Conformal thin film packaging for sic sensor circuits in harsh environments
In this investigation sputtered silicon carbide annealed at 300°C for one hour is used as a conformal thin film package. A RF magnetron sputterer was used to deposit 500 nm silicon carbide films on gold metal structures on alumina wafers. To determine the reliability and resistance to immersion in harsh environments, samples were submerged in gold etchant for 24 hours, in BOE for 24 hours, and in an O2 plasma etch for one hour. The adhesion strength of the thin film was measured by a pull test before and after the chemical immersion, which indicated that the film has an adhesion strength better than 108 N/m2; this is similar to the adhesion of the gold layer to the alumina wafer. MIM capacitors are used to determine the dielectric constant, which is dependent on the SiC anneal temperature. Finally, to demonstrate that the SiC, conformal, thin film may be used to package RF circuits and sensors, an LC resonator circuit was fabricated and tested with and without the conformal SiC thin film packaging. The results indicate that the SiC coating adds no appreciable degradation to the circuits RF performance.
Study on a 3D packaging structure with benzocyclobutene as a dielectric layer for radio frequency application
A new wafer-level 3D packaging structure with benzocyclobutene (BCB) as interlayer dielectrics (ILDs) for multichip module fabrication is proposed for the application in radio frequency. The packaging structure consists of two layers of BCB films and three layers of metalized films, in which the monolithic microwave IC (MMIC), thin film resistors, strip lines and microstrip lines are integrated. Wet etched cavities fabricated on the silicon substrate are used for mounting the active and passive components. BCB layers cover on the components and serve as ILDs for interconnections. Gold bumps are used as electric interconnections between different layers, which eliminate the need of preparing vias by costly dry etching and deposition process. In order to get highly qualitied BCB films for the subsequent chemical mechanical planarization (CMP) and multilayer metallization processes, the BCB curing profile is optimized and the roughness of the BCB film after CMP process is controlled lower than 10nm. The thermal, mechanical and electrical properties of the packaging structure are investigated. The thermal resistance can be controlled below 2 °C/W. The average shear strength of the gold bumps on the BCB surface is around 70 N/mm2. The performances of MMIC and interconnection structure in high frequency are optimized and tested. The S-parameters curves of the packaged MMIC shift slightly showing perfect transmission character. The insertion loss (S21) change after the packaging process is less than 1db range at the operating frequency. And the return loss (S11) is less than −8 dB from 10GHz to 15 GHz.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.