Chip Scale Review - May June 2020

20 Chip Scale Review May • June • 2020 [] Testing AiP modules in high-volume production for 5G applications By Jose Moreira [Advantest] he ar rival of 5G promises enhanced mobile broadband (eMBB), massive machine- type communication (mMTC), and ultra- reliable low-latency communication (URLLC). eMBB is already available t o c on s ume r s , who c a n pu r c h a s e 5G-capable handsets that can operate on 5G networks deployed by carriers in many regions throughout the world. But as 5G rolls out, the test community faces challenges and opportunities. That’s particularly true regarding the antenna arrays that will connect handsets to base stations. Most initial 5G deployments will take place only using the sub-6GHz frequencies used for previous generations of cellular technology, but they will be enhanced later with the possibility of short-range high data rate connections using mmWave frequencies. 5G new radio (5G NR) def i nes two ranges, frequency range 1 (FR1) and frequency range 2 (FR2). FR1 includes the sub- 6GHz frequencies, but FR2 opens up mmWave frequencies above 24GHz for 5G deployment. 5G NR leverages the FR2 frequencies to achieve larger modulation bandwidths (for example, 800MHz). But because of the high transmission loss at these frequencies, it is necessary to use antenna arrays for multiple-input and multiple-output (MIMO) functionality and to focus the transmission beam (beam forming) in both the base station and the consumer’s handset. These arrays come in the form of antenna-in-package (AiP) modules, which are a critical part of the current 5G wireless communication wave. For the handset, these AiP modules will usually have an a r r ay of dual polarized patch antennas for top firing and, in some instances, also an array of dipole antennas for side firing as shown in Figure 1 . To keep RF losses to the antenna radiators to a minimum, the AiP module includes an RF integrated circuit that provides the modulated mmWave signals to the AiP antenna array with the needed gain and phase to each radiating element. The module would then usually only requi re power, digit al cont rol signals, and modulated intermediate frequency (IF) signals. AiP modules for 5G handsets need to be extremely small to fit into the modern cellphone form factor, and a multiple of them need to be used in a single cellphone because the user’s hand position has a significant impact on the transmitted beam loss. Also, the AiP modules in a cellphone might not be all equal, but in fact have different antenna configurations depending on the handset design. The 3GPP standard def ines th ree methods for the over-the-air (OTA) standard compliance testing of AiP modules: direct far field, indirect far field (e.g., compact antenna test range, or CATR), and near-field to far-field transformation. Each of these methods have advantages and disadvantages, but they all require relatively large test chambers and a complex manipulator to rotate the AiP device under test (DUT) or the measurement antenna. For high-volume production testing, the objective is to check functionality of the DUT and not its compliance to the standard. Low cost of test is critical because most of the end applications are consumer-oriented. Also, to keep costs down it is important to be able to reuse as much as possible the test cell infrastructure already used for testing RF integrated circuits. From a test engineering point of view there are multiple possible steps in testing an AiP module. First, the RF chip (e.g., a wafer-level chip-scale package [WLCSP] part), is tested at wafer level. This test can be either very simple and low cost (e.g., mainly DC or even a wire loopback on the mmWave ports), or it can also include f ull mmWave pa ramet r ic measu rements usi ng an appropriate probe card and automatic test equipment (ATE) system. After the AiP module is assembled, the same question on the types of tests to be performed on the AiP module can be evaluated. It might consist of a simple low-cost DC test or even some kind OTA loopback test, to a full parametric OTA test with a measurement antenna. Finally, after integration in the end product (e.g., a 5G handset) a system-level type of test might also be performed. The test strategy at each stage depends on the overall test strategy for the AiP module. Another important point is the AiP module calibration. For proper beam steering of the AiP module, it is critical that one is able to accurately set the gain and phase at each antenna element. If this accuracy cannot be guaranteed by design or a built-in self-test (BIST) c a l i b r a t i o n t e c h n i q u e , t h e n t h i s calibration step needs to be performed at one of the test stages of the AiP module testing, or worst case at system level when the handset is assembled. We will now concentrate on the case that a parametric OTA test is required for an Ai P module, but because of T Figure 1: Drawing of an example of a generic antenna array module, comprising 12 dual-polarized patch antenna elements and seven dipole antenna elements.