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40 The PCB Magazine • April 2017 devices like very high-power amplifiers (PA), designers have no option but to use bare die. These are invariably gold-wire (or preferably ribbon) bonded (to provide interconnect) and typically 50–100 µm thick. Similarly, managing signal loss and wire inductance are key to per- formance and limiting the length of the gold wire (or ribbon) is an important consideration in mmW PCB assemblies viz. the shorter the connection, the lower the losses, and induc- tance. Ribbon bondwires have superior perfor- mance but in most cases two or three parallel single bondwires are good enough. An established practice is to recess the dies within cavities that are either mechanically milled or laser-ablated. A ground plane of cop- per will form the floor of the cavity. The ground plane is required by the MMIC for both low impedance electrical grounding and for good thermal coupling from the die. The thermal dissipation of a single PA mounted on a PCB cavity can be several watts in an area <10 mm2 which needs very efficient cooling through the PCB laminate. In such instances, thermal via and embedded coins are features that can sat- isfy the thermal management aspects. In a 100 µm thick copper-clad laminate there is the con- venience to remove the dielectric and allow the MMIC device (with wire-bond pads atop) to be flush (or near flush) with connecting bond-pads on the PCB. As its apparent the proximity of MMIC bond-pad to PCB bond-pad is a key factor for short bond-wires. The use of laser ablation works well in this aspect as positional accuracy of it is typically better than that of mechanical- ly milling. In MiWaveS work the gap between PCB bond-pad and cavity wall was typically ≤25 µm. Consider too mechanical milling requires process tolerance in Z-plane and burring is prevalent. Figure 3 shows an image of a laser- ablated MMIC cavity with the PCB bondwire pads meeting the top of the cavity wall. Waveguide transitions are a key feature of mmW PCBs. Often the mating waveguide (in a transceiver assembly the waveguide will lead to the antenna) will mechanically locate with side 2 of the PCB and the RF energy is fed by a mechanically milled conduit. An etched reso- nator on side 1 completes the transition. In Mi- WaveS mechanical depth-milling was used to form the cavity in the bonded PCB; an end-mill was used to machine within ~50 µm of the side one resonator. Often the walls of the cavities are plated and that adds to the complexity of manufacture. In such circumstances two depth- milling steps are required to reveal a non-metal- ized opening to the side 1 resonator. The Z-axis depth-milling capability required being ±35 µm. Figure 4 shows images of a depth-milled cavity from top and bottom perspectives. In circumstances where plated vias require "capping" the accepted practice of via-filling (with epoxy-based pastes) is impractical because mechanical planarization can mechanically distort the non-reinforced base materials. The sequential plating steps can also hamper con- ductor definition because of the overall copper thickness result. Galvanic copper via-filling is the only practical route. Here the via geometries (height versus diameter) require careful consid- eration to maximize the copper filling process. This is a process not unique to mmW PCB prod- uct but is a trend prompted by the availability of mmW packaged devices, particularly BGA types. Plating and Finish Requirements It is generally accepted in the PCB indus- try that the panel plating process, whereby all surfaces of a drilled panel are copper-plated, re- PCB TECHNOLOGY REQUIREMENTS FOR MILLIMETER-WAVE INTERCONNECT AND ANTENNA Figure 3: Micrograph of laser-machined MMIC cavity in 100 µm LCP dielectric depicting circuitry and bondwire pads defined in layer 1 and exposed cavity floor in layer 2 (ground).