The Smallest Form Factor GPS for Mobile Devices

2017 IEEE 67th Electronic Components and Technology Conference The Smallest Form Factor GPS for Mobile Devices Eb Andideh 1, Chuck Carpenter 2, Jason Steighner 2, Mike Yore 2, James Tung 1, Lynda Koerber 2, David Schnaufer 2, Jeff Moran 1, Suwanna Jittinorasett 1, Bharati Ingle 1, Anousa Sengsavanh 1, and Otto Berger 3 1 Qorvo-OR, USA, 2 Qorvo-FL, USA, 3 Qorvo-Munich, Germany Abstract We have developed a GPS product to eliminate the enclosure by creating a hollow cavity between active dice and the package substrate. To create the hollow cavity, we use a 2-layer sheet overmold to prevent mold compound encroaching under the dice. Flipped SAW filter and GaAs low noise amplifier (LNA) dice with gold stud bumps are attached to Ultra Low-footprint CSP (ULC) co-fired ceramic substrate using Gold-Gold Interconnect (GGI) thermosonic die attach. This results in the unit s low cost with significant potential profit margin. This is the first time a GaAs LNA has been integrated in a hollow cavity to accommodate the SAW filter requirement. This GPS product is targeted for the mobile RF and handset products. Measurements show the module meets the application-targeted noise figure, in-band gain, and excellent off band rejection. The module has also completed our stringent standards suite of reliability with resolvable minor concern regarding gold stud bump landing pads surface pre-treatment process. The SAW filter s interdigitated transducers are enclosed in a cavity housing, creating an enclosure to avoid contamination of the transducer surface. Any contamination on the surface of transducers can shift frequency response of the filter, causing GPS unit failure. However, presence of the enclosure increases the die area and packaged unit thickness. This paper presents the integration and packaging processes followed by reliability and performance assessment of our front end GPS module. location of the personal belongings, package tracking, young drivers location, jogging distance, pet, etc. The GPS function may be integrated on the heterogeneous module along with many other functions. However, in many cases customers want stand-alone GPS function in a small form factor (SFF) and at low cost. We have fabricated the smallest GPS module to date by integrating GaAs E/D-PHEMT LNA and a SAW filter into an ultra-small footprint ceramic package with Gold-Gold Interconnects (GGI). TAM and cost analyses to date show the best profit margin at the lowest total cost as shown in Figure 1 compared to our competitors product [2, 3]. This product addresses the form factor requirement with excellent performance at low cost for many applications listed above. Our targeted front end module (FEM) GPS product required creation of a cavity under the SAW and LNA dice assembled in a small FF package without mold compound underfill. No_underfill for GaAs devices is new to us and the risk of no-underfill was assessed. The module operates between 1.8 V to 2.8 V bias and consumes less than 6 ma with center frequency of 1585 MHz making it suitable for use in low-power applications and during low-battery situations. Our module covers three bands: GPS, GNSS and Beidou as show in Figure 1. This module has the highest gain of 16 db at the smallest size 1.5X1.1 mm 2 size compared to competitor s product listed in Figure 1. I. INTRODUCTION II. FABRICATION PROCESS The Total GPS market is expected to reach $26.67 billion by 2016 according to a press release by MarketsandMarkets.com. Personal and pet tracking GPS Market which may require small form factor (SFF) GPS devices is estimated to reach $2 Billion by 2021 according to ABI Research. Our targeted GPS unit requires a heterogeneous integration of a SAW filter with a GaAs Low Noise Amplifier (LNA). All smartphones contain GPS modules. In some cases, all functions are integrated on one Si die with an increased die size penalty, and increase in overall packaged module size [1]. Consequently, users of mobile devices expect to have GPS functionality on their handheld devices and phones. GPS tracking devices are also in high demand for monitoring A. Gold Stud Bump Formation, Wafer Thinning and Dicing Assembly processing involves gold stud bump formation on gold bond pads of the GaAs LNA. LNA bond pads are normally are fabricated on organic dielectric (BCB). However, the gold stud bumps formations require ultrasonic energy for bump attach. Presence of soft BCB will cause loss of ultrasonic energy with loss of adhesion between bond pad and gold stud bump. For this reason, pads are located on solid surfaces with gold via to support the stud bump formation resulting in strong adhesion. We experimented with forming Au stud bumps on bond pads with BCB under them, however this resulted in poor adhesion of the stud to the bond pad as shown in Figure 2. resulting in poor reliability result. SAW filter device gold landing pads are on solid inorganic hard 2377-5726/17 $31.00 2017 IEEE DOI 10.1109/ECTC.2017.12 1437

