A DC to 40 GHz Ceramic MMIC Package
Dielectric Laboratories Inc.
Today's MMICs are being produced for applications in the mm-wave region including digital radios, local multipoint distribution systems, satellite communications and multipoint video distribution systems. One of the performance-limiting factors of these devices has been the package that houses the MMIC chips. A new ceramic package utilizing low temperature cofired ceramic on a metal core (LTCC-M) has been developed that exhibits good performance from DC to 40 GHz. The model M20001 DiPak™ ceramic package provides low loss, broadband performance and low cost in a 0.33" ¥ 0.39" MMIC package that rivals bare chip performance when measured at the package's input/output terminals. The package utilizes a copper-molybdenum-copper (CuMoCu) core to provide good thermal conductivity and RF grounding. Typical insertion loss at 40 GHz is 0.3 dB.
LTCC-M technology provides an alternative packaging method to thick-film, LTCC, high temperature cofired ceramic and other multilayer packaging solutions. The proprietary materials used in the DiPak LTCC-M package provide a low dielectric constant, low loss tangent and metal core for shielding, making this technology desirable for microwave and mm-wave applications as well as for high speed digital and mixed-signal environments. The material's low dielectric constant minimizes the package resonance effects and reduces parasitic capacitance. The CuMoCu base material allows good thermal management and makes the package suitable for higher power applications as well.
LTCC-M eliminates the manufacturing issues related to three-dimensional shrinkage while providing the benefits of a tape-based technology. With LTCC-M construction, the core constrains the shrinkage to the Z direction only. No nonuniform shrinkage or shrinkage-tolerance problems are associated with the LTCC-M technology.
All of the high conductivity conductors used are silver materials that provide low loss for high frequency and high power applications. Any exposed conductor is NiAu plated using either an electrolytic or electroless plating process. The top surface conductors are both wire bondable and solderable. The tape dielectric has a low dielectric constant and low loss tangent, which produce low losses at mm-wave frequencies.
The material's thermal coefficient of expansion (TCE) of 5.5 x 10-6 matches GaAs, Si and SiGe closely so that large devices may be mounted in the packages. The metal core (13Cu/74Mo/13Cu) was chosen as the package platform because of its TCE match to GaAs and Si, and because of its high thermal conductivity for heat dissipation.
The DiPak packages are constructed using a tape lamination process. The individual layers are prepared appropriately and laminated to the core. The assembly then is cofired, separated where necessary and plated.
The DiPak Package
Figure 1 shows an outline drawing of the M20001 DiPak package. A 0.010"-thick nonmetallized lid is available with a nonconductive B-staged epoxy printed on it for lid attach. Lid attachment requires that the lid be held to the package with a clamping force during the cure cycle of 15 minutes at 150°C, although this schedule can change depending on time and temperature.
The printed conductors (both on the surface and buried) are thick-film silver nominally 400 microinches thick (10 m m). The current package does not have side wall or lid grounding, and its mm-wave feedthru performance is enhanced as a result of this unshielded configuration. The low dielectric constant and package design produce high isolation performance, making shielding unnecessary in many applications. Generally, RF isolation inside the package will be degraded by shielding at least at cavity resonance frequencies. If desired, ground vias can be designed within the side wall to add RF shielding and topside grounding. The lid also may be metallized ceramic or Kovar. (These options are available from the factory.)
Wire or ribbon bonding is the usual method for external interconnection to the DiPak package. The plating of the surface conductors is solderable for user leading. Leaded and surface-mount variations are under development.
The DiPak package is compatible with most common assembly methods. The package can be mounted to the next level of assembly using epoxy, low melting point solders or eutectic solders (such as AuSn). Due to the nickel gold plating, process temperatures above 500°C should be avoided.
Figure 2 shows the setup used for testing the M20001 DiPak package. A probe station with ground-signal-ground probes was used to connect to the device under test. Figure 3 shows the measured return and insertion losses for one side wall of the M20001 DiPak package. The interconnect substrates were not calibrated out of the data. At 40 GHz, the insertion loss of back-to-back launchers measured 0.15 dB. This insertion loss must be subtracted from the total loss to yield the true port insertion loss of 0.25 dB at 40 GHz. The launcher loss is linear from 0 dB at 0.05 GHz to 0.15 dB at 40 GHz. Figure 4 shows return loss and insertion loss for the entire package with a thru line installed. The thru line is thick-film gold on 10-mil alumina.
The M20001 DiPak ceramic package provides a low cost, high performance means of packaging today's wideband MMIC chips to preserve their high frequency characteristics. The package adds as little as 0.3 dB of insertion loss at 40 GHz while matching the thermal expansion characteristics of GaAs and Si, making it suitable for microwave and mm-wave IC applications. The package is offered complete with a ceramic lid with B-stage epoxy already applied for $9.50 each in 1000-piece quantities. Delivery is three to five weeks. Custom versions utilizing the same outside dimensions and custom internal cavities are available in prototype and moderate quantities for a small, nonrecurring engineering fee.
Dielectric Laboratories Inc.,
Cazenovia, NY (315)