Heterogeneous integration of a miniaturized W-band radar module

The development of miniaturized and cost effective packaging technologies for radar systems gained momentum since radar systems are mandatory for comfort features as adaptive cruise control and safety features as emergency brakes. Technologies for the embedding of 77 GHz radar systems into PCB have been described earlier [1], leading to maximum miniaturization but have not met the maturity demands of automotive applications at that time. Today the use of even higher frequencies is gaining more interest, millimeter-waves (mmW) are becoming increasingly important for many applications. While the wavelength decreases linear in relation to increasing frequency, the required size and volume of a system decreases with quadratic and cubic relation respectively. This allows building systems that are more compact by using higher frequencies. On the other hand, production cost of systems operating at 100 GHz and above are still very high, as the utilized waveguide technology is not only expensive in production but also huge and heavy.

New substrate materials, such as liquid crystal polymer (LCP), have been demonstrated to provide sufficient performance for W-band applications [2]. This enables the development of cost-efficient and lightweight systems, as the RF signals can be handled directly onboard.

Instead of bulky horn antennas, broadband antennas can be directly integrated planar on the PCB as either stacked patch antenna [3], [4] or even more easily to realize as Vivaldi antenna [5], [6]. Due to the progress in development of compact and energy-efficient signal processors, the digital signal processing can be integrated into the module. This is essential to reduce the output data rate, to avoid the need of external computers for processing the raw data and to enable high measurement repetition rates. Another point in radar distance measurement is to provide high accurate and reliable results.

In this paper, a miniaturized W-band radar module has been designed based on linear frequency-modulated continuous-wave operation. Design of this system is described in [7], [8]. The block diagram showing the components of this system is depicted in Figure 1.

The resulting system with a description of the components used is shown in Figure 2. The technological building blocks for the miniaturization achieved are the hybrid combination of standard PCB technology for low frequency signals and high frequency-adapted LCP laminate for RF routing & integration of a 50 μm thick edge emitter antenna.

Also the high precision die attach of a 50 μm GaAs MMIC into a laser milled cavity with surface levelling of die and substrate plus extremely short loop “zero-tail” Au-wedge-wedge-wire bonds, allowing minimal attenuation of the radar signal (while transmitting from the IC to the antenna) are key elements for maximum miniaturization. Electronic components for data processing on the hybrid RF-module have been assembled using automated equipment. The combination of solder-paste jetting and high speed & precision assembly allows the demonstration of cost effective volume manufacturing compatibility without sacrificing of the design flexibility. The technological path chosen for the assembly of the Radar modules is detailed below.

Four main aspects concerning assembly and interconnection technology for the integrated radar modules had to be considered; (1) RF-and thermal performance, (2) Cost efficiency, (3) Integration density and (4) Design flexibility.

Considering these criteria, the following assembly was designed: Two microelectronic boards stacked by SMT-micro socket connectors constitute the modular radar system, including RF-frontend, analog and digital signal processing and digital interface. One of these PCB contains the low frequency digital signal processing components, which can be standardized for different RF-front-end modules and is assembled by standard SMD-technology on FR4.

The process flow for the manufacturing of the radar modules was:

1. Manufacturing of Hybrid RF/NF PCBs

2. SMD Assembly

3. Die Attach

4. Wire Bonding

5. Assembly & Test

Key process steps and technological findings are described below.

Based on the 7-layer substrate design developed by IAF a manufacturing process flow for the manufacturing of the hybrid LCP-FR4 layer setup was developed at IZM. Both, the low-frequency-core consisting of FR4 base material and the RF-Layer were manufactured separately. Using a prepreg sheet, low frequency board and RF layer were joined by a standard lamination process.

For the RF layer a variety of materials were considered but only LCP provided the combination of RF performance, fine line structures down to 50 μm line/space plus the availability of 50 μm thick LCP layers with 5 μm Cu metallization on both sides. This LCP was not only suited for the RF signal routing but also allowed the manufacturing of a free standing Vivaldi antenna and the laser drilling of a 50 μm deep cavity for the assembly of the 50 μm thick MMIC, designed and manufactured by IAF. The cavity, together with numerous blind vias, required for RF-coplanar line shielding are fabricated by pico-second laser milling in one processing step, which enables very fine and precise structures as well as cost-effectiveness due to avoidance of mechanical degradation of expensive micro drills.

