5G RF 제품 테스트 서비스 당면 과제

April 19, 2021 in Semiconductor Story by Vineet Pancholi
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5G 무선 주파수(RF) 표준이 빠르게 시행되고 있습니다[1]. 지난 4 ~ 6 분기 동안 시장에 소개된 간행물과 제품에 대한 관심이 증가하고 있습니다. 휴대전화, WiFi, 자동차, 사물 인터넷(IoT), 위치 서비스 등이 가장 인기 있는 RF 생태계 애플리케이션의 예입니다. 사물인터넷(IoT)은 한정된 데이터 양만 필요로 하는 반면, WiFi와 휴대전화 서비스는 데이터 집약적입니다.

4G 이동 통신사의 유닛 볼륨 메트릭스에 근거하면, 유닛 볼륨 달성(그림1)에 대한 신뢰도 수준과 5G 표준 정의의 총 적용가능한 마켓(TAM)이 높은 것으로 나타났습니다.

그림 1: 5G 제품의 견고한 성장 전망 , 출처: 사물인터넷(IoT) 비즈니스 뉴스

제품을 개발하는 전 세계 지역에서 개발되는 제품의 양부터 5G 사양까지 유사한 양상을 보이고 있습니다. 5G RF 유닛 볼륨이 많이 질수록 테스트해야 할 유닛 수가 늘어납니다. 인프라 구조 개발 및 개발은 사용자 장비 도입을 선행할 것으로 예상합니다. 그림 2에서와같이 일반적인 휴대전화 애플리케이션에는 범위 영역 내에서 다양한 사용자의 휴대전화를 각각 지원하는 휴대전화 타워 기지국이 포함됩니다.

Figure 2: Key ingredients of a two-way RF communication block diagram include an application processor (AP), baseband integrated circuit (IC), and radiofrequency integrated circuit (RFIC).

Since base stations have a coverage area to support multiple user equipment, the RF power requirement is higher relative to the user equipment. Base stations are powered by plug-in power, while the user equipment is designed to be power efficient because they are mobile and battery-powered. Since the magnitude of data downloaded on a typical cellphone is a couple of orders of magnitude higher than the data uploaded, the number of receive channels is typically larger than the number of transmit channels. Concepts like multiple-input, multiple-output (MIMO), and carrier aggregation (CA) [1] are employed at a protocol layer to increase the effective bandwidth. Receive channels employ diversity [1] to improve spatial performance. Even though these concepts are not the direct focus of this article, product architecture and design do have an impact on test requirements and test methodology. WiFi technology-based applications are typically within the home/office. Their maximum RF power is limited, yet the dynamic range is not, and their bandwidth is typically higher relative to cellphones.

The recent introduction of the 5G 3GPP standard [1], identifies carrier frequencies in two separate carrier frequency spectrums. As shown in Figure 3, FR1 carrier frequencies are in the 410 MHz to 7.125 GHz range and the FR2 carrier frequencies are in the 24 GHz to 52 GHz range. The allowable bandwidths exceed 100 MHz up to 2 GHz. The sub-carrier spacing is compacting and hence the need for tighter constraints for phase noise and gain flatness.

Figure 3: The 5G carrier frequencies are defined in the 3GPP specifications [1].

5G NR(New Radio) 변조 방식

There are two 5G NR signal modulation schemes – cyclic prefix orthogonal frequency division multiplexing (CP-OFDM) and discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) [1] (Figure 4).

Figure 4: A 256 quadrature amplitude modulation (256-QAM) 5G NR constellation plot captured on an Advantest V93K.

CP-OFDM is for downlink (D/L), with quadrature phase-shift keying (QPSK), 16-QAM, 64-QAM, and 256-QAM. It has high spectral efficiencies and is compatible with MIMO and 4G LTE definitions. DFT-S-OFDM is for uplink (U/L), with π/2- binary phase-shift keying (BPSK), 16-QAM, 64-QAM, and 256-QAM. It has a more complex implementation and has less flexible resource assignments compared to CP-OFDM and it is not used in combination with MIMO. The five sub-carrier spacings for 5G NR are between 15 kHz to 240 kHz. Figure 4 shows a 256-QAM plot.

5G RF 제품 & RFIO

Modern direct and heterodyne converter architectures [2] have digital baseband I/O. The digital baseband feeds the data to a digital-to-analog converter (DAC) that creates the analog in-phase and quadrature (I/Q) waveforms. These waveforms, when mixed with a local oscillator (LO) signal, up-convert the data to produce the modulated intermediate frequency (IF) or RF signal that is transmitted to the receiver (Rx). The signal transmission occurs over a coaxial shielded cable or over the air. Prior to transmission, especially when it is over the air, the signal may require signal amplification. Also, the receiver may require the received signal to be amplified prior to supplying the signal for down-conversion. The down-converted signal is fed to an analog-to-digital converter (ADC) that converts the signal to digital baseband for processing by the application processor. Figure 5 shows these steps.

Figure 5: Simplified transmitter (Tx) and Rx RF chain blocks.