dielectric layers and thermosonic stud bump formation result in good adhesion. During the gold bump formation, wafer temperature was maintained below 50 o C. Height of the stud bump was targeted at 50 um. Figure 2 shows a SEM photo of the stud bump. Wafer thinning was performed next. Figure 3. Gold Stud Bump on GaAs LNA landing pads Figure 2. SEM photo for an example of poor adhesion between gold stud bump and GaAs LNA bond pad. GaAs LNA wafer back grind was done on a glass carrier with a spacer material used to attach the wafers to glass carrier. Thickness of the organic spacer material was properly decided to avoid deformation of the Au stud bump such as coining of tip of the stud bumps. SAW filter wafers were laminated with back grind tape for thinning. We back grind both wafers to the targeted thickness of 250 um. for both GaAs LNA and SAW filter. The common-side dimensions were 700 um. These dimensions were used to avoid overmold creep under the dice. We used IR laser ablation dicing process for GaAs LNA singulation and stealth dicing was used for the SAW filter. 1438

B. Assembly and Packaging Thermosonic bonding was used to attach both dice onto the high temperature co-fired ceramic (HTCC). Figure 4 shows the structure after die attach. As shown in the figure 3, interface between Au stud bump and device pads and between the bumps and laminated substrate is without any voids and encroachment. After die attach on the HTCC substrate, the dual-layer overmold compound is applied to form a hollow cavity between substrate and front surface of the dice. This process step is critical for the performance of the SAW filter to avoid surface loading of the interdigitated resonator. For this reason, we must avoid encroachment of the mold compound under the SAW filter which could load the resonator surface and cause a shift in operating frequency. The shift in SAW resonator frequency response will make the module useless for the GPS functionality. Another factor which could impact SAW filter operating frequency is outgassing of the substrate laminate. Organic laminate dielectric layer will outgas at high end of the operating temperature of the filter. For this reason, we decided to use HTCC substrate. To accomplish the hollow cavity requirement, we use the aforementioned dual-layer sheet mold compound. The bottom layer has a high viscosity while the upper layer has a lower viscosity. Figure 5 shows the post-mold compound application. In addition to the use of dual layer overmold, we also targeted thickness of both SAW filter and GaAs LNA to be the same at 250 um thickness to mitigate possible molding encroachment under the SAW filter concern due to dissimilar height of the die as shown in Figure 5b 3D x-ray image. This process creates a hollow cavity under both dice without surface loading of the SAW filter resonator and impact to performance of the GPS module. We measured die to substrate attachment strength by die shear force measurement. Results show equivalent or higher shear force compared to our Cu/Sn bump devices in production. All failure interfaces show the failed interface is in die layers or within Au which is an indication of strong GGI interfaces as show in Figure 6. The standoff height between die and substrate is 15 to 20 um after die attach due to flattening of the Au stud bump during assembly. LNA and SAW filter must go through the plasma treatment to remove any foreign organic material on the bond pad. Lack of proper cleaning may result in poor adhesion between the Au stud bump and Au pad. This plasma treatment could include an inert gas and oxygen at low pressure. IV. PERFORMANCE RESULTS Electrical performance tests show this lead-free product with only 2 external inductor components can cover all three GPS bands (Beidou, GLONASS and GPS) with good receive signal sensitivity and noise figures < 2.5dB (1.8dB min) at 5mA. Noise Figure for all 3 bands at -25 o C up to 90 o C is shown in Figure 8a for mobile applications. Our integrated flip chip SAW filter and the advanced flip chip GaAs phemt LNA provide an in-band gain of 16dB indicated in Figure 8b. Excellent out of band attenuation is shown in Figure 8c which indicate excellent rejection outside the required frequency range. These results meet product performance for the intended mobile GPS application. Figure 9 show the raw data collected for 1160 modules from multiple lots to represent the narrow distribution of primary parameters. This figure shows a gain of greater than 13 db, I d_on below 6 ma, Noise figure <3.2dB, and off-band rejection less than -38dB. We attempted to produce all these performance results but we would intend to improve upon the values. III. BLOCK DIAGRAM, PACKAGE PINOUT AND RELIABILITY ASSESSMENT Our 1.5 x 1.1 x 0.55 mm packaged module requires only two external matching inductors. Value of these inductors are optimized for the best performance of the module and for minimizing the final assembled module dimensions. The block diagram and package pin-out is shown in Figure 7 including location of the external inductors. Reliability assessment was carried out using our standard suite of quality assessment and completed as shown in Table 1. Few marginal fails detected during the module level reliability were traced to the insufficient LNA pads plasma treatment cleaning as was shown in Figure 2. Gold stud bump landing pads for GaAs Figure 4. Die attach process show good adhesion between stud bump and LNA and HTCC bond pads without any void. 1439

