Aerosol Jet Printing Process Considerations for Radio Frequency Packaging Applications

Aerosol Jet (AJ) printing has experienced increasing attention from the research community over the past years. The exploration of new fields of application and new printable materials lead to increasing usage in industrial manufacturing. AJ printing is a flexible, highly precise, and contactless deposition technology. In addition, it is capable of printing onto non-planar substrates. Those properties make it appealing for applications in the realm of radio frequency engineering.<br/><br/>Recent advances in semiconductor technologies enable operating microwave integrated circuits (MMICs) at frequencies up to several hundred GHz. Those frequencies are needed to fulfill the ever-growing need for higher data rates, more precise sensors, and smaller packages. However, there is a lack of packaging technologies suitable for 100 GHz and more. Bond wires suffer from severe losses and impedance mismatches at those frequencies. The latter ones can be corrected but only by sacrificing bandwidth, hence jeopardizing data rates and radar resolution, which counteracts the benefits of higher frequencies. Alternative solutions are extremely expensive (split block packaging) or limited in frequency and unsuitable for prototyping (EWLB). Aerosol Jet Printing is a promising technology for remediating this lack of packaging technologies as interconnects can be printed directly from MMICs to transmission lines on the radio frequency (RF) substrate. Those interconnects are shorter than bond wires, which form loops, and reduce losses. They can also be shaped to match the line impedance on- and off-chip and thereby form broadband, well-matched interconnects. Additionally, the digital nature of AJ printing makes this approach appropriate for prototyping and small batch sizes, which enables faster and cheaper development and customized RF applications for healthcare, industry 4.0, and communication.<br/><br/>In previous research, we have demonstrated AJ printed microstrip transmission lines on polyimide for D-Band (110-170 GHz) with a loss of only 0.12 dB/mm at 110 GHz. This work confirms that AJ printing is indeed suitable for millimeter-wave packaging. We are currently extending our work to the aforementioned interconnects. Throughout the course of our research, we have tested a large variety of silver ink formulations, process parameters, and sintering approaches. We want to share the lessons we have learned and the design constraints we could establish. This talk covers the manufacturing process we used for our D-Band transmission line and the improvements we made since then. It focuses on the impact of the printing parameters and ink formulation on surface roughness, minimum feature size, and overspray. Additionally, the influence of various ink solvents will be discussed, including water, ethanol, TGME, and more. Aside from ink formulation and printing parameters, post-processing influences the print quality. We compare flash sintering in a Novacentrix PulseForge 1200, laser sintering directly in the Optomec Aerosol Jet printer, and thermal sintering. We give a comparison of the resulting conductivity for those methods. We also show that some sintering approaches can cause cracks where others yield perfect results for the same printing process.<br/><br/>Apart from the actual printing process, we present our approach to AJ printed MMIC packages: A cavity is laser machined in an RF substrate. Subsequently, the MMIC is placed inside this cavity so that its top side and the surrounding substrate top are on the same height. Next, the remaining gap between MMIC and substrate is filled with a polymer, onto which the actual transmission lines can be printed. This approach is optimized for short interconnects hence low losses. It also minimizes the distances between MMIC and substrate bottom, where a heatsink can be mounted. Therefore, the AJ printed package promises excellent electrical and thermal performance.