Showing papers by "Rao Tummala published in 2021"

A Review of 5G Front-End Systems Package Integration

Abstract: Increasing data rates, spectrum efficiency, and energy efficiency have been driving major advances in the design and hardware integration of RF communication networks. In order to meet the data rate and efficiency metrics, fifth-generation (5G) networks have emerged as a follow-on to 4G and projected to have $100\times $ higher wireless date rates and $100\times $ lower latency than those with current 4G networks. Major challenges arise in the packaging of radio frequency front-end modules because of the stringent low signal-loss requirements in the millimeter-wave frequency bands, and precision-impedance designs with smaller footprints and thickness. Heterogeneous integration in 3-D ultrathin packages with higher component densities and performance than with the existing 2-D packages is needed to realize such 5G systems. This article reviews the key building blocks of 5G systems and the underlying advances in packaging technologies to realize them.

Laminated Glass-Based, Compact Inline Stepped-Impedance Resonator Bandpass Filters for 5G New Radio Modules

Abstract: Miniaturization of millimeter-wave (mm-wave) filters relies on innovative designs that require fine lines and spacings. These designs drive the need for precision fabrication that cannot be achieved with traditional laminates or cofired ceramics. This gives unique performance justification to glass substrate technologies for future integrated mm-wave components because of both superior performance and 3– $5\times $ reduction in volume. In this letter, laminated glass-based compact inline stepped-impedance resonator (SIR) filters are presented for 5G new radio (NR) modules. The frequency bands of interest are 5G NR bands: n257 and n260. The filters are designed using microstrip transmission lines with the fourth-order quasi-elliptic response with skew-symmetric feed. The enhanced transition responses of these filters provide transmission zeros (TZs) on either side of the passband. When these filters are designed for mm-wave applications, their dependence on the accuracy of metal patterns increases significantly due to narrow linewidth and spacing requirements to realize moderate fractional bandwidths. Two inline SIR filters are fabricated using a large-area, panel-compatible semiadditive patterning (SAP) process on precision redistribution layers (RDLs) of a laminated glass substrate stack up. The sensitivity of these filters to the dimensional variations of patterned copper is analyzed, and the discrepancy between simulated and measured response of the filters is studied by performing postfabrication simulations.

High-density low-loss millimeter-wave package interconnects with the impact of dielectric-material surface roughness

Abstract: Heterogeneous package integration and chiplet approaches are the key technology to enable next-generation high performance small form-factor packages for emerging applications. Millimeter-wave packaging for fifth-generation and upcoming sixth-generation platforms also need to meet the high-density low signal-loss interconnect specifications utilizing advanced conductor and dielectric materials. This article presents the comparison of the liquid-based photoimageable dielectric (PID) and dry-film dielectric materials in terms of interconnect path losses that are critical in mm-wave frequency bands. The conductor loss being more dominant in the frequency bands and in thinner dielectric structures, we assess daisy chains and microstrip lines on 15-μm dielectric by measuring the S-parameters to quantify the impact of the surface roughness at around 28 GHz. Measured results from the daisy chain and microstrip line structures exhibit that the smooth surface of the liquid-based PID (3 nm) leads to 8%–32% lower signal loss in the dB scale than the 325-nm rough dry-film dielectric. The study provides comprehensive experimental results that the different material forms with various surface roughness largely impact the package-level interconnect loss.

Measurement and Analysis of Through Glass Via Noise Coupling and Shielding Structures in a Glass Interposer

Abstract: In this article, we first measured through glass via (TGV) noise coupling and the effectiveness of shielding structures in a glass interposer. To analyze the noise coupling between signal TGVs, an open-ended structure is adopted. Glass interposer test vehicles were fabricated to verify the noise coupling between signal TGVs. With these test vehicles, noise transfer functions between signal TGVs were measured. Based on these measurement results and the equivalent circuit model, the noise coupling between signal TGVs was analyzed. To suppress this TGV noise coupling, shielding structures for the TGV noise coupling were proposed and verified. The proposed shielding structures include the variation of signal TGV pitches and the number of grounded shield TGVs, ground pads, and guard rings, respectively. The effectiveness of the proposed shielding structures was verified up to 20 GHz in frequency-domain measurements. Using the proposed shielding structures, the noise transfer function decreased by 9.4 dB at 5 GHz. Also, the effectiveness of the proposed guard ring structure was verified by a time-domain coupling noise simulation with clock signals at frequencies of 1 GHz. The proposed guard ring successfully suppressed the clock noise coupling between signal TGVs by 60.5% and 69.2% when a signal TGV pitchis 300 and 900 $\mu {\text{m}}$ , respectively.

Effect of latching force on socketed BGA packages with Ni-Au coated solder spheres

Abstract: Solder BGA packages, when used in sockets, face multiple reliability challenges such as damage to the solder and increase in contact resistance due to surface oxides on the solder and intermetallic formation at the interface with socket paddle. To overcome these challenges, a new socket to package interface technology was proposed, which involves the surface modification of solder spheres with diffusion barrier-noble metal coatings, to provide a stable mechanical contact with the socket paddle and mechanical damage resistance. This paper evaluates the effect of socket latching force on the mechanical responses of BGA packages fabricated with solder spheres coated with Ni-Au coating. Effect of different paddle geometries, flat and curved, was also evaluated and results were compared with those for packages with uncoated solder spheres. It was understood for both paddle geometries, that for latching forces beyond 0.2 N per sphere, the equivalent stress generated in the Ni coating layer was greater than the compressive strength of Ni, which would lead to cracking of the Ni layer. It was also confirmed that the contact diameter changed marginally within a range of 2–3 um with the increasing latching force, which was attributed to the very high modulus of Ni which resisted deformation of the sphere.

Reliability Modeling of Micro-vias in High-Density Redistribution Layers

Abstract: The ever-increasing demand for high-bandwidth interconnects has given rise to the need for high IO-density package redistribution layers (RDL). This necessitates scaling down RDL critical dimensions as well as microvias. There are numerous challenges pertaining to scaling down microvias below $5\ \mu \mathrm{m}$ diameter. The main challenge is the thermomechanical reliability of vias in polymer dielectrics. Modeling and design for reliability in various polymer dielectrics is the key to achieve mechanical reliability. This paper presents a model for the prediction of micro-via failure. The effects of via geometry such as-via angle and height as well as material properties such as-CTE and elastic modulus on via failure are presented. Furthermore, modeling results are correlated with experimental results to verify the accuracy of the model. Using this model, it was determined that the conventional via geometry reaches an engineering limit at $2 \mu \mathrm{m}$ of via diameter. Below this size, it becomes difficult to achieve reliable vias in polymers as they do not survive 1000 thermal cycles. Based on the modeling studies, a novel method is proposed for enhancement of reliability of vias below the engineering limit of $2\ \mu \mathrm{m}$ .