A DC to 50 GHz and Beyond MMIC Carrier
HEI Broadband, HEI Inc.
With frequency spectrum demands at a premium many of today's wireless applications are moving higher and higher in frequency. Many of the systems now operate in the Ku and Ka frequency bands. The components and subsystems used at these high frequencies share some common requirements. In order to meet these demands at high frequencies they must have good interconnection matching between devices up to millimeter-wave frequencies and be cost- effective.
Fig. 1 Side view of a traditional chip-wire-on-board structure.
Interconnections at millimeter-wave frequencies pose particular problems. For example, a wire bond between a MMIC and its module circuit behaves as a parasitic inductance. This inductance can significantly degrade signal integrity at these frequencies. Traditional interconnection methods, using a chip-on-board topology, involve the use of ribbon bonding and chip-well structures to reduce the parasitic inductance and discontinuity. However, ribbon bonding and chip-well construction are not methods that lend themselves well to low cost, high volume manufacturing. Modern manufacturing techniques for lower cost and higher volumes dictate surface-mount technology (SMT) for millimeter-wave components. The traditional ribbon bonding methods are too slow and require complex machines that do not fit well into this mass production environment.
A New Interconnection Structure
Fig. 2 Simulated frequency responses of a traditional wire bonds.
A novel interconnection structure for high performance and high volume DC to millimeter-wave chip carrier applications has been developed. The T-Ball™ structure contains a shunt distributed capacitance attached to a wire bond that connects between a module and its MMIC circuitry. This new structure can be designed to be frequency-independent by choosing the appropriate capacitor value to match the predetermined wire bond length. Thus, a chip carrier utilizing this interconnection structure results in a low cost, high performance millimeter-wave package, suitable for high volume production.
Figure 1 shows a side view of a traditional chip and wire construction. A standard 1 mil wire diameter wire ball bond, a 1 mil wedge bond and a 2 mil ribbon bond were used for the simulated performance test shown in Figure 2 . Using 15 dB return loss as a criteria, the resulting cut-off frequency is 18.4 GHz for the wire ball bond, 23.2 GHz for the wedge bond and 40.2 GHz for the ribbon bond. The simulated results clearly show that the traditional wire ball bond is not suitable for applications beyond 18 GHz. The results are considerably worse when there are multiple ports involved. The wedge bond provides minimum acceptable performance. Beyond 25 GHz only the ribbon bond is acceptable.
Fig. 3 T-Ball wire bond pad structure circuitry arrangement.
Figure 3 shows the concept of the T-Ball structure. Here an additional T-section is added in the wire bond area to create a specific distributed shunt capacitance to the bond wire. The combination of the T-section, wire bond and bonding pad on the MMIC establishes a predetermined characteristic impedance (50 W) over a certain frequency range. Figure 4 shows the simulated frequency response of the T-Ball structure.
As can be seen, the T-Ball interconnection response has been improved to 50 GHz using a regular wire ball bond. The wedge bond T-Ball is usable to 65 GHz and the T-Ball ribbon bond extends beyond 100 GHz.
T-Ball MMIC Chip Carrier
Using the T-Ball interconnection structure, a SMT DC to millimeter-wave chip carrier has been developed. Figure 5 shows a cross section of the packaged MMIC mounted on a mother board. The MMIC is mounted on a chip carrier using standard wire bonding. The MMIC then makes electrical connection with the 50 W lines of the mother board through the T-Ball structures and filled vias, while the backside of the MMIC is grounded to the chip carrier and mother board ground pads, forming a good electrical and thermal ground path.
Fig. 4 Simulated frequency response of the T-Ball structure with different type wire bonds.
This new configuration has been used to develop a line of low cost, laminate-based MMIC chip carriers and packages for frequency applications to 50 GHz and beyond. The first package to be developed provides the functionality of a pick-and-place component while maintaining a near electrically transparent platform. The result is a SMT MMIC package with performance that closely resembles the response achieved by direct-attach methods. Figure 6 displays the typical performance achieved with a 50 W microstrip line mounted inside the package with ball bonds.
In addition to the previously described T-Ball interconnect structure, the thermal characteristics of the package have been improved by using a thin profile design that incorporates top and bottom plating connected by multiple vias. The vias are gold/copper plated and filled with a thermally conductive material for environmental sealing.
The package is designed for traditional ball-wire bonding, as opposed to the more costly wedge or ribbon bonding. In addition, every package is electrically tested to ensure performance and supplied in a tape-and-reel format, ready for surface-mount assembly. A variety of package sizes to fit MMICs ranging from 1.4 to 3.4 mm in length are available.
As an example, the HFC17L package is designed to accept a 1.7 mm MMIC, while the HFC34L accepts a 3.4 mm MMIC. Custom designs incorporating multiple chips or sizes beyond the standard range are also possible.
Fig. 5 Cross section of the packaged MMIC mounted on a motherboard.
The HFC**L series utilizes Rogers' RO4003 1/2 oz. copper, 8 mil thick laminate as material for the carrier. Package specifications include an insertion loss of 0.2 dB typ., 0.25 dB max. through each signal port at 40 GHz. Isolation between RF ports is 37 dB min, and return loss with a 50 W line is -15 dB typ, -10 dB max. The minimum parasitic resonance is beyond 50 GHz.
Fig. 6 Packaged 50W line wire bonded within chip package.
The packages are capable of handling 5 W max. dissipation and the reflow temperature is 250°C max. Operating temperature is -40° to +85°C. Figure 7 shows the package outline for the HFC17L chip carrier and its associated mother board. The complete package measures 0.20" x 0.20" x 0.040".
A novel MMIC chip carrier has been introduced that features a broadband response to 50 GHz and beyond in a low cost, SMT configuration. The new package has excellent high frequency performance and high thermal conductivity for microstrip applications where pick-and-place assembly is desirable. HEI has received patent allowance for the T-Ball structure and has numerous additional patents pending on various aspects of these MMIC carriers.Qualification of the MMIC carriers has been completed and volume production has begun.
Fig. 7 Typical chip carrier; (a) bottom view and (b) motherboard layout.
HEI Broadband, HEI Inc.,
Victoria, MN (952) 443-2500,
Circle No. 301