V. CONCLUSION We have demonstrated integration of a GaAs LNA and SAW filter in a ULC HTCC package with a Hollow Cavity to prevent frequency shift due to loading the SAW filter resonators for the 1 st time. Hollow cavity eliminates the filter housing, resulting in smaller SAW filter, and reducing height of the module. In addition, using GGI interconnect, enables narrower gap (stand-off) between die and the substrate which reduces the chance of mold compound encroachment under the SAW filter. Our assembly process reduces cost and cycle time. The integrated GPS module is optimized for size, performance and suited for battery-powered mobile devices due to low power consumption. We have also shown passing performance results and the required accelerated reliability stress evaluation. The only external components are 2 inductors to address the small size requirements for today s space-constrained mobile devices. During this process we overcame the challenges such as dissipation of the thermosonic energy during gold stud formation on LNA die and die attach, avoidance of gold stud coining during wafer back grind and surface contamination with the use of this hollow cavity process. Figure 5. (a) Drawing of the complete packaged product showing the 2-layer sheet mold compound and cross section of the HTCC substrate layers, (b) 2-D X-ray of the final product showing hollow cavity under LNA and SAW filter. REFERENCES [1] H. Kim, et. al. A single-chip multi-mode RF front-end circuit and module for W-CDMA, PCS, and GPS applications, 8 th European Conference on Wireless Technology 2005, Paris, pp 367-370. [2] YH Chow, et.al. Aminuature LNA-Filter GPS receiver Frontend Module Combining FBAR and E-mode PHEMT Technology, 2008 IEEE MTT-S International Microwave Symposium Digest, pp 1525-1528 [3] YC Hsu, et. al. A Fully Integrated GPS LNA Module, Proceedings of APMC 2012, pp944-9 Figure 6. Die shear test for SAW filter and GaAs LNA after die shear showing favorable delamination interface. 1440

Figure 7. (a) Block Diagram of the module showing 2 matching inductors and location, (b) Package Pinout Table 1. Standard Suite of Reliability Assessment Results. The resolved fail modules after stress were associated with insufficient pre stud bump attach clean on LNA pads. 1441

Gain>13 db Gain (db) I on (V dd ) 2 4 6 8 10 (ma) N b f M d l Noise Fi Rejection < -38 db Figure 8. Preliminary design test results: (a) Noise Figure vs. operating temperatures for Beidou, GPS and GNSS bands, (b) In band Module Gain showing greater than 16dB gain, (c) Module Wide Gain at Vdd=2.7 V showing the out of band attenuation. Reject i Figure 9. Raw Test data RF Probe of one lot- (1160 site from multiple lots) measurement of Gain, Noise factor, I on, and Rejection 1442