RF front-end filters and modularization are the biggest constraints

With the trend toward higher frequencies, the frequency band spacing of smartphones is gradually shrinking, the difficulty of frequency band isolation is increasing, and the performance requirements such as bandwidth, insertion loss and size are also further improved, all of which pose higher challenges to the design and production of RF filters.

On the other hand, from 2G to 5G, smartphones are becoming thinner and lighter, and the battery capacity is getting larger and larger. More and more RF front-end devices are integrated in smartphones, which makes the space left for RF front-end PCB inside the phone more and more limited. RF front-end modularization will become a long-term trend. As domestic RF front-end devices continue to mature, miniaturized and integrated filter resources have not only become scarce resources in module design, but also the most prominent shortcoming of domestic RF front-end modules.

In this context, filters have become the key to breaking through for domestic RF front-ends to move from low-end discrete devices to mid-to-high-end modules. Founded in 2016, Hangzhou Zuolan Microelectronics Technology Co., Ltd. (hereinafter referred to as "Zuolan Microelectronics") is becoming a pioneer in this wave of modularization.

All categories of RF filters are shipped steadily, moving towards both discrete devices and modularization

Facing the new opportunities brought by the 5G era, as the only company in China that has achieved the research and development of all categories of RF filters, Zuolan Microelectronics has developed multiple product series with a focus on mid-to-high-end products to meet the different needs of high, medium and low frequency bands, including: conventional SAW, medium and high-performance TC-SAW and high-performance PESAWTM and PEBAWTM. The product categories mainly include Rx filters, Tx filters, dual filters, duplexers and highly integrated RF front-end modules.

With its outstanding technical strength, Zuolan Microelectronics' filter products have passed the project testing and certification of more than 100 domestic manufacturers, and continue to receive orders for batch delivery, with shipments exceeding 100 million units. Under the trend of modularization, Zuolan Microelectronics successfully entered the modular track in 2021. On the basis of consolidating the batch shipment of filter products, it launched two diversity antenna RF link DiFEM module products with integrated RF switches and filters: SPDM001 and SPDM003. These two products have the advantages of small size, high integration, strong performance, and stable reliability.

According to Bruce, the sales manager of Zuolan Microelectronics, the SPDM001 diversity receiving module of Zuolan Microelectronics adopts LGA package with a package size of 3.2mm*3.0mm, adopts the standard MIPI-RFFE V2.0 communication protocol, supports WCDMA/LTE network bands B1, B2, B3, B8, B26, B40, B41, and supports B1+B3 downlink carrier aggregation. At the same time, 4 AUX paths are integrated inside the module to support customers to expand the frequency band according to system requirements, and it is widely used in 4G/5G smartphones, smart wearable devices and module products.

Figure: Diversity module SPDM001

The SPDM003 diversity receiving module also uses LGA packaging with a package size of 3.7mm*3.2mm and adopts the standard MIPI-RFFE V2.0 communication protocol. The module integrates a dual antenna port switch to support WCDMA/LTE network bands B1, B3, B7, B8, B26, B34, B39, B40, B41, and supports B1+B3+B7/40/41, B34+B39+B41, B7+B40 and MHB+LB carrier aggregation. At the same time, the module integrates 4 LB_AUX and 3 MHB_AUX paths to support customers to expand the frequency band according to system requirements, which can meet the application requirements of 4G/5G mid-to-high-end smartphones and module products.

Figure: Diversity module SPDM003

It is worth mentioning that both SPDM001 and SPDM003 use WLP SAW packaging. At present, the main packaging processes used for SAW filters include wafer-level packaging (WLP) and chip-scale packaging (CSP). Among them, the CSP package is larger in size, can integrate partial matching, can protect the filter wafer, and has lower requirements for the module packaging process, which has certain cost advantages. "In contrast, the WLP package can achieve a smaller size, can be jointly designed with PA, LNA, etc., can realize wafer-level integration of multiple filters, has advantages in the design of other modules in the module, and is the future development direction of filters in modules." Bruce said.

In Bruce's opinion, Zuolan Microelectronics has three major advantages in modular products:

The first is to have diverse design capabilities. On the one hand, Zuolan's module design is based on the company's rich conventional SAW, TC-SAW, PESAWTM and PEBAWTM filter product lines, and can flexibly select the best design solution for customers' different frequency bands and performance requirements. For example, the use of conventional SAW can meet low-cost requirements, and the use of filters with other high-performance technology routes can bring customers better insertion loss, temperature coefficient, carrier aggregation and other performance indicators.