Integrated device manufacturer (IDM) customers bring a variety of RF products for assembly and test services. This includes, and is not limited to transceivers, low-noise amplifiers (LNA), power amplifiers (PA), digital step attenuators (DSA), filters, and mixers. Depending on the target application, the number of RF input and output channels may be different. Bandwidth, phase noise, intermodulation distortion (IMD), phase and amplitude resolution/accuracy, and other test requirements may vary as well.

The device under test’s (DUT’s) transmitter characteristic specifications for production testing includes transmitting power and RF spectrum emissions (occupied bandwidth, out of band emissions, adjacent channel leakage ratio (ACLR), and IMD). DUTs Receiver characteristic specifications for production testing include receiving sensitivity, maximum input levels, adjacent channel selectivity, blocking, spurious response, and IMD [1].

5G RF 하위 시스템을 이용한 자동화된 테스트 장비(ATE) 테스터 및 툴링(Tooling)

Advantest, Teradyne, National Instruments, and Cohu have recently publicly released an upgrade path for their mature ATE offerings. Amkor utilizes ATE’s RF sub-systems hardware and software instrumentation infrastructures to test customer products in production factories.

ATE vendors typically architect a universal superset configuration of instrument resources for customer test application development. The number of arbitrary waveform generators (AWGs), digitizers (DGTs), LOs, filters, amplifiers, tone combiners, transmit signal splitters, receive signal switches and their wide bandwidth and dynamic range of operation present trade-offs that must be considered for every new 5G RF application for each customer. Phase noise at application-specific frequencies and amplitudes from the instrument design has a direct impact on the error vector magnitude (EVM) test. Phase noise of -110 dBc/Hz at an offset of 100 kHz and -10 dB or better is acceptable (typical) at 5G range of continuous wave (CW) frequencies. In typical broadband customer product applications, there is a need to switch frequencies and amplitudes. Switching times impact the overall test list execution times. Testers with the smallest switching times are the most efficient in production testing. Figure 6 shows an ATE block diagram.

Figure 6: A simplified ATE block diagram.

Custom tooling (probe cards and/or load boards) must be developed to help route tester resources to devices, pins, or bumps. For wafer probe services, probe card vendors deliver probe pin technologies. For 5G RF carrier frequencies that are above 50 GHz, the challenges include impedance matching and pin to pin and site to site signal isolation. For packaged parts, load board, socket, and socket-pin technology vendors deliver pin technologies. For 5G RF carrier frequencies, challenges are similar to the ones described for probe pins. Acceptable levels of insertion loss (S-parameter S21) at these frequencies are typically no more than -10 dB and return losses (S11) over the frequency range are typically better than -10 dB. Acceptable levels of pin-to-pin isolation for typical applications are better than -45 dB over the range of frequencies.

RF performance and accuracy specifications are guaranteed by the supplier to the test head signal delivery interface. The tester supplier develops and delivers calibration systems (hardware and software) to calibrate, verify, and diagnose performance within documented specifications. RF instruments’ accuracy specifications are sensitive to temperature fluctuations. In most cases, a ±5°C (or tighter) change in temperature triggers the instrument’s self-calibration routines. Power, signal (digital, analog/RF), and clocks require moving the calibration plane from the test head to the device pin. This path includes the traces on the probe card or load board. We have a unique benefit of either employing de-embedding techniques and using loopback or custom-developed Short, Open, Load, Through (SOLT) structures to help deliver the required RF signal accuracies to the device under test. Developing custom standards for calibration in most cases requires additional efforts, however, with in-house package designs, the avenue does exist. In most cases, golden loopback DUT techniques have been sufficient to achieve the desired accuracies.

Assembly-Test Attach

Our Assembly and Test divisions work closely to enable 5G RF engineering development followed by production testing. The benefit is a complete assembly and test turnkey solution offered from the same factory location. 5G packages offering antenna in package and antenna on package (AiP/AoP) SIP were first produced by Amkor in July 2018 and announced in a public press release in 2019 [3].

With the recent advancements in assembly and packaging technologies, RFICs, like 5G transceivers and RF Front End (RFFE) devices, may have antennas embedded within the package. Similarly, System in Package (SiP) devices have integrated relevant components like processor, memory, RFIC peripherals, discrete components including power amplifiers, low noise amplifiers, phase arrays, and antenna structures within the IC package [4]. The antenna forms the critical component of the front-end and requires tuning for the specific frequency band of operation. The 5G NR FR2 compliant customer products being designed today have their performance-tuned in specific operating bands, as defined in the 3GPP specification [1]. Data-intensive applications may warrant packing multiple radios within the package and hence the need for multiple antennas tuned per frequency band of operation.

