Performance levels of high-frequency monolithic-microwave integrated circuits (MMICs) are often compromised within packages and circuits. Fortunately, a novel technique for mounting MMICs in microstrip circuits helps optimize device performance even at millimeter wavelengths by eliminating many of the drawbacks of traditional mounting techniques. This patented new approach helps designers achieve MMIC performance promised during wafer probing but often lost in the transition to microwave/millimeter-wave circuitry.

In this new approach, a MMIC is inserted into a laser-drilled pocket in a polytetrafluoroethylene (PTFE) circuit board, establishing continuity with the groundplane and reducing or eliminating jumper wires. A secondary FR4 circuit board is bonded to the bottom of the PTFE board, from which DC control lines are connected to the chip through viaholes. In addition, the MMIC is flush with the top of the board (rather than on top of it) where it is not subject to damage from handling. Test results indicate that performance of pocket-mounted MMICs differs little from manufacturers' bare-die specifications.

The transition between a MMIC and its supporting microstrip circuit elements and RF and DC connections is the crucial factor in determining its performance when mounted in a circuit. MMIC manufacturers supply copious performance specifications for their die, which in an ideal mounting situation could be perfectly preserved. However, the typical microstrip circuit board onto which a MMIC is bonded presents a less-than-ideal environment, since the path to ground is routed to the chip from the groundplane with via holes. The resulting discontinuities cause significant mismatch, and parasitic capacitance and inductance caused by bond wires are extremely difficult to remove. Consequently, the specified performance of the device can be significantly reduced. The severity of this situation increases with frequency, and becomes a major problem at millimeter wavelengths.

A bare-die MMIC mounted on top of a circuit board is also vulnerable to damage, since it is higher than the surrounding surfaces. This issue can be addressed by incorporating the device in a ball-grid-array (BGA) or low-temperature-cofired-ceramic (LTCC) package, but in addition to being more expensive and difficult to assemble, these packages can increase insertion loss. Faced with a customer requirement for use of bare die to achieve the highest level of performance in a millimeter-wave assembly, KDI Integrated Products (Whippany, NJ) investigated various ways to mount MMICs that would retain their performance, creating a nearly pure resistance between the MMIC and its connections.

The solution involved creating a pocket for the device in the microstrip laminate just slightly larger than the device itself. This pocket not only provides electrical benefits, but also lowers a 6-mil-thick MMIC to the level of the board, reducing susceptibility to damage. To create the pocket, the top center conductor of the board is removed by laser drilling in the spot where the MMIC will be placed. The drilling continues through the dielectric layer below the center conductor, revealing the main groundplane used by the remainder of the circuit. The surface of the center conductor and groundplane are plasma-etched to remove burned material.

The microstrip and ground lines are then plated with a 0.05-mil layer of gold where the bond wires are to be connected and via holes will be placed. The metal patch on the bottom of the MMIC is attached to the groundplane with liquid silver-filled epoxy, which provides high mechanical strength and an excellent conductive path from the device to ground. Solder that is not affected by metal plating (such as indium solder), can also be used in place of epoxy, in which case the solder is placed in the pocket, and the chip is placed in the pocket over the solder. The entire assembly is heated to the solder's melting point to achieve the necessary mechanical and electrical bonding of the chip to the groundplane. The bond wires are then connected to the chip and metal-plated areas.

Pocket-mounting allows the groundplane used by the rest of the circuit to become the groundplane for the MMIC as well (Fig. 1). This direct ground connection eliminates the traditional need to provide a ground path to the bottom of the chip from the underlying groundplane through via holes. The discontinuity created by this noncontiguous path degrades circuit performance by creating inductance. This series inductance, when added to the inductance incurred from the RF input and output connections, makes tuning extremely difficult. Tuning of the device when pocket-mounted requires attention only to the wire bonds.