On the other hand, Zuolan's professional "module + SAW collaboration" design model can effectively reduce the matching loss of filters and other components in the module, effectively shorten the product design iteration cycle, and maximize the use of space. The filters, duplexers and other products involved in the module have completely independent intellectual property rights, and corresponding patents and integrated circuit layout designs have been applied for.

Secondly, Zuolan has a complete simulation and test design process. In the early stage of design, accurate simulation models and methods can quickly and accurately estimate device characteristics, speed up R&D, reduce design error rates and design modification costs. The finished product undergoes comprehensive testing and reliability verification to ensure that the product's electrical performance and quality reliability meet customer requirements.

In addition, Zuolan has a stable product yield. In the manufacturing process, Zuolan's existing module products all use mature WLP SAW technology, which can bring stable and reliable quality assurance; the matching circuits all use high-Q SMD components, which can provide the module with lower power consumption RF characteristics and stability (high-frequency insertion loss can be improved by about 0.3dB) compared with the integrated matching components in the substrate; relying on the stable tape-out manufacturing and packaging and testing capabilities of leading domestic and foreign foundries, the product front-end tape-out baseline yield is above 98%, and the back-end packaging baseline yield is above 95%. In terms of module partners, the RF switches and other devices in the module are provided by well-known domestic manufacturers with mature and stable design and manufacturing capabilities. For example, the yield of RF switches can generally reach more than 99%. Mature and stable manufacturing, packaging and testing, and close module partners determine the high consistency and reliability of Zuolan's module products.

Figure: SAW foundry tape-out yield

According to Jiwei.com, the two latest DiFEM modules have been tested in many customer terminal projects and will soon be shipped in large quantities. "These are the first two of the six module products planned by Zuolan. The next step is to plan LFEM, PAMiD and other aspects, and plans to launch related products in the next two years." Bruce further stated, "Even Japanese manufacturers are not very good at medium and high frequency multiplexers, so domestic manufacturers have little chance in medium and high frequency PAMiD modules in the past two or three years. However, there are great opportunities in low frequency PAMiD modules, and China is more mature in supporting devices such as PA, so Zuolan's strategy is to first make the filters and duplexers required for low frequency PAMiD modules, which is also the breakthrough point chosen by Zuolan based on the current situation of the domestic industry."

Filters have become a hindrance to the modularization of domestic RF front-ends. How can we move towards the mid-to-high-end market?

With the popularization of 5G smartphones, the RF front-end solutions of flagship models mainly adopt Phase7L and Phase 7LE architectures. Both diversity and main sets are mainly modular products, and there are fewer discrete devices. The RF front-end solutions of mid- and low-end 5G mainly adopt Phase5N architecture, with diversity modules as the main receiving end and discrete filters and duplexers as the main transmitting end. The miniaturization of devices is becoming more and more widely used. Market research firm Yole predicts that the market size of RF front-end modules will reach US$16.2 billion in 2025, accounting for about 69% of the total capacity of the RF front-end market, of which the market size of diversity receiving modules will reach US$4.572 billion, with an average annual compound growth rate of 13%.

my country's RF front-end chip manufacturers are still in the early stages and have limited market voice, but the demand for domestic chips continues to grow after the Sino-US trade war, and domestic supply chain manufacturers are expected to usher in major development opportunities. At the same time, in the RF front-end market dominated by international giants, modularization has become the main arena of future competition. Domestic PA, LNA, switches, antennas, etc. have all developed relatively maturely, but filters, duplexers and multiplexers have become the shortcomings of weak domestic production capabilities.

"Looking at the domestic RF front-end market, while the global market has been moving towards integration and modularization, the domestic market is still dominated by discrete devices, and each market segment is highly homogenized. Most of them are competing for the mid- and low-end market and are in urgent need of upgrading. In particular, China is generally not strong in advanced filter technology and processes, and modularization capabilities." Bruce pointed out, "Parts of the RF front-end market have even moved from 'domestic substitution' to 'domestic substitution', but low-price substitution alone is not enough. It is necessary to be able to provide products with performance levels comparable to foreign products in order to truly win the market competition. Starting from mid- and low-end products and moving towards high-end during the 5G window period has become the development path for domestic manufacturers to break through, and it is also the choice of Zuo Lan."