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1. What is a SAW device?
Surface acoustic waves (Surface Acoustic Wave: SAW) are waves that propagate in the vicinity of the surface of an elastic body. Electronic devices that apply these waves are called SAW devices.
The difference between crystal unit and SAW devices is described below. Although the crystal element vibrates mechanically in various modes, commonly used are AT-cut and BT-cut for thickness-shear. (The AT cut has the tertiary temperature characteristic, and the BT cut has the secondary temperature characteristic.) In addition, since frequencies are inversely proportional to the thickness as shown in Table 1, the crystal element of crystal unit becomes thinner when corresponding to high frequencies, making processing difficult.
Table 1 Crystal element's mode of oscillation and frequency ranges
On the other hand, as shown in Fig.1, the SAW filter has soot electrodes on the piezoelectric substrate that excites and receives surface waves. When an AC voltage is applied to the soot-shaped electrodes, the piezoelectric effect causes distortion in the piezoelectric substrate between adjacent electrodes and excites the surface waves.
Fig.1 Basic structure of SAW filter (transversal type)
The interdigital electrodes is arranged periodically as shown in Fig.2, and the surface wave is excited most strongly when the wavelength λ and the electrodes period of the surface wave are equal to each other. Between the mid-band frequency fo and the propagating velocity v of the surface wave fo = v / λ and the frequencies are determined by the electrodes intervals formed on the surfaces. For this reason, photolithography can easily accommodate high frequencies.
Fig.2 Basic architecture of everyone electrodes (transversal type)
2. SAW device lineup
Nihon Dempa Kogyo Co., Ltd. has a lineup of airtight PKG-type SAW devices for discrete-type CSP (Chip Scale Package), industrial, automotive, and space applications, including WLCSP (Wafer Level Chip Scale Package for built-in modules. Available frequencies range from 40 MHz to 3 GHz.
3. About Automotive SAW Devices
Automotive SAW devices are mainly used as filters in RF circuits for RKE (Remote Keyless Entry) and TPMS (Tire Pressure Monitoring System receivers and navigation systems (Global Navigation Satellite System: GNSS). RKE and TPMS filters, which mainly use frequencies in the 300 MHz to 400 MHz band, are often used in airtight PKG-type products of 3 mm by 3 mm because of their low frequencies and restrictions on SAW chip sizes. For high-frequency navigation systems, not only airtight PKG-type but also CSP-type products (WFF93A1582UE) are used. This section describes the structure of the in-vehicle CSP that requires high reliability.
4. Structure of In-Vehicle CSP
Automotive products require AEC-Q200 reliability, and must be adapted to a wider range of temperatures than consumer products. In conventional CSPs, SAW chips are mounted on ceramic substrate by flip-chip bonding via gold bumps. Thermal shock resistance is -40°C to +85°C for 200 cycles due to differences in temperature coefficients of materials. On the other hand, as a result of improving thermal shock resistance by making the temperature coefficient of the materials used nearly the same, we confirmed that there was no problem at temperatures between -55°C and +125°C in thermal shock resistance tests and 1000 cycles (details will be described later). In addition, a WLCSP using solder balls is used to connect to the base substrate, and resin is filled around it and molded again to enhance reliability by adopting the manufacturing method of module manufacturers to improve environmental resistance performance.
The structure difference between conventional CSP and automotive CSP is shown below.
Fig.3 Conventional structure CSP
Fig. 3 shows the conventional CSP structure. As described above, the SAW chip is flip-chip bonded to the ceramic substrate via gold bumps and sealed with sheet resin to form a hollow SAW propagating part. Due to the large differences in the thermal coefficients between the ceramic substrate and the SAW tip, repeated thermal shocks can stress the gold bumps and damage the connections, resulting in loss of electrical continuity and failure.
Fig.4 shows the structure of the CSP for automotive use. The WLCSP with hollow space in the propagating part shown in Fig.5 is flip-chip bonded to the substrate, and the sealing resin is filled up to the lower surface of the SAW propagating part.
The same thermal coefficients for SAW chips, substrate, and sealing resins are used to reduce the stress on the bumps due to thermal shock. Thermal shock tests have confirmed that there are no problems in the -55°C to +125°C range for 1000 cycles, and other requirements are included in the AEC-Q200 standards. (Fig. 6)
Fig.6 Results of automotive CSP thermal shock test
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