The onboard, anti-podal Vivaldi antenna is used as an edge-emitter and needs to be free standing. The FR-4 core PCB has mechanically milled cavities in the area of the antenna, which are covered by the LCP substrate by a customized lamination process, resulting in a free-standing LCP membrane carrying the antenna structure. Sacrificial materials need to be used to protect the membrane in the following plating and structuring processes. The structure of the hybrid board is shown in Figure 3.

The SMD-component assembly was fabricated using a combination of stencil printing for smallest depots and of solder-paste jetting for larger solder volumes. This combination was used when solder depot sizes needed for the process do vary too much for one stencil design. Solder paste application is followed by standard SMD pick & place and reflow processes, enabling again high flexibility in RF-frontend design and cost-efficiency for small series production. Printing was done using a DEK galaxy stencil printer, for jetting a Micronics MY500 solder paste jetting system is used allowing the dosing of solder depots down to 330 μm in diameter. For component placement an ASM Siplace CA3 fully automated pick & place system was used to obtain a high yield, highly repeatable prototype assembly. Figure 4 illustrates the SMD assembly flow.

For best RF performance a planar signal routing is targeted in the project. For the MMIC assembly this implies a chip mounting with the die surface level with the substrate metallization. To achieve this, the MMIC was embedded into the cavity in the LCP layer described above. This direct integration of the 50 μm thin and fragile GaAs-chips on the substrate avoids complex and expensive split-block technology, which is common in the 94 GHz range. As the MMIC, as well as the LCP layer have a thickness of 50 μm, together with a thin conductive adhesive layer, the metallization of the MMIC is at the same height level as the top copper layer.

To achieve this extremely tight tolerance in die placing, a high-precision die-attach process using thin layers of pre-applied non-flowing electrically conductive film adhesive for minimizing adhesive bleed-out and contamination risk of MMIC circuit and for optimal Z-positioning of MMIC-bond pads exactly on the same level as the pads on the LCP-board were developed. This adhesive film is typically applied on wafer level to the wafer backside prior to dicing. Then dies and die attach film or DAF are diced in a single step and the dies with DAF are assembled using a thermode bonding step. As only singulated dies were available for this project, a process was developed to pick up the DAF with the die and subsequently placing it into the cavity with a precision of ±10 μm. This process needs a tight temperature and force control during pick up and actual placement. A process draft is given in Figure 5.

The MMIC to board interconnection was done by wire bonding – to yield the shortest bonds possible an Au-wedge-wedge wire bond was selected. Almost all wire bonds were connecting pads of low frequency. Only one bond had to carry the 94 GHz signal, which was the one connecting MMIC and coplanar line to Vivaldi antenna. For this bond not only planarity was an issue but also the reduction of bond wire tails as these “antennas” lead to radar signal attenuation due to parasitic transition induction. So for the short loops “zero-tail” wedge-wedge-wire bonds were developed, that is depicted in Figure 6.

Based on this RF/NF hybrid substrate a fully functional radar module was built consisting of circuitry for signal processing, a Radar lens adapted to the 50 μm thick Vivaldi antenna (see Figure 3) and a metal housing that served as mechanical fixture, circuitry protection and heat sink.

Using this module, performance testing was done yielding high precision measurements confirming the system architecture chosen.

A miniaturized 94 GHz radar module has been developed, that is both performant and miniaturized, with an evaluated distance measurement accuracy of 5 ppm and a weight of only 160 g as reported in [8]. The advanced packaging technologies described in this paper allowed building such miniaturized modules and proving the actual performance.

Next step targeted in radar packaging is the evaluation of embedding processes for use with RF circuitry as first tested within KRAFAS [1] but potentially combined with current technology trends as FO-WLP/PLP with integrated RF interfaces. First results indicate that to meet this end chip-package co-design is a key issue.

This work was supported by the Fraunhofer Society Internal Programs under Grant No. WISA 824 631.