BY RONALD O. BUB
Packaging for high-frequency components has never received much attention. Historically, high-speed millimeter wave integrated circuit (MMIC) devices were developed primarily as a resource for the United States military. Their applications required extremely dense, high-reliability packaging that dictated the use of multi-chip modules or more conventional chip and wire hybrid assemblies. These processes work well for low-volume applications that can justify the relatively high costs associated with manual assembly. The modules require hand tuning to achieve optimal electrical performance, and the thermal performance and physical robustness must also be optimized to withstand the rugged conditions to which they are exposed.
However, these predominantly military uses have been overshadowed by the explosive growth of commercial sector applications, such as point-to-point and point-to-multipoint radios, VSAT terminals, automotive radar, high-speed digital communication networks (2.5, 10, and 40 Gbps digital amplifiers and mux/demux devices), handset amplifiers, and direct broadcast satellite television applications. The last two applications have the highest volume usage by far and are packaged almost exclusively in plastic.
These device packages are used in both high-speed digital and microwave applications through 50 GHz.
These commercial opportunities differ from the earlier military applications because most of them require very high-volume production of the MMICs and their assemblies. Even some of the military uses are moving into the tens and hundreds of thousands of devices (particularly the phased array radar applications), while most of the commercial telecommunications applications have already moved into the hundreds of thousands (or more) per year range. Commercial end users are demanding chips that are completely packaged and tested so they are ready to be soldered to the circuit board. These requirements have appeared because it is not economically feasible to manufacture high-volume commercial assemblies in cleanroom facilities or to handle bare die, which would be necessary if the MMICs were not fully packaged.
There are five common package requirements to consider:
- It must provide electrical interconnection between components and subsystem.
- It must protect the circuit from any corrosive environment (even air or water is corrosive to some circuit components).
- It must protect the circuit from general contamination or foreign material (that is, particles that may damage components or change electrical response).
- It must protect the circuitry from mechanical damage.
- It must provide or assist in thermal management of heat generated by the circuit.
Besides these requirements, there are several additional factors that drive the design of the package. Suitable materials must be selected to package high-speed devices. Thermal and mechanical considerations must be made for the MMIC, package and assembly to work in concert. For example, the package body, metal base and chip materials must all have similar coefficients of thermal expansion for the package to operate over a wide temperature range. The techniques to assemble and test the package must be addressed and, for commercial applications, low cost is essential. Most important, the package must be electrically transparent to the device. Mismatches caused by the interconnections from the internal circuitry to the next assembly must be minimized using sound microwave transmission line design.
Previously, chip manufacturers were reluctant to consider using a package for their devices because they were concerned about losing the high-frequency electrical capabilities of their chips and did not want to compromise the chip's performance. With the packages available today, that no longer is an issue. Package design and manufacture have become more sophisticated – protecting the chip against adverse environmental conditions and dissipating the tremendous amount of heat produced, yet maintaining the electrical integrity.
A variety of packages are available, and requirements vary according to the frequency of the device being packaged. Conventional transfer molded plastic packages can be used for handset applications through 1-2 GHz and as high as 10-12 GHz for other applications where electrical performance can tolerate the losses because of the plastic. Low-cost hermetic glass-wall packages that can handle the thermal requirements that plastic does not are
available for amplifiers through about 12 GHz. Ceramic packages designed for both low- and high-power devices are available for applications up to, and even above, 50 GHz.
Integration of the package into the next level assembly is a major issue for commercial applications. Microwaves and millmeter waves are not as forgiving as lower-speed signals, and they require special interconnect attention. While surface mount connects have worked in low power and lower microwave frequencies (up to an estimated 15 GHz), devices producing high power and higher band performance provide interconnect challenges. However, advances in these areas are coming quickly. Leaded power packages, which allow the leads to be soldered in place, are available up to Ka-Band frequencies. Leadless power packages can operate in excess of 50 GHz but require wire or ribbon bonding as the external means of attachment. For high-speed digital applications, the same operating frequency rules apply but with much higher lead counts.
Into the Future
So, what is on the horizon in high-speed packages? The need for greater data transmission will create explosive growth in several commercial applications. Lower cost products must be developed to meet the needs of consumer applications. Continued product development will be necessary to increase the upper operating frequencies to 100+ GHz. And, development of easy-to-manufacture interconnect technology must continue to keep pace.
Today's high-speed device packages fulfill the current needs of microwave, millimeter wave and high-speed digital assemblies. Chip manufacturers, materials companies, packaging designers, and assembly and test houses are working together with the OEMs who demand these packages to identify emerging market needs and drive technology developments toward the packaging of the future.
For more information, contact Ronald O. Bub, president, at StratEdge, 4393 Viewridge Avenue, San Diego, CA 92123; 858-569-5000; Fax: 858-560-6877; E-mail: [email protected].