All production testing of previous and present generations of RF devices has been conductive. RF I/O from and to the DUT are electrically connected with impedance-controlled paths over cables and shielded printed circuit board (PCB) micro-traces to the tester’s RF instrumentation. As described above, all ATE suppliers developing 5G RF test solutions include a conductive RF coaxial interconnect. To enable high-volume production testing of packages with an embedded antenna, the test methodology requires an interconnect that can transmit or receive RF energy with minimal and controlled signal loss. Antenna transmission theory [7], requires minimal spatial separation between the transmitter and the receiver. This separation depends on the carrier frequency. The number of RF I/O channels and multisite test requirements add to the production test complexity. The test options that are presently being explored, include patch and horn antennas, beamforming ICs (BFICs), embedded directional couplers, and waveguides. None of these solutions is high-volume manufacturing friendly nor are they scalable as the number of antenna increases. This is primarily due to the physical space requirement in the handler at the tester interface.

IDMs have been architecting design structures that allow loopback Design for Excellence (DfX) modes on transceivers to help simplify and make production test equipment requirements economical. While an antenna embedded within the package offers added miniaturization and overall integration, it does take away the flexibility of final performance tuning of the application for the new 5G NR operating band of carrier frequencies. The company continues to partner with suppliers and customers to solve the over-the-air (OTA) test challenges for production testing.

부가가치 제안

상위 레벨에는 두가지 주요 생산 테스트 운영 모델이 있습니다. 첫번째 테스트 운영 모델은 고객이 테스트 컨텐츠와 5G RF 테스트 장비를 선정하여 앰코가 생산에 사용하도록 하는 것입니다. 두 번째 테스트 운영 모델은 생산 테스트를 가능하게 하는 고객 요청 엔지니어링 서비스를 제공합니다. 이 경우, 테스트 개발 팀은 고객과 긴밀히 협력하고 각 고객의 테스트 개발 엔지니어링(TDE) 요구 사항에 맞는 맞춤형 요구사항을 충족합니다. 부가적인 TDE 서비스의 예는 다음을 포함하지만 이에 국한되지 않습니다:

  • 일치하는 5G 지원 테스터 선택
  • 일치하는 프로버 및 핸들러 선택
  • 적절한 테스터 리소스 할당, 특히 다중 사이트 생산 테스트를 위한 일치하는 5G테스트 툴(프로브 카드, 로드 보드) 설계
  • Developing and debugging production test programs, test patterns, and test waveforms per the customer’s functional test specification,
  • 제품 검증,
  • 제품 특성 테스트 루틴
  • Yield optimization, low yield failure analysis, and product design feedback. (Failure analysis may, for example, require X-ray or de-lamination to determine the root causes of fabrication and assembly packaging defects.),
  • 맞춤형 백 엔드 플로우로 완제품을 효율적으로 처리할 수 있습니다.

The RF test development engineering group has significant experience developing test solutions and test content for previous and present generations of RF technologies and continues to build upon this expertise to solve 5G test challenges described here. The group is actively engaged in creating and proposing test solutions for base station and mobile 5G RF products in both FR1 and FR2 RF spectrum. These test solutions make use of the 3GPP standard-compliant ATE hardware and software test tools described above.

Internal production test processes have matured over the years and allow the implementation of design for manufacturing (DFM) rules to 5G RF production testing. Collecting, analyzing, and retaining manufacturing test results of 5G RF production tests is vital for incremental improvements to test methods, flows, and content. In specific cases, test engineers provide valuable feedback to the IC design and fabrication process engineers. Established statistical bin limits (SBL) for 5G RF test results for multisite across the test equipment fleet can help identify systemic equipment-related false failures and help with the elimination of such factors. This ensures optimal test equipment utilization and improves the overall production throughput.

A good portion of customers have products with critical time-to-market (TTM) goals and are sensitive to intellectual property (IP) contamination and security. Mature systems and processes are in place to handle all such customer concerns.

Amkor production test has been preparing to test the large number of 5G products that are expected in the coming years. This includes the 5G base station and infrastructure equipment that is expected to precede user equipment (mobile devices) growth.

요약

The 5G RF production test business is substantial in size and growing rapidly. Our production test teams have been working closely with assembly packaging, ATE suppliers, and customers to ensure that holistic 5G RF production test services are made available to meet and exceed all test capability and capacity challenges.

참고 자료

  1. 3GPP TS 38.101-1 V16.1.0 (2019-09).
  2. Wide Band RF Architecture options – Peter Delos, Analog Devices.
  3. Amkor Device Packages
  4. Antenna In Package/Antenna On Package
  5. Amkor Antenna in Package – Article.
  6. Amkor Packages – Press Release 2019
  7. Fresnel Far Field Region or Antenna Theory

작성자 정보

Vineet Pancholi, Sr Director Test Technology at Amkor Technology, Inc. in Tempe, AZ. Vineet joined Amkor in January 2019 and currently leads test technology development for 5G RF and high-speed digital production test methodologies. Before joining Amkor, Vineet worked in test development at Microchip Technology. Prior, he spent 19 years at Intel in a variety of test roles, including tester supplier management, test technology development (burn-in, final and system level test) and RF tester architect. Vineet holds a patent on semiconductor device testers and has earned master’s degrees in physics and electrical engineering from Arizona State University.