Navy radar projections.

The first MMIC Flip Chip ManTech project began in September 2002 and will conclude in March 2005. This first project has focused on transition of MMIC bumping from a “development” wafer fabrication facility to “production” fabrication. The project has also focused on initial transfer of key characteristics of the bumping process to an additional supplier.

The transition of MMIC bumping to production has reduced cost and increased throughput at the Raytheon RF Components (RRFC) facility in Andover, Massachusetts. This transition to a high volume RRFC production line has also reduced cost through reduced labor and increased yields. The RRFC production line has increased throughput to handle some of the projected increases in bumped MMIC demand. Reliability is also improved due to less equipment downtime and fewer single points of failure.

Bumped devices have been shear tested to demonstrate process robustness. Multiple bumped daisy chain test coupons containing four die and also live devices have been qualified through thermal cycle testing. This led to the development of a more stable process with less variability.

Insertion of silver bumped flip-chip MMICs into the T/R module production line for the F-18 AESA radar is in progress, with other platforms due to benefit as well. Finally, a second supplier fabrication foundry has been established, providing the U.S. Navy with multiple sources of silver bumped MMICs.

The Manufacturability of Bumped RF Devices for DoD Applications project will provide transition and validation of a bump process for MMIC devices intended for microwave modules and assemblies. The objectives are to transfer the process from the first project and validate the key characteristics of the board-level manufacturing process, followed by electrical, material, and reliability analysis to verify process robustness.

In addition, the EMPF and TriQuint Semiconductor will work together to develop a flip-chip bumping attachment manufacturing process for MMIC die and perform thermal cycling and failure analysis. The key characteristics of this attachment process will be validated and publicized to assist the qualified second supplier foundry to develop a production-worthy process. The EMPF and TriQuint Semiconductor will support efforts for ongoing work in Navy radar applications in addition to supporting the needs of other DoD, US Government, and commercial entities.

TriQuint Semiconductor is a leading supplier of products in the RF and MMIC markets. In addition to completing Navy ManTech development work on plated bump technology, they are collaborating with Amkor Technology, Inc. to commercialize a low-cost flip-chip assembly process for MMIC gallium arsenide (GaAs) semiconductors based on TriQuint CuFlip™ bumping technology. The high-density interconnect capability of CuFlip™ enabled a 40% size reduction and improved RF performance compared to its predecessor. The direct thermal connection to the surface of the substrate provided by CuFlip™ bumps is especially valuable in MMIC and high power amplifier integrated circuit die design. It enables smaller die sizes and improved long-term reliability due to lower operating temperatures.

At the EMPF, the Demonstration Factory is used to test the latest equipment for component placement and to develop processes for component attachment including flip-chip. Two different pick and place machines are currently in use for attachment of surface mount devices including flip-chip die. The “Advantage1” surface mount device (SMD) placement machine from MIMOT has a SMD placement rate of 6,000 components per hour with fast-flash and parallel-processing for fast component centering. It contains a vision on-the-fly system for die and fine pitch components with sizes from 0201 to 55mm square. The CLM9000 pick and place system from Essemtec is used for production of small runs and prototyping. It is highly flexible, placing devices from tape, stick feeders, and palettes, using laser centering for device sizes from 0402 to 32x32mm. Both machines have been used to complete commercial customer flip-chip placement prototyping jobs.

In previous military projects, the EMPF has also performed flip-chip placement for redesigns of electronic boards. In particular, the Electronics Miniaturization for Missile Applications (EMMA) Program developed processes including four flip-chip devices that were assembled onto a customized test vehicle.

The future of MMIC flip-chip technology relies on new and novel development of process technologies that will address the high power, performance, parasitics, cost, size, and feasibility considerations required for successful products. State of the art investigations like this Manufacturability of Bumped RF Devices for DoD Applications project, involving military programs, commercial technology, and equipment suppliers are vital to the success of RF MMIC based electronics.