The Design of Multi-Chip SMT Front-End Modules for mm-Wave 5G Applications
by Graham Pearson*, Liam Devlin* and Mike Geen†,
*Plextek RFI Ltd and †Filtronic Broadband Ltd
The order-of-magnitude increase in data rates, and seemingly infinite capacity, promised by 5G communications systems will be facilitated by the use of mm-Wave frequencies, where large bands of contiguous spectrum are being made available. Although the mm-Wave bands for 5G have yet to be confirmed, development work is already underway in several of the candidate bands, including the FCC licensed bands at 28 GHz, 37 GHz and 39 GHz, and also in Europe in the band around 26 GHz band, which has been identified by the EU’s Radio Spectrum Policy Group (RSPG) as the “Pioneer Band” for 5G.
The agreement of the mm-Wave bands for 5G will be finalized at the World Radio Conference in 2019 (WRC-19). The candidate bands  where development work is already taking place are shown in Figure 1, and include the FCC licensed bands at 28 GHz (27.5 – 28.35 GHz), 37 GHz (37 – 38.6 GHz) and 39 GHz (38.6 – 40 GHz)  and the 26 GHz European Pioneer Band (24.25 to 27.5 GHz).
This article describes an integrated packaging technology that allows the realization of custom multi-chip SMT compatible components. Details of the design, implementation and measured performance of an SMT packaged multi-chip front-end component covering the 26 GHz Pioneer Band are presented, although the design approach and technology are equally applicable to other mm-Wave 5G bands. The FEM comprises a low-noise amplifier (LNA), power amplifier (PA) and transmit/receive switch housed in a custom laminate surface mount (SMT) package measuring 10mm x 10mm. Low loss RF filtering has also been integrated into the package structure.
Design and Implementation
A single SMT component has been developed which includes the mm-Wave blocks of a Front-End Module (FEM) to cover the full 26 GHz 5G band (24.25 to 27.5 GHz). A block diagram depicting the functionality of the FEM is shown in Figure 2. It includes three GaAs MMICs (LNA, PA and Tx/Rx switch) and two filters (low-pass filter after the PA for harmonic rejection and bandpass filter after the LNA).
The FEM is realized as a custom laminate package suitable for SMT assembly. A photograph of one of the assembled FEMs, prior to lidding, is shown in Figure 3. The three MMICs (PA, LNA and switch) are assembled into pockets in the laminate substrate material that forms the package. The backside of the ICs therefore sit on the metal base of the package, providing a good thermal contact to the PCB on which the FEM is ultimately mounted, and a low inductance interconnect to the PCB ground. It also means that the surface of the die is approximately level with the laminate surface inside the package, thus minimizing RF bond wire lengths and associated parasitics.
An advantage of the custom laminate package approach is the ability to integrate filters within the package structure, which helps to reduce both the size and cost of the FEM component. Such filters have been integrated into both the receive path (bandpass filter after the LNA) and the transmit path (low pass filter after the PA). In order to keep the package size small, the layout of the filters needs to be made as compact as possible, and careful design and EM simulation is required to account for discontinuity and proximity effects.
The PA die can be seen in the top left of Figure 3, followed by the integrated low-pass filter. This was designed to have a very low pass-band loss (around 0.2dB) and to provide good harmonic rejection (in excess of 20dB). The filter also forms the RF routing track that is necessary to connect the RF output of the PA to the Tx/Rx switch, and hence does not significantly increase the overall component size. The PA includes on-chip power detection that can be used to monitor the RF power transmitted by the FEM. The Tx/Rx switch at the common port of the FEM, on the right side, is a PIN diode MMIC, which is a technology that is well suited to the realization of low-loss, high linearity mm-Wave switches .
The LNA die is in the bottom right of the module, with the RF path moving from top to bottom of the die in the Y direction. The bandpass filter after the LNA is clearly visible as a coupled line structure. The coupled sections have been designed as curves rather than straight lines to allow for a more compact implementation. The shaped tracks at the input and output of the coupled line structure are part of the filter, and are critical in ensuring optimum performance. Detailed EM simulation and optimization of the layout was required to obtain the desired low insertion loss of just 0.7dB.
Careful design of the RF ports of the FEM package was necessary to obtain optimum RF performance of the transition from the PCB, on which the component will be mounted, to the internal short 50Ω routing lines within the package. The parasitics of the vertical transitions through the laminate package and the PCB pads on the motherboard were accounted for using EM simulation, and the overall structure was optimized to compensate for the parasitics and to ensure a good RF transition.
The RF interface to all MMICs within the package was also EM simulated and compensated to ensure optimum performance for the packaged MMIC. The “V” structure and short bond lengths at the RF ports of each MMIC are evident from the photograph. This helps to minimize the parasitic inductance of the RF bond connections and eases the process of optimizing the RF performance of the whole FEM.
The resulting FEM component was evaluated on a PCB designed on a representative low-cost laminate material, suitable for the realization of high-volume mm-Wave modules. The FEM components were assembled onto the evaluation PCB, which was attached to an aluminum carrier to provide ease of handling. Figure 4 is a photograph of the evaluation PCB; the FEM is mounted on an array of vias on the evaluation PCB, which provides a low inductance ground contact. Edge mounted mm-Wave connectors are used to interface to all RF ports. The three 6-way SMT connectors are used for applying control and bias voltages to the FEM.
The performance of the FEM was measured on the evaluation PCB with the reference ports set at the package. TRL calibration structures realized on the same PCB material were used to facilitate this. The small signal measured performance of a typical receive path is plotted against frequency in Figure 5 along with the simulated performance (dotted traces). The band-pass shape of the gain response is predominantly defined by the bandpass filter. The receive amplifier (LNA) itself has a much broader band response.
It can be seen that the measured gain response is in close agreement with the simulated response, demonstrating the accuracy of the filter performance predicted in the EM simulations used to design the filter, and confirming the low loss of the bandpass filter integrated into the package.
The measured NF of the receive path of three different parts is plotted against frequency, along with the simulated (dashed red trace) in Figure 6. The mid-band NF is 3.6dB, dropping to 3.1dB at the top of the band and rising slightly at the lower end of the band. The switch loss is low (around 0.8dB) and relatively constant across the band; the NF response is dominated by the LNA.
Figure 7 shows the measured small-signal performance of the transmit path versus frequency compared against the simulated (dashed traces). As with the receive path, there is good agreement between simulated and measured performance. The harmonic filter starts to roll off significantly above 40 GHz (providing around 20dB of rejection at the 2nd harmonic) and so its rejection performance is not evident in this plot. The close agreement between the in-band measured and simulated gains does, however, confirm the low insertion loss of the harmonic filter as predicted by the EM simulations used in the design process.
The linearity of the transmit chain was evaluated by measurement of the output-referred third order intercept point (OIP3). This measurement was made with the level of the two intermodulating tones set to +12dBm at the common port of the FEM. The measured OIP3 versus frequency is plotted in Figure 8, and varies between +35dBm and +37dBm, depending on frequency. The saturated output power of the transmit chain varies between 27dBm and 28dBm over the operating band so the IP3 measurement is backed off by around 12dB from saturation.
An integrated packaging technology suitable for the realization of mm-Wave multi-chip components in an SMT package has been described. The technology is based on low cost laminate PCB material, and can be used to integrate custom mm-Wave filters within the body of the package along with MMICs. The development of an FEM — comprising LNA, PA and Tx/Rx switch — using this technology has been described. It covers the full 26 GHz 5G band and incorporates low loss transmit and receive filters integrated into the package body. The transmit filter provides greater than 20dB of harmonic rejection after the PA, and has a loss of 0.2dB. The receive filter is a bandpass structure located after the LNA, and has an insertion loss of 0.7dB. Measured performance of the FEM mounted on a representative PCB has been presented, and shows good agreement with simulated. Receive path gain is 20dB with a NF of around 3.5dB. Transmit path gain is 19dB with an output referred third order intercept point (OIP3) of +36dBm.
 Components for mm-Wave 5G: https://www.plextekrfi.com/mm-wave/mm-wave-5g/
 Rules to Facilitate Next Generation Wireless Technologies, FCC.
 Devlin L.M., Dearn A.W. and Pearson G.A., “Low Loss MM-Wave Monolithic SP4Ts,” proceedings of the 2001 “Workshop on Design for Broadband Wireless Access,” Cambridge, England, 3rd May